Columbia Kermit

home *** CD-ROM | disk | FTP | other *** search

/ Columbia Kermit / kermit.zip / bbcmicro / bbcasmdoc.txt < prev next >

Wrap

Text File | 2020-01-01 | 143KB | 5,082 lines

USING THE 65C02 ASSEMBLER (Version 1.50) This guide describes how to use the 65C02 Assembler produced by Alan Phillips of the Department of Computing, Lancaster University. The reader is assumed to be familiar with the concepts of programming at assembly level. The 65C02 Assembler is copyright (C) Alan Phillips 1986. It may be passed on by anyone, to anyone, and used for any peaceful purpose. No licensing or permission is needed. It may be distributed in any way provided that it is not sold (apart from reasonable handling and media costs), that due credit is made for authorship, and that this paragraph is brought to the attention of the recipient. Edition 2.0 November 1986 Alan Phillips CONTENTS 1. INTRODUCTION 1.1 Installing the Assembler 1.1 The Assembler's CLI mode 1.2 Creating source files 1.3 Starting an assembly 1.3.1 The command line options 1.3.2 Alternative syntax forms for the command line 1.4 Assembly on a second processor 1.5 The object code buffer 1.6 Returning to the calling language 1.7 Use of control keys during assembly 2. SOURCE PROGRAM FORMAT 2.1 Syntax of the line 2.1.1 The label field 2.1.2 The opcode field 2.1.3 The operand field 2.1.4 The comment field 2.2 The syntax of expressions 2.2.1 Numeric expressions 2.2.2 Strings 2.2.3 String expressions 3. THE OUTPUT LISTING 3.1 The source listing level 3.2 Source listing format 3.2.1 The code listing level 3.3 Symbol table listing format 3.4 Error reports 3.5 Output page control 3.5.1 Setting page dimensions 3.5.2 Setting the page title 3.5.3 Setting the timestamp 3.5.4 Control of output layout 3.6 Controlling the output destination 4. DIRECTIVES DEFINING DATA AND CONSTANTS 4.1 Defining constants 4.2 Byte and word directives 4.2.1 Byte directives 4.2.2 Word directives 4.2.3 Using repeat counts 4.3 Character string directives 4.3.1 The ASC and STR directives 4.3.2 The CASC and CSTR directives 4.3.3 Planting special characters 4.4 The HEX directive 4.5 The CODE directive 5. OBJECT FILE CONTROL DIRECTIVES 5.1 Defining the current address 5.2 Using dummy sections 5.2.1 The DS directive 5.2.2 Reading the current address 5.3 Defining object load and execution addresses 5.3.1 Specify the low bytes of the addresses 5.3.2 Specifying the processor 6. SOURCE FILE CONTROL DIRECTIVES 6.1 Chaining source files 6.2 Including one file into another 7. CONDITIONAL ASSEMBLY 7.1 Assembly conditional on expressions 7.2 Assembly conditional on the existence of symbols 7.3 Nesting conditionals 7.4 Listing conditionals 8. PROGRESS REPORTING DIRECTIVES 8.1 The DISP, DISP1 and DISP2 directives 8.2 The WAIT, WAIT1 and WAIT2 directives 8.3 The QUERY directive 8.4 The STOP directive 9. WRITING SIMPLE MACROS 9.1 Using macro parameters 9.2 Specifying macro parameters 9.3 Nesting macros 9.4 Redefining opcodes and directives 9.5 Labels within macros 10. THE MACRO PROGRAMMING LANGUAGE 10.1 Sequence Symbols 10.2 Assembly Time Variables 10.2.1 Creating Assembly Time Variables 10.2.1.1 Local and global ATVs 10.2.1.2 String and numeric values 10.2.1.3 Efficient use of memory 10.2.2 Simple substitution of Assembly Time Variables 10.3 Writing complex macros 10.3.1 Programming macro loops 10.3.1.1 Loops controlled by counter 10.3.1.2 Loops accessing macro parameters 10.3.2 Changing macro parameters 10.3.3 Listing control for macros 10.3.4 Exiting a macro prematurely 10.4 System ATVs Appendices A1. Opcodes and addressing modes A2. Assembler directives A3. Differences from the ADE Assembler Acknowledgements Thanks are due to Dave Morriss, Neil Mercer, Alan Baker, Peter Vince and Mike Tubby for many helpful suggestions and comments during the development of this release of the Assembler. USING THE 65C02 ASSEMBLER 1. INTRODUCTION The 65C02 Assembler runs in sideways RAM or as a sideways ROM on the BBC Models B, B+, B+128, Master 128 and Master Compact, with or without a 6502 Second Processor. It supports all the opcodes of the 6502 and 65C02 processor families, and contains a powerful Macro Programming Language. The Assembler is a disc-oriented system. Source files must be held on disc, and the object code will be written to disc. It can be used with the Acorn DFS, ADFS, NFS and any other Acorn-compatible disc or network filing system, but not on cassette-only machines. There are no source editing facilities contained in the Assembler. However, it is able to accept source produced by any text editor or word-processor program. 1.1 Installing the Assembler The 65C02 Assembler code is completely unprotected. If you have a machine with sideways RAM, you will be able to load it to a RAM bank and run it; if not, you will need to program it into a 27128 EPROM and fit it into a sideways ROM socket. 1.1.1 The Assembler's CLI mode If you have the Assembler fitted into the highest priority ROM slot of your machine, pressing CONTROL-BREAK or powering on will enter it as the current language. In this case the Assembler is said to be in "CLI (Command Language Interpreter) mode". You will see a "*" prompting you for input, and any lines you type will be sent to the BBC Operating System as MOS commands. In this mode there is no need to type a "*" yourself in front of the commands, although it won't matter if you do. From CLI mode you can of course start an assembly with the *ASSEMBLE command described below. Alternatively, you could enter another language by typing, say, *BASIC or *WORDWISE Naturally, you are able to issue DFS commands in CLI mode, or you could *TYPE files, and so on. You can't use the BASIC statement MODE to change screen mode, though, since the current language is not BASIC. However, for convenience the Assembler will respond to a command *MODE, so you could type 1 USING THE 65C02 ASSEMBLER *MODE 3 to set mode 3 if you wished. This command is available either from the Assembler's CLI mode or from any other language. 1.2 Creating source files Though the Assembler does not contain any editing facilities, it will accept files produced on just about any word processor or source editor. The source you create should contain only printable characters, spaces and TAB characters, and each line should end with a carriage return byte (with optionally a line feed character before or after it). You can create suitable files easily with WordWise or View: a. Using WordWise Files saved by menu option 1 can be used directly as input to the Assembler, provided that you have not included any embedded commands. The WordWise TAB character, which appears on the editing screen as a right-pointing arrow, will be accepted as a TAB by the Assembler. Remember to press RETURN at the end of each source line. Alternatively, you can spool the file to disc first using menu option 8 - but this will create a bigger file, that will take longer to assemble. b. Using View A file created by the View SAVE, WRITE or EDIT commands can be directly input to the Assembler. You should not include rulers, stored commands or highlight codes in this file. You should always remember to press RETURN at the end of every source line: it will be necessary to turn off the Format and Justify options when typing the source, otherwise View will interfere with this. 1.3 Starting an assembly The Assembler is started with the *ASSEMBLE command. This is followed by a number of "option flags" that tell the Assembler exactly how you wish it to operate. The simplest use of the command would be, for example: *ASSEMBLE SOURCE 2 USING THE 65C02 ASSEMBLER where the source file name is SOURCE. This format performs a "syntax check run" - no object code is produced. This will give you the fastest possibly assembly, and it can be very useful when checking a large program for errors. To produce an output file, you need to specify a "-O" option: *ASSEMBLE SOURCE -OPROGRAM where the "-O" is followed by the name of the object file you wish to produce. After assembly, the Assembler will return you to the language you were running when you issued the *ASSEMBLE command. If you have a machine with shadow screen capability, the Assembler will automatically turn it on to give you the maximum space for symbols. It will return the shadow screen to its initial state when it finishes, and since this involves clearing the screen in a mode change, it will pause for you to press a key before it does so. 1.3.1 The command line options The various options you can specify to the command are listed below. Each option flag begins with a "-" character; some options must be followed immediately by numbers or filenames. You must separate one option from another by spaces. Options can be given in any order, but the first parameter to the command must always be the name of the first (or only) source file. -A You must specify this option to assemble the source for BBC KERMIT, and it is recommended for sources originally written for the ADE Assembler. It makes the syntax of labels ADE-compatible, and treats only the first 6 characters of each as significant. This option implies a -R. -B Specifies that a memory buffer is to be used for object code. This must be followed by a number in the range 1..16 (see section 1.5) -C Specifies the default code listing level (see the CLST directive). This must be followed by 0, 1 or 2. the default value is 1. -G Specifies that the Assembler is to restart the calling language immediately the assembly is over. (See section 1.6) -L Specifies the default source listing level (see the LST 3 USING THE 65C02 ASSEMBLER directive). This must be followed by 0, 1, 2 or 3. The default value is 0. You can also control the list level during the actual assembly with function keys 0, 1, 2 and 3. -M Instructs the Assembler to change screen mode before it starts to process the source. The number of the mode you wish to use should follow the letter M, and must be in the range 0..7. -O Specifies the name of the object file, which must follow the option flag. If you omit this, the Assembler performs a syntax check only and does not produce an object file. -P Specifies that the listing is to be sent to the printer only. If you omit it, the listing is sent only to the screen. You can change the destination of the listing at any time during assembly by pressing CONTROL-P. -R If this is specified, 65C02 opcodes and addressing modes are rejected. If you omit it, all 65C02 codes are assembled. -S Specifies that lines skipped in conditionals are not listed (see the SFCOND and LFCOND directives) -W Specifies that the Assembler should wait after displaying a line containing an error, so you can note down the details. To resume assembly, press any key. This option has no effect if you are sending output to a printer. You can change the setting of this option at any time during assembly by pressing CONTROL-W. As an example, the line: *ASSEMBLE SOURCE -OPROG -P -L2 -B8 will assemble from file SOURCE, putting the object code into file PROG. The default listing level used will be 2, and the listing will be sent to the printer only. An 8 kilobyte memory buffer is to be used for the object code to speed up assembly. The Assembler will leave the screen in whatever mode you have selected, so that for large assemblies you may need to change to mode 7 to provide room in memory for the symbol table. Shadow mode will automatically be selected if it is available, so that on the B+, B+128 and Master series you will always have the maximum space available. 4 USING THE 65C02 ASSEMBLER 1.3.2 Alternative syntax forms for the command line The example given above, and the output the Assembler will produce if you type *HELP ASSEMBLER detail the full form of the command line syntax. As you gain experience in using the Assembler, however, you might wish to take advantage of various ways of specifying the options in a more compact form. Firstly, the Assembler allows you to specify the options all at once, without the need to put the "-" flag before each of them. Thus, you could type either *ASSEMBLE SOURCE -M3 -W -L2 or the shorter, but less readable *ASSEMBLE SOURCE -M3WL2 Secondly, the "-O" flag to specify an object file is optional. The Assembler will take the second parameter of the command as an object file name, unless it starts with a "-" character. Thus *ASSEMBLE SOURCE -OPROG and *ASSEMBLE SOURCE PROG are identical in effect. If you should ever need to start the object file with a "-" character (e.g. if you wish to use a temporary file system on a Master 128) you will have to specify the "-O" flag explicitely. 1.4 Assembly on a second processor The 65C02 Assembler is compatible with both the external 6502 second processor, and with the Master Turbo-Card. Using a second processor will give a faster assembly due to the higher clock rate of the processor, and will also give you more memory for the symbol table. A second processor allows you to organise your source files to take advantage of a source file memory buffer. Any source file that is less than 14 kilobytes in size is *LOADed into memory and assembled from there: since the Assembler does not then need to wait for data to be fetched from disc, assembly will speed up dramatically. It is well worth arranging your program so each of its source files will fit into this buffer area. 5 USING THE 65C02 ASSEMBLER The source buffer is automatically switched on when a second processor is in use. 1.5 The object code buffer The Assembler allows you to select an object code buffer in memory. Code bytes generated from the source are written into this buffer, rather than being sent directly to the object file, and this gives a substantial increase in speed of assembly. Whenever the buffer is full, the assembly will pause in reading source to write the contents to the object file. It then resumes assembly until the buffer again fills, and so on. You can specify the buffer size with the "-B" option. The flag is followed by a number between 1 and 16, giving the buffer size in kilobytes. Note that using a buffer means that there is correspondingly less space available for the symbol table. The fastest assembly will be achieved when the object code fits entirely into the buffer. In this case, the Assembler will produce the object file with *SAVE, which is substantially faster even than writing it in large blocks. 1.6 Returning to the calling language When an assembly finishes, the Assembler will always restart the language ROM that was in use when you issued the *ASSEMBLE command. If this language was BASIC or the Assembler's own CLI mode, there will be no difficulty: however, some languages such as WordWise or View immediately clear the screen when they start, so you may not have time to read the final lines that the Assembler displayed on the screen. To overcome this, the Assembler is able to pause when it ends until you press a key. It will do this whenever the previous language is not BASIC or the Assembler's CLI mode. If you don't wish it to pause (you may for example not want to read the final screen, or the language may not clear the screen), simply specify a "-G" option in the command line. Now the Assembler will immediately restart the previous language without a pause. There are two cases where the Assembler will always pause regardless of the "-G" option or the previous language. These are: - When an error is detected in the *ASSEMBLE command you typed or - On a machine with a shadow screen that was turned off when you began assembly. The Assembler will turn the shadow screen on automatically, and will always turn it off at the end: as this requires a mode change which will clear the screen, it will always 6 USING THE 65C02 ASSEMBLER pause. 1.7 Use of control keys during assembly During an assembly you can use various control keys to check on progress or give commands to the Assembler. - Function keys f0, f1, f2 and f3 will force the Assembler to change the source listing level to 0, 1, 2 or 3 respectively. Once you have pressed one of these keys the level you set is locked: the Assembler will process LST directives in the source, but will not implement them. This lets you, for example, change the list level during an assembly to check on what's happening, or to suppress a listing you decide you don't want. To cancel the locking of the list level and return to that set by the last LST directive processed, simply press CONTROL-L. - CONTROL-H displays some help information to remind you about the use of control keys. The Assembler will pause after displaying the information: to resume simply press any key. - CONTROL-N and CONTROL-O turn the BBC Computer's paged scrolling mode on and off respectively. You could use these keys, for example, to pause a listing on the screen after every screenful at some point during an assembly. - CONTROL-P controls whether the Assembler sends output to the printer. If it is printing, pressing CONTROL-P will stop it: if it is not printing, CONTROL-P will start it sending output to the printer. - CONTROL-Q lets you find out where the Assembler is in the current file. It will tell you whether it is on pass 1 or 2, then give you the number of the line it is about to process. If the number is followed by "(M)", the source line is a macro call and the Assembler is currently expanding that macro. - CONTROL-W reverses the current "wait after error" action set by the command line's "-W" option. - SPACE pauses the assembly. You can use this to examine a listing on screen at your leisure. To resume assembly, simply press any key. 7 USING THE 65C02 ASSEMBLER 2. SOURCE PROGRAM FORMAT The Assembler will read source from files produced by any editor or word-processor program, as described in section 1.2. The source is seen as a number of lines, each ending with a carriage return byte ($0D). Line feed characters are ignored, as are all other control characters and any with the most significant bit (bit 7) set. The general format of a line is: label opcode operand ;comment Depending on circumstances, the label and operand fields may be optional or mandatory; or they may need to be omitted. All lines may have a comment field, which is begun with a ";" or a "\" character. If you include a label field, it must begin in the first character of the line. You may separate the fields from each other with any number of spaces and/or TAB characters (code $09). No line may exceed 132 bytes in length; any that do will be truncated. Any line whose first non-space character is a "*", a ";" or a "\" is treated as comment. 2.1 Syntax of the line 2.1.1 The label field A label consists of a string of characters, starting with a letter, and containing any combination of letters, numbers and the characters ".","$" and "_" (underscore). Any lower-case letters are translated to upper-case. Labels may be of any length, and all are significant unless you have specified the "-A" option in the command line, when only the first 6 are significant. A label may be terminated with a ":" character that is not considered part of the label. Examples of valid labels are: OSWRCH osbyte Program.start ITEM_33: A label may be written in a line on its own: the value it is set to will be the value of the current address. A variant of the label is the Sequence Symbol, described in section 10.1. This has the same format as a normal label, with the exception that the first character must be a "%" character. 8 USING THE 65C02 ASSEMBLER Where the label is a macro name (i.e. in the MACRO directive) it may not exceed 8 characters in length. 2.1.2 The opcode field The opcode field, if present, is separated from the label by one or more spaces or TABs. If there is no label, the field must be preceded by at least one space or TAB. The opcode field can contain either a normal opcode mnemonic (such as LDA), an Assembler directive or pseudo-op (such as SFCOND) or the name of a macro that you have defined previously. 2.1.3 The operand field For some opcodes and directives an operand field may be supplied. This will consist of one or more elements, separated by commas and optional spaces. Except within strings delimited by the single- or double-quote characters, spaces are not significant in the operand field. Thus you can use them to make complex numeric expressions more readable: for example you could write LDA TABLE+(1+FRED/3*(BERT+$1E)) as LDA TABLE + (1 + FRED/3 * (BERT + $1E) ) if you wished. 2.1.4 The comment field The comment field can be used to annotate the line. It is begun with a ";" or a "\" character: anything following is not processed by the Assembler. 2.2 The syntax of expressions Throughout this guide you will see references to "expressions", for example in the definitions of directives. The syntax of these is as follows: 2.2.1 Numeric expressions These are indicated in definitions as "<expr>", and consist of terms separated by arithmetic operators. For example, a valid expression might be 9 USING THE 65C02 ASSEMBLER FRED*(2+BERT) The Assembler performs all calculations using 16-bit arithmetic. Any results or intermediate values that overflow will be truncated, and no warning will be given. The various elements that you can use in numeric expressions are these: a. Symbols These are the labels that you define in the label fields of lines, and in the evaluation of the expression they are replaced by the value of the label. b. Decimal constants These are numbers, composed of the characters '0'..'9'. For example, 123 c. Hexadecimal constants These are numbers, composed of hexadecimal digits '0'..'9' and 'A'..'F', preceded by a "$" or a "&" character. For example, $AF34 d. Binary constants These are numbers composed of binary digits '0' and '1', preceded by a "%" character. For example, %10100111 e. Character constants These are single characters, enclosed in single-quotes. The Assembler will use the ASCII code of the particular character. You can specify that a character value is to be used with bit 7 set by preceding the character with a "^", and can specify a control character by preceding it with a "|". Thus the code for an "A" with bit 7 set is represented as '^A' and that for CONTROL-A as '|A'. The character codes for "^" and "|" are obtained by writing 10 |USING THE 65C02 ASSEMBLER| the characters twice (i.e. as '^^' and '||'). Note that you must always include the final quote character. For example, 'X' f. Operators A number of arithmetic operators can be used in expressions. They are divided into groups of varying priority, which, in decreasing order are: () Parentheses - Unary minus ~ Unary 1's complement (NOT) & Bitwise AND ! Bitwise OR = Equality # Inequality > Greater than < Less than * Multiplication / Integer division + Addition - Subtraction The "=" and "#" operators return values of -1 and 0 for TRUE and FALSE respectively. Additionally, expressions may be prefixed with two unary operators > and <, which select the low-byte and high-byte of the result respectively. These operators have the lowest priority of all. g. Current address The current value of the address counter may be represented in expressions by the "*" character. For example: *-3 2.2.2 Strings 11 USING THE 65C02 ASSEMBLER These are indicated in definitions as <string>, and consist of one or more ASCII characters, enclosed in delimiters. Only the single- and double-quote characters (' and ") can be used as delimiters, and the start and end delimiter must be the same. For example: 'This is a string' "And so, you see, is this" To include the ' or " character in a string, use the other as the delimiter. 2.2.2 String expressions These are indicated in definitions as <stringexpr>, and all involve comparisons between strings. The general format is <string> <op> <string> where <op> is one of the operators = Equality # Inequality > Greater than < Less than 12 USING THE 65C02 ASSEMBLER 3. THE OUTPUT LISTING The assembly listing is produced in pass 2. You can control whether it appears or not, whether it goes to screen or printer, and the amount of detail it contains. 3.1 The source listing level The main control you have over the listing is through the "source listing level". This is a value in the range 0..3 that you can set either with the LST directive, the -L command line option, or with function keys 0 to 3 during the course of an assembly. The listing levels have the following meaning: 0 This level suppresses all listing except the reporting of errors. 1 This level will list all source lines that originate in source files, but not the expansions of macros. 2 This level lists all lines from source files and macro expansions, but not Macro Programming Language statements such as AIF. 3 This level lists all source lines. By default, the Assembler sets the source list level to 0, so all you will see will be error reports. You can change the default setting in the *ASSEMBLE command by using the "-L" option: thus *ASSEMBLE SOURCE -L2 would cause the Assembler to start with a default listing level of 2. Within the source, you can control the list level with the LST directive. Thus: LST 1 would set the list level to 1. Simply writing LST with no operand field will reset the value to the default, without you needing to build this value into the actual source. Additionally, you can override the list level in use while an assembly is in progress. Pressing one of function keys 0 to 3 will force the corresponding list level to be adopted, so that, for 13 USING THE 65C02 ASSEMBLER example, you could force the Assembler to show you parts of an assembly to monitor what was being done. Pressing one of these keys will "lock" the list level to that selected, so that LST directives in the source will not be able to turn the listing off again. The directives are noted, though, and you can return listing level control to them at any time by pressing CONTROL-L. 3.2 Source listing format The listing begins with the source lines, including any code bytes they have generated. The first character on the line indicates where the line was read from. If it came from an INCLUDE file the character will be "I"; if the line is from a macro it will be "M". Otherwise, the character is a space. The next element is the line number, reset to 1 at the start of each source file. Next comes the address field. This will normally contain the current value of the address counter; in some directives, though, such as LST, it contains the value of the operand expression. Then comes a hexadecimal representation of the first 3 object bytes generated by the line; this is followed by the source line itself. By default, if the source line generated more than 3 object bytes, these will be listed on subsequent lines. For example, a listing line might show: I 23 AB34:C9 0A CMP #10 where the source line is the 23rd line in an INCLUDE file and reads "CMP #10". The code bytes generated are $C9 and $0A, and they are planted starting at address $AB34. 3.2.1 The code listing level As mentioned above, the first three code bytes generated by a source line are listed on that line, and any others will appear on subsequent lines. This may produce a lot of output, particularly if you define lots of strings, so the Assembler gives you control on how much it lists. The amount is controlled by the "code listing level". This is a value in the range 0..3 that you can set either with the CLST directive or the -C command line option. 14 USING THE 65C02 ASSEMBLER The code listing levels have the following meaning: 0 Only the first 3 bytes generated by a line are listed. 1 All bytes are listed from all lines except the CODE directive. 2 All bytes are listed, however they are generated. By default, the Assembler sets the code list level to 1. You can change the default setting in the *ASSEMBLE command by using the "-L" option: thus *ASSEMBLE SOURCE -C0 would cause the Assembler to start with a default code listing level of 0. Within the source, you can control the code listing level with the CLST directive. Thus: CLST 2 would set the list level to 2. Simply writing CLST with no operand field will reset the value to the default, without you needing to build this value into the actual source. 3.3 Symbol table listing format If the source listing level is not zero at the end of the assembly, the Assembler will list the symbol table, showing you the value of all the symbols defined in the source. The symbols are listed in alphabetical order, and only the first 13 characters of each will be shown. The value of each symbol is shown in hexadecimal. If the value is shown as "????", the symbol was used but was not defined. If the value is "****", the symbol was defined more than once. A "-" character after the value indicates that the symbol was defined, but was not used anywhere else in the source. 3.4 Error reports The Assembler will list all lines that generate errors, regardless of the listing level. The errors are reported with self-explanatory 15 USING THE 65C02 ASSEMBLER text messages following the line. Where appropriate, the Assembler will attempt to indicate where on the line the error occured. The text you might see would look something like: ****** Err : Undeclared symbol at about character 19 This indicates that the erroneous symbol is at about the 19th character in the line, which starts at character 1. Remember, though, that the number refers to the source line: the listing line may have expanded any TAB characters you used, and the number will then not correspond to what you see on the page. 3.5 Output page control The Assembler provides a number of directives that you can use to specify the exact format of the output page, and to let you tailor the listing to your exact needs. 3.5.1 Setting page dimensions The PAGE directive defines the size of the page you are using. It takes two parameters, which are the total depth of the page in lines, and the width in characters you wish printed. Thus, PAGE 66,132 tells the Assembler that the page is 66 lines deep, and that you wish lines to contain a maximum of 132 characters. The Assembler will always allow a small gap at the end of the page to avoid the perforations, and will truncate any lines longer than the maximum width you specify. The value of the width you define with PAGE becomes effective at once. The depth, though, is not used until the next page throw, so you would normally follow the directive with a TTL or a SKP H directive. By default, the Assembler will print lines of 80 characters on paper that is 66 lines deep. It moves to the top of a new page by sending a Form Feed ($0C) character to the printer. 3.5.2 Setting the page title The TTL directive lets you set up a page title that is printed on 16 USING THE 65C02 ASSEMBLER the top of every listing page. The title can be up to 20 characters long, and can contain any printable text. For example, TTL 'Screen Dumper' will print "Screen Dumper" at the top of each page. The TTL directive will cause a page throw to occur immediately, and the next page output will use the title it defines. 3.5.3 Setting the timestamp If you are using a Master 128, the Assembler will automatically print the current date and time at the top of each output page, taking the values from the built-in real-time clock. The other BBC models do not contain clocks, so the TIME directive lets you simulate the effect by defining the string used. The TIME directive allows you to define a string of up to 25 characters that will appear in the page header. The string can contain any printable characters: it is not constrained to be a date and time value, so you can use it as a sub-title if you wish. For example, you could use TIME 'Friday at 1030' or TIME 'First Module' The directive is ignored on a Master 128, so there is no need to change a source that is being assembled on both a model B and a Master 128. 3.5.4 Control of output layout There are a number of directives that let you lay the output page out in more detail, so you can delimit sections of source for easier inspection. The REP directive provides a convenient way of separating sections of code in the listing. For example, REP 80 17 USING THE 65C02 ASSEMBLER will print a line of "*" characters in the listing for you (and takes much less space in the source file than that line itself would). The CHR directive lets you change the character used to make the line, so that, for eaxmple you could select lines of "-" characters with REP - The SKP directive lets you break the listing up for easier reading. For example, SKP 5 would leave a 5 line space on the page. The format SKP H causes the Assembler to start a new page. Note that the SKP directive itself will never be listed unless it contains an error. 3.6 Controlling the output destination By default, the Assembler sends the output listing to the screen, in whatever mode you have selected. If you wish, you can direct output to the cuurently-selected printer by specifying the "-P" option in the command line. Thus, for example, *ASSEMBLE SOURCE -L2 -P would assemble from source file SOURCE and send the listing, at listing level 2, to the printer only. At any time during an assembly, you can change the output destination by pressing CONTROL-P. This will switch the output destination between printer and screen: you can do this as often as you wish. 18 USING THE 65C02 ASSEMBLER 4. DIRECTIVES DEFINING DATA AND CONSTANTS The Assembler contains a number of directives that allow you to define values in the object file. You can specify byte values, two-byte or word values, and character strings, and also set up symbols to hold constants. 4.1 Defining constants Assigning symbols to various constant values is a considerable help in writing assembly-level programs, both in terms of making a source more readable and also in helping when you need to change it. For example, you could refer to an Operating System routine by writing its address $FFEE every time, or you could declare a symbol OSWRCH to stand for that name. You can declare symbols to use as constants with the EQU directive. In the example above, you would write OSWRCH EQU $FFEE and then call the routine as, say JSR OSWRCH You can write any numeric expression in the operand field of the EQU directive, but the expression must not contain any forward references. 4.2 Byte and word directives There are 6 directives that plant byte and word values. DFB plants bytes, and DFW and DFDB plant word values. For convenience, and compatibility with other asemblers, the directives can also be written as DB, DW and DDB respectively. All the directives can be given one or more expressions in the operand field, separated by commas, and these expressions give the values of the bytes or words to be planted. If you specify a label in the label filed of the line, it is set to the address of the first byte of the first value planted. 4.2.1 Byte directives The directive DFB (or its equivalent DB) allows you to define single-byte values which are written directly into the object file. The operand field is one or more expressions, each of which 19 USING THE 65C02 ASSEMBLER should lie in the range 0..255 . For example, DFB 1,2,$FF,FRED+1 will write 4 bytes to the object file: the values will be 1, 2, 255 and the value of FRED+1. 4.2.2 Word directives The directives DFW (or its equivalent DW) and DFDB (or its equivalent DDB) allow you to define two-byte or word values which are written directly into the object file. The operand field of each is a list of one or more expressions, each of which should lie in the range 0..65535. The difference between the directives lies in the order in which the two bytes are written to the object file. The DFW directive writes the bytes in low-byte, high-byte order; the DFDB directive writes them in high-byte, low byte order. Thus, for example, DFW $1234,$ABCD will output the bytes $34, $12, $CD, $AB to the object file in that order, but DFDB $1234,$ABCD will output the bytes $12, $34, $AB, $CD. 4.2.3 Using repeat counts Occasionally, you will need to use the byte or word directives to plant a number of bytes or words containing the same value. For example, you might need to write DFB 5,5,5,5,5,5,5 to plant 7 bytes containing the value 5. For small numbers of repeats, this is no problem, but it can be onerous if, say, you needed 49 bytes containing 5. The Assembler gives you a very convenient shorthand way of repeating values in all the byte and word directives. Every expression you use can be prefixed with a "repeat count", which tells the Assembler to plant the value itself more than once. The 20 USING THE 65C02 ASSEMBLER repeat count is specified in "[]" brackets: thus, to plant the 49 bytes mentioned above, you could simply write DFB [49]5 Like the value itself, the repeat count can be any expression, but it must not contain forward references. Thus you could write DFB [(COUNT+27)/2]$FF You can freely mix values with and without repeat counts in any of the byte and word directives. 4.3 Character string directives The four directives ASC, CASC, STR and CSTR allow you to plant character string values into the object file. All the directives require you to specify a character string, enclosed in single- or double-quotes, in the operand field. If you include a label in the label field of the line, it is given the value of the address of the first byte planted. 4.3.1 The ASC and STR directives These directives plant simple strings in the object file. ASC plants the string exactly as you specify it; STR plants the string, and then automatically plants a carriage-return byte (code $0D) immediately after it. Thus ASC 'ABC' plants the bytes $41, $42, $43 in the obect file, and STR 'ABC' plants the bytes $41, $42, $43, $0D. 4.3.2 The CASC and CSTR directives These directives plant "counted strings" in the operand field. They are similar to ASC and STR, except that the bytes planted are preceded by a single byte giving the length of the string. Thus, to use the same examples as above, CASC 'ABC' 21 USING THE 65C02 ASSEMBLER plants the bytes $03, $41, $42, $43 in the object file, and CSTR 'ABC' plants the bytes $04, $41, $42, $43, $0D. Note that the count byte planted by CSTR includes the $0D byte that the directive adds itself. 4.3.3 Planting special characters The strings you give as the operands to ASC, CASC, STR and CSTR must of course be made up of printable characters in the source file, but the Assembler allows you a way of specifying control characters and characters with the most-significan-bit set to 1. To specify a control character, you should precede the character itself with a "|" character. Thus, if you wish to plant the value of CONTROL-A, the string should contain "|A". This method is identical to the way in which you specify control characters in Operating System commands such as *KEY. To specify a character with bit 7 (the most significant bit) set, precede it with a "^" character: thus, to plant an "A" with bit 7 set, specify it in the string as "^A". For example, ASC 'A|AB^B' plants the bytes $41, $01, $42, $C2 in the object file. Should you need to specify the "|" or "^" characters, you should double them in the string (i.e. write them as "||" and "^^") 4.4 The HEX directive This directive is rather a hybrid of the DFB and ASC directives. It plants a series of byte values into the object file, but you specify the values as a string of hexadecimal digits. For example, HEX '1234ABCD' plants the byte values $12, $34, $AB, $CD into the object file. The string you supply must contain valid hexadecimal digits (i.e. '0'..'9' and 'A'..'F'). You can give the letters in either upper- or 22 USING THE 65C02 ASSEMBLER lower-case. If the string contains an odd number of characters, the Assembler will automatically add a '0' to make the final hexadecimal value complete. 4.5 The CODE directive The code directive gives you a convenient way of including things like screen dumps or already-compiled relocatable subroutines, into your programs. Suppose, for example, you had prepared a mode 7 screen dump with some Teletext editor system, and wanted to use this as a banner page when your program started. One way of including this into the code would be to examine the dump and convert it into the appropriate DFB directives, planting each byte explicitely by hand. However, this might be a very time-consuming process, and would be very error-prone. Using CODE, though, removes the need to translate the dump into DFB or similar directives. If, for example, the dump was in file BANNER, writing CODE BANNER would cause the Assembler to read it in and copy the bytes directly to the object file, without processing them in any way. By default, the listing will show only the first 3 bytes included from the CODE file, since otherwise a considerable amount of paper might be used. If you do want to see all the bytes in the listing, use the CLST directive to set the code listing level to 2 before using the CODE directive itself. 23 USING THE 65C02 ASSEMBLER 5. OBJECT FILE CONTROL DIRECTIVES The Assembler contains a number of directives that control the object file. They let you define the actual address that the code generated will run at, map symbol definitions into workspace, and set the load and execution addresses for the final result. 5.1 Defining the current address In almost all programs you assemble, you will need to tell the Assembler the actual address at which the resulting code is to run. It will then be able to assign the correct address values to labels. You define the current address with the ORG directive. For example, you could write ORG $1900 before the first code line in the source. This would set the address value to $1900, and the Assembler would produce code based on that address. You can specify the address in the operand field of the ORG directive as any expression you like: however, since the Assembler must know the value to use on pass 1, the expression cannot contain forward references. You can use as many ORG directives as you wish within your program source, and you can move the current address value to anywhere you wish. However, you should remember that the ORG directive has no effect on where in the object file bytes are written. Thus, the source lines ORG $1900 LDA #1 ORG $3000 LDA #2 would set the address to $1900, and write the two bytes for the LDA instruction to the object file. It then sets the address to $3000 and writes the bytes for the second LDA instruction: however, the four bytes will be contiguous in the object file - the Assembler will not generate a gap. You could use this effect, for instance, to write an object file containing many sections of overlay code - each section would be assembled to run at the same address, but they would lie one after the other in the object file. If you do require to leave a gap in the object file, you can use the DS directive to generate the required gap yourself (see section 5.2). There is no way of making the Assembler move backwards in the object 24 USING THE 65C02 ASSEMBLER file. 5.2 Using dummy sections A "dummy section" is a convenient way of laying out symbols within the workspace that your program needs. For example, suppose your program needed some page zero workspace. You could build the actual numeric addresses into the source, but it's much better programming practice to define some symbols to identify the locations. One way of doing this would be to use the EQU directive, which simply sets up a symbol and gives it a value. Thus you could write WORK0 EQU $00 WORK1 EQU $01 WORK2 EQU $02 to set up three symbols. This is a perfectly adequate technique, but suffers from the disadvantage that you would have to change a lot of the source if, say, you ever wanted the page zero workspace to start at some place other than address $00. A better technique is to define workspace in a dummy section, which is a region of the source between DSECT and DEND directives. DSECT instructs the Assembler that a layout is being defined: it will process anything that follows it, but it won't generate any code for the object file. All it will do is work out what addresses any labels will take. Thus, you could write the above example as DSECT WORK0 DFB 0 WORK1 DFB 0 WORK2 DFB 0 DEND Here each of the workspace locations has been defined as one byte long with a DFB directive. Since the Assembler generates no code within a DSECT, the actual value you place in the operand field is quite arbitrary. This technique also lets you see clearly that you've set the locations up as one-byte values. If you wished, say, to make them two-byte values, you could change the DFB directives to DW. The Assembler would then allow two bytes to each label, and the source would show this clearly. In the example above, we have implicitely taken advantage of how the Assembler handles the address value within a DSECT..DEND block. When it meets the first DSECT in a source, the Assembler makes a note of the address value it is using in the code, and resets it to zero 25 USING THE 65C02 ASSEMBLER for the DSECT..DEND block. When it meets the DEND, it resumes using the address it saved at the start. On the second occurence of a DSECT, the effect is the same, except that now the address within the DSECT..DEND block resumes at the value it reached in the last block. If you don't wish to operate in this way, you can set the address value with the DSECT..DEND block yourself with the ORG directive. Thus, if you wanted your page zero workspace to begin at some other location, you could write the source as DSECT ORG $70 WORK0 DFB 0 WORK1 DFB 0 WORK2 DFB 0 DEND 5.2.1 The DS directive Quite often you will need to define a large area of workspace within a DSECT..DEND block, for example as a source line input buffer. The DFB and DW directives are convenient ways of laying out byte and word values, but there are no built-in directives to lay out, say, a 256-byte buffer. To do this, you can use the DS directive, which lets you lay out any number of bytes. For example, to lay out a 256-byte buffer you could write DSECT ORG $A00 BUFFER DS 256 DEND The operand field of the DS directive tells the Assembler how much space you wish to lay out. Normally the DS directive is used in DSECT..DEND blocks, but you can also use it in the code area of a program as well. In this case, the Assembler will write the appropriate number of bytes containing the value zero to the object file. 5.2.2 Reading the current address You can use the value of the current address counter within expressions by referring to it with the special symbol "*". This can be used exactly as any other symbol in expressions, so that, 26 USING THE 65C02 ASSEMBLER say, you could work out the size of a data table with TABLE DFB 1 DFB 2 DFB 3 T.SIZE EQU *-TABLE-1 where T.SIZE will be set to the number of bytes defined in the preceding table. 5.3 Defining object load and execution addresses Every machine-code program on a BBC Computer needs to have a load address and an execution address associated with it. These values are stored in the file's catalogue entry: when you then *RUN the file, the filing system will load it to memory starting at the load address, and will enter it with a JSR instruction going to the execution address. The addresses also tell the filing system whether the program is to be run on a second processor, or on the BBC Computer itself. The filing system regards load and execution addresses as 4 byte (32 bit) values. The lower two bytes or word of each address represent an actual address in memory and the top two bytes or word of the address is a value that indicates which processor the file is to be loaded on. The Assembler can handle only two-byte values in its arithmetic, so it is necessary to set the two parts of each address separately. 5.3.1 Specifying the low word of the addresses There are two ways of setting up the low words of the addresses when you assemble programs. By default, the Assembler will set the low words of both the load and the execution addresses to be the value set by the first ORG directive that is not in a dummy section, and in many cases this is all you need do. If, though, you require something else, you can use the LOAD and EXEC directives to set the low word of the load and execution addresses respectively. Thus, for example, LOAD $1900 EXEC START would set the low word of the load address to be $1900, and the low word of the execution address to be whatever address the label START is defined as. 5.3.2 Specifying the processor 27 USING THE 65C02 ASSEMBLER If the top word of the load address is $FFFF, the file is loaded into the BBC Computer's own memory, whether or not you are using a second processor. If the top word is 0, though, the file will be loaded into the second processor if you are using one. By default, the Assembler produces object files with the top word of the addresses set to $FFFF, so the files will load to the BBC Computer's memory. The MSW directive, though, lets you change this value: thus MSW 0 lets you change the address to 0. You should always include this directive if you write programs to run on a second processor. 28 USING THE 65C02 ASSEMBLER 6. SOURCE FILE CONTROL DIRECTIVES The Assembler provides a number of directives that allow you to control the source files which you are assembling. This lets you split your source up into more than one file. 6.1 Chaining source files If you are writing a large program, you will find it convenient to split the source into a number of small files, rather than keeping it in one large one. This approach not only makes editing easier, but also helps you structure the source to reflect the organisation of the program. Additionally, if you have a second processor, keeping source files to less than 14 kilobytes in size will give a very considerable increase in assembly speed (see section 1.4). You can instruct the Assembler to assemble multiple files with the CHN directive. This tells it to close the current source file and open the one specified in the operand field. Thus, you could write CHN FILE2 as the last line in FILE1. The Assembler will close FILE1, and continue assembly with FILE2. Note that if the CHN directive is not the last line a file, anything following it will be ignored. You can use CHN as many times as you wish in an assembly. 6.2 Including one file into another A different way of using multiple source files is to use the INCLUDE directive. This tells the Assembler to start assembling another source file; unlike CHN, however, the first file is not closed. When the Assembler comes to the end of the included file, it resumes in the original file at the line after the INCLUDE directive. One use of this directive might be to include a file of standard definitions or routines into a source. You might then write INCLUDE STDSUBS to assemble the source of some standard routines. Lines that result from assembling an included file are marked in the listing by having an 'I' in the first column. Note that you cannot use an INCLUDE directive inside a file that 29 USING THE 65C02 ASSEMBLER you are including already. Nor can you use the CHN and CODE directives. 30 USING THE 65C02 ASSEMBLER 7. CONDITIONAL ASSEMBLY The 65C02 Assembler has full conditional assembly facilities, allowing you easily to change the source being compiled. 7.1 Assembly conditional on expressions Conditional assembly that depends on the value of an expression is achieved by using the IF, ELSE and FI directives. The most general form of construction is this: IF <expr> . TRUE block . ELSE . FALSE block . FI <expr> can be any numeric expression, as described in section 2.2, but it must not contain any forward references. If <expr> is non-zero, the Assembler takes the condition as TRUE, and will assemble the lines in the TRUE block. On coming to the ELSE it will ignore all the subsequent lines, until it reaches the FI; here assembly will continue normally once more. If <expr> is zero, the condition is FALSE. The Assembler will ignore the lines that follow until it reaches the ELSE; it then will assemble the lines in the FALSE block. If you wish, you can omit the ELSE and the FALSE block of code, to form IF <expr> . TRUE block . FI Here, if the condition is FALSE, the Assembler will ignore all lines up to the FI, so no code is assembled in this case. 31 USING THE 65C02 ASSEMBLER For example, you might have a symbol DEBUG.MODE defined at the very start of the source to show if debug code is to be assembled or not. This might be defined as DEBUG.MODE EQU -1 ;use -1 for TRUE, 0 for FALSE Then, throughout the rest of the source, you can put the debug code in an IF condition of the form IF DEBUG.MODE . debug mode version . ELSE . non-debug mode version . FI Depending on circumstances, you may wish to reverse the condition tested to include some code if debug mode is not selected. You can use the unary 1's complement or NOT operator here: thus IF ~DEBUG.MODE . non-debug mode version . FI includes code if DEBUG.MODE is set to 0. The IF..ELSE..FI construction can also be written using the directives DO..ELSE..FIN (or with any combination of them) 7.2 Assembly conditional on the existence of symbols Two variants of the IF directive, IFDEF and IFNDEF, allow you to test for the existence or otherwise of symbols. IFDEF is TRUE if the symbol in the operand field does exist; IFNDEF is TRUE if it does not. Thus the example above might have been written as 32 USING THE 65C02 ASSEMBLER IFDEF DEBUG.MODE . debug mode version . ELSE . non-debug mode version . FI where the mere existence of the symbol DEBUG.MODE would select the debug code, regardless of its value. 7.3 Nesting conditionals The Assembler allows you to "nest" conditionals. Within a nested condition, an ELSE directive will be associated with the immediately preceding IF, IFDEF or IFNDEF. 7.4 Listing conditionals By default, the Assembler will list all the lines that it skips in a conditional, printing an "S" in the address field. To save paper, or make the listing clearer, you can suppress listing of skipped lines with the SFCOND directive, or by specifying the "-S" option in the command line. you can re-instate the listing of false conditional branches at any time with the LFCOND directive. 33 USING THE 65C02 ASSEMBLER 8. PROGRESS REPORTING The Assembler supports several directives that allow you to output progress reports to the screen to show the course of an assembly. 8.1 The DISP, DISP1 and DISP2 directives These all display a message on the screen. DISP displays the text on both passes; DISP1 and DISP2 display the text on pass 1 only and pass 2 only, respectively. You can use INFO as a synonym of DISP2 is you wish. The use of the directives is the same: for example, you might write DISP 'Assembling debug code' to display ---- Assembling debug code on the screen. The message you display can contain printable text, and you can also include some control characters. You specify them in the same way as you would in any other string, by preceding them with a "|" character. The control characters you can use are: |M and |J - start a new line |G - make a beep sound For example DISP 'Line 1|MLine 2|G' displays a 2 line message and rings the bell. Additionally, the text can contain numeric expressions that are evaluated as the message is output. For example, this would let you output a message to report the size of a section of code. Within the text, you can specify an expression in one of two ways. %D(<expr>) will evaluate <expr> and display the result in decimal, and 34 USING THE 65C02 ASSEMBLER %X(<expr>) will display it in hexadecimal. You can write any numeric expression you wish within the parentheses, but it must not contain forward references. For example, to report the size of a section of code between labels START.CODE and END.CODE, use: DISP2 'Size of code is %X(END.CODE-START.CODE) bytes' 8.2 The WAIT, WAIT1 and WAIT2 directives These output messages in the same way as DISP, DISP1 and DISP2, but then suspend the assembly until you press a key on the keyboard. You could thus use these directives to enable you to change source discs during an assembly. 8.3 The QUERY directive This offers a convenient way of setting up conditions for conditional assembly. QUERY operates only on pass 1 of the assembly. It displays a message on the screen in the same way as the directives above, then waits for you to type in a line. The line is treated as a numeric expression and evaluated, the result being set as the value of the symbol specified in the line's label field. If the expression is invalid, or contains a forward reference, the question will be repeated. For example, suppose that your source contains optional debug code, and you select this with IF conditions of the form IF DEBUG.MODE . debug code . FI The value of DEBUG.MODE would normally be -1 to select debug code, or 0 if it were not needed. 35 USING THE 65C02 ASSEMBLER Rather than define DEBUG.MODE in the source, and thus have to edit the source every time you wished to change it, you can make the Assembler ask for the value at the start of an assembly. A line such as: DEBUG.MODE QUERY 'Use debug mode' will output the question ---- Use debug mode? on pass 1. You can now type in -1 to select debug code, or 0 to suppress it. Since the line you type is an expression, and can contain symbols, you can make this more friendly by changing the source to include YES EQU -1 NO EQU 0 before the QUERY directive. Now you can reply either "YES" or "NO" to the question: the Assembler will evaluate the line and take the value of the symbol you type. 8.4 The STOP directive STOP provides a useful way of abandoning an assembly part-way through. It operates on pass 1, and like DISP1, it displays a text message. Afterwards, though, it aborts the assembly with the error message "Stopped" 36 USING THE 65C02 ASSEMBLER 9. WRITING SIMPLE MACROS The Assembler contains a powerful macro facility, allowing you to write very complex and sophisticated macros. In this chapter, we shall examine the simpler aspects of macros. A macro is a way of producing a number of Assembler source lines merely by specifying its name in the opcode field of a line. For example, your source might contain many occurences of the lines: LDA VAR CLC ADC #2 STA VAR You could shorten the source, and make it easier to follow, by replacing each occurence with a macro call: you might set up a suitable macro with the statements ADDTWO MACRO LDA VAR CLC ADC #2 STA VAR ENDM Then, instead of writing the four lines of code, simply write one line ADDTWO The Assembler will expand the macro definition, and will automatically generate the four lines for you. 9.1 Using macro parameters Although this is useful in itself, the macro as shown above is rather limited. It couldn't, for instance, be used to add 2 to the variable COUNT instead of VAR. In order to provide this sort of flexibility, you can pass information to macros as "parameters". You could re-write the macro above to read 37 USING THE 65C02 ASSEMBLER ADDTWO MACRO LDA @P1 CLC ADC #2 STA @P1 ENDM Here we have replaced the occurrences of VAR by the string "@P1". Now, when the Assembler expands the macro, it will replace all occurrences of "@P1" with parameter number 1 of the macro call. To add 1 to VAR, you would then write ADDTWO VAR and to add 1 to COUNT, you would write ADDTWO COUNT You can supply up to 9 parameters when you call a macro, and they are indicated in the body of the macro by @P1, @P2, @P3 and so on. (Note that you can also specify the parameters as @1, @2, @3 and so on, omitting the "P". This is adequate for existing code; however, for new programs you should use the "@P1" form, as this is necessary when you come to use the Macro Programming Language) 9.2 Specifying macro parameters You can specify anything you want as parameters to macros. A macro can have up to 9 parameters, separated by a comma and optional spaces. A macro with 3 parameters could be called with lines like CHECK 1,FRED,27 CHECK 1 , FRED , 27 and so on. Normally, the Assembler will remove leading and trailing spaces from each parameter. If you require leading or trailing spaces, or if the parameter has to include a comma, you will need to specify it as a string, delimited by single- or double-quote characters. Thus, a macro call might look like THING 'Here, is a comma' and "@P1" will be replaced in the macro body with the characters Here, is a comma Note that the string delimiters are not taken as part of the parameter proper. 38 USING THE 65C02 ASSEMBLER 9.3 Nesting macros You can call macros from within macros, up to a depth of 5. If you attempt to nest deeper than that the Assembler will flag an error. 9.4 Redefining opcodes and directives The Assembler allows you to set up macros to redefine any opcode or directive. For example, you might want to redefine the JSR (Jump-to-Subroutine) opcode to automatically save the registers before entering the subroutine. You could do this by declaring a macro called JSR thus: JSR MACRO PHA TXA PHA TYA PHA JSR @P1 ENDM Now, whenever the Assembler comes across a line with JSR in the opcode field, it will expand the macro JSR rather than obeying the opcode. It will plant the code to save the registers, and then will come to the line JSR @P1 in the macro. Here, because it is already in a macro, the Assembler will not use the macro JSR. Instead it will assemble the opcode JSR, planting the code to enter the subroutine. 9.5 Labels within macros Suppose you wish to write a macro that includes a branch of some sort. You might write the macro definition as: 39 USING THE 65C02 ASSEMBLER THING MACRO LDA @P1 BEQ ZERO EOR #$FF ZERO STA @P2 ENDM The first time the macro is called, it plants the code bytes, and defines the label ZERO as the destination of the BEQ instruction. On a subsequent call, though, the macro will produce the same code, and will attempt to define the value of ZERO again. This of course will fail, since it already exists from the first macro call. The Assembler provides a way round this problem, by giving you a way of generating unique labels. Every time a macro is called, the Assembler sets up what you can regard as a special parameter on your behalf, which contains a string that is different for every macro call. This string is substituted, in the same way as ordinary parameters, by writing "@$MC" in the line. Thus, the above macro could be changed to be: THING MACRO LDA @P1 BEQ ZERO@$MC EOR #$FF ZERO@$MC STA @P1 ENDM Then, on the first macro call, every occurrence of ZERO@$MC might be changed to ZERO1X1. On the next call, they become ZERO2X1, so that there is no clash between the macros. 40 USING THE 65C02 ASSEMBLER 10. THE MACRO PROGRAMMING LANGUAGE A very powerful feature of the Assembler is its Macro Programming Language. This allows you considerable control in how macros are expanded - you can construct loops, manipulate macro parameters and perform several other functions that allow you to build macros of great power. Although this facility is mostly intended for use within macros, many of its facilities can also be used outside macros to great effect, as this section will explain. The Macro Programming Languages's facilities build on two source language features known as Assembly Time Variables and Sequence Symbols. 10.1 Sequence Symbols These are "place markers" within your source files or within macros that the Macro Programming Language uses in loops. Using directives such as AGO and AIF, you can make the Assembler move up or down within a file or a macro, letting you repeatedly assemble some parts of the source or totally omit others. Sequence Symbols are very similar to the labels that are part of the source proper, and they can contain the same characters. To distinguish them, Sequence Symbols always begin within a "%" sign in the first character of the line. The Sequence Symbol should be the only thing on the line: if you do put anything else there the Assembler will ignore it. To take an example of how Sequence Symbols could be used, suppose your source file contained the lines AGO %SKIP ASC 'These lines will never ' ASC 'get assembled' %SKIP ASC 'But this one will' The Assembler will encounter the AGO directive, and will then ignore everything in the source file until it finds the Sequence Symbol %SKIP. It will then resume its normal processing. Although this example will actually work, the technique isn't greatly useful, as ignoring source lines can be done just as easily with the IF..ELSE..FI construction. However, AGO (and the various conditional skips such as AIF) also allows you to go backwards in the 41 USING THE 65C02 ASSEMBLER source or macro - there is no other way of achieving this. The Sequence Symbols used in any file or macro are quite independent of those in any other; thus you can safely use ones of the same name in every file and macro, if you wish. 10.2 Assembly Time Variables Assembly Time Variables, or ATVs, are string variables that the Assembler itself uses while assembling your source files. As it reads the program source (either from a file or from the definition of a macro) the Assembler continually looks for references to ATVs. These are replaced - before the line is assembled - with the contents of the ATV, thus allowing you to vary the source that is actually processed. You can use ATVs in many ways. For example, the first line of a source might set an ATV string to hold the version number of the program you are assembling; the Assembler will then automatically replace every reference to that name with the string. Some ATVs are created by the Assembler itself, and let you incorporate such things as the name of the source file into the source itself. The main use, though, is in controlling loops within macros and within source files. 10.2.1 Creating Assembly Time Variables ATVs have names similar to the variables that form part of the source proper, and you can manipulate them with various directives. 10.2.1.1 Local and Global ATVs There are two types of ATV: local and global ones. a. Global ATVs exist for the whole of the assembly, and can be used anywhere. They are created and manipulated with the ASTR, ASET, ASTRIP and ALEN directives, which you can use even inside macros - the ATVs will continue to exist after the macro finishes. b. Local ATVs are created and manipulated by the MSTR, MSET, MSTRIP and MLEN directives. These can only be used inside macros, and the ATVs can be used only within the macro - they cease to exist at the end of the macro expansion. You would use these, for example, in controlling loops within a macro. In fact, you have already seen local ATVs in use in section 6.1 on "Simple Macros". Whenever you invoke a macro, the 42 USING THE 65C02 ASSEMBLER Assembler creates local ATVs with names P1, P2, P3 and so on, each holding one of the parameters you supplied on the macro call. For example, the source line NAME ASET 'OSWRCH' will set up an ATV called NAME, whose contents are the string 'OSWRCH'. Since the ASET directive has been used, the ATV is global, and can be used anywhere in the assembly. You can create local and global ATVs of the same name if you wish. However, if you wish to refer to the local ATV, you should be careful to always use the "M" form of the directives. The "A" forms will always create global ATVs, even if local ones of the same name already exist. 10.2.1.2 String and Numeric Values All ATVs are stored by the Assembler as printable character strings. However, in many cases you will find you use them as counters for controlling loops: to make this easy, the Assembler will automatically convert ATVs from numbers to strings whenever necessary. The rule used is quite simple. When processing ASET or MSET directives the Assembler examines the first character of the operand field. If it is a string delimiter, the operand is taken as a string. If it is any other character, the Assembler treats the operand as a numeric expression, evaluates it, and converts the result into a string for storage. Thus, the line COUNT ASET 15+3 will set up the ATV COUNT, containing the string "18", but COUNT ASET '15+3' sets up the string "15+3". The operand can be any expression, provided it does not contain any forward references: thus COUNT ASET ADDRESS+10 sets COUNT to hold the string form of the result of adding 10 to the address label ADDRESS. 43 USING THE 65C02 ASSEMBLER The ALEN and MLEN directives similarly convert from number to string for you. Here the operand must be a string: for example, SIZE ALEN 'A short string' sets SIZE to hold the string "14" - the length of the operand string, converted from a number into a string. 10.2.1.3 String slicing The ASET and MSET directives also allow to you to "slice" strings, or extract characters from inside them. You perform this by adding some parameters in the operand field: for example SLICE ASET 'abcdefghij',3,2 will set SLICE to hold the string "cd". The second operand parameter specifies the position of the first character to be sliced out (the string starts at character 1), and the third parameter specifies the number of characters to be sliced. Occasionally, string manipulations such as slicing may result in strings that have leading or trailing spaces, and you may not be able to tell beforehand how many. The ASTRIP and MSTRIP directives remove all leading and trailing spaces, so that NEW ASTRIP ' abcd ' would set the string NEW to be "abcd". 10.2.1.4 Efficient use of memory The strings contained by ATVs are held by the Assembler in the symbol table, and so compete for memory with other symbols. You can change the contents of an ATV to a string of different length as often as you wish: however, every time the string becomes longer the Assembler will allocate a new block of memory for it. The previous block cannot be used again, so continually increasing the size of an ATV can be extremely wasteful of memory. To overcome this, the ASTR and MSTR directives let you pre-declare a block of memory big enough to hold the maximum size string you will use. For example, NAME ASTR 50 sets up a block long enough to hold a 50-byte string without the need to get more memory. The minimum amount of memory the Assembler will allocate is 5 44 USING THE 65C02 ASSEMBLER bytes: this is enough to hold the largest possible numeric value, so that loop counters will not cause memory wastage as their values grow - you need not use ASTR and MSTR for them. 10.2.2 Simple Substitution of Assembly Time Variables Once you have set up ATVs, you can use them to create variable source lines. We have already come across this concept in section 6.1 on "Simple Macros", in the discussion of macro parameter substitution. There, we saw that if the source of the macro contained the line LDA #@P1 the Assembler would replace "@P1" with whatever you had specified as the first parameter of the macro when you called it. ATVs are substituted into source lines in exactly the same way, and as we saw, macro parameters are in fact local ATVs. The rule the Assembler uses is quite simple: whenever it detects an "@" sign in the source (other than in a comment line) it expects an ATV name to follow. The "@" and the name itself are replaced by the contents of the ATV before the line is assembled. For example, suppose you set an ATV with the statement NAME ASET 'OSWRCH' The Assembler will then replace "@NAME" anywhere in the source by "OSWRCH". You might then have a line that in the source read JSR @NAME This would be changed by the Assembler into JSR OSWRCH before assembly, and you will see this second line in the listing. There are some more complex uses of ATV substitution, and we shall discuss these later in section 7.3 on "Writing Complex Macros". Some useful points to note on substition are: 45 USING THE 65C02 ASSEMBLER a. If you want the "@" character to appear in the line the Assembler processes, your source must contain two of them. Thus, if the line you really want to assemble is ASC 'VAT @ 15%' write it in the source file as ASC 'VAT @@ 15%' b. Once the Assembler finds an "@" character in the source, it assumes that what follows is an ATV name. The end of this name is assumed to be the first non-alphanumeric character that it meets, or the end of the line. In almost all cases, this will cause no difficulty, but occasionally will, usually in complex macros. As an example, suppose you had declared at ATV called EXPR, holding the string "10+". A subsequent source line might then read LDA #@EXPR3 and the intention is for this to be transformed into LDA #10+3 In this case, though, the substitution will fail, as the Assembler will look for an ATV called EXPR3. To force it to do as you want, write the source line as LDA #@EXPR/3 The "/" character enables the Assembler to detect the end of the ATV name, so it will look for EXPR as it should. The "/" will be discarded, so that the resulting line will be LDA #10+3 as intended. If you need a "/" character in the resulting line, write it as "//". There are two other techniques you might use in these circumstances. The Assembler does not regard spaces as significant in numeric expressions, so that you could write LDA #@EXPR / 3 to achieve the same result. Also, you could adopt a technique normally used in more complex cases (described in section 10.1.3.2) and write the line as LDA #@(EXPR)/3 46 USING THE 65C02 ASSEMBLER c. No ATV substitution is performed in comment lines, in lines containing Sequence Symbols, or in the definition of macros (i.e. between the MACRO and ENDM directives). Apart from these, though, substitutions can be made at any place in a line - you can substitute for labels, opcodes, operands or any parts of them. 10.3 Writing Complex Macros 10.3.1 Programming macro loops Mostly, you will use the Macro Programming Language to program macro loops, controlled by various conditions. 10.3.1.1 Simple loops controlled by counter The simplest form of loop is one which is executed a fixed number of times, and needs only a counter to control it. As an example, suppose that we need a macro to plant a number of bytes containing $FF with the DFB directive, the number being specified by the first parameter. (There are much easier ways of doing this than with a macro, of course - this only shows a general technique). The macro definition might then be: PLANT MACRO COUNT MSET 0 %LOOP DFB $FF COUNT MSET @COUNT+1 AIF @COUNT<@P1,%LOOP ENDM To see how this works, we can examine each line in turn, assuming that the macro was called with a line PLANT 7 Line 1 : This is the macro definition line. Line 2 : This line sets up a local ATV called COUNT, and gives it a string value of "0". Line 3 : This is a Sequence Symbol marking the top of the loop. Note that there is nothing else on the line with it. Line 4 : This is the DFB line that plants the $FF byte required. 47 USING THE 65C02 ASSEMBLER Line 5 : This line increments the value of COUNT. As the Assembler reads the line, it encounters "@COUNT", which it replaces with the current string value of the ATV. Thus the first time this line is encountered, the Assembler will generate COUNT MSET 0+1 The second time, it generates COUNT MSET 1+1 and so on. Line 5 : This tests whether the Assembler is to loop round once more. As with line 4, the Assembler will replace "@COUNT" with the current value of the ATV. "@P1" is, of course, replaced by the first parameter of the macro. The first time round, the line processed is AIF 0<7,%LOOP which is true, so the Assembler skips backwards in the macro to line 4 and resumes processing from there. 10.3.1.2 Loops accessing macro parameters Another frequently-needed form of loop is one in which all the parameters of the macro are accessed in turn. Suppose, for example, you need to write a macro THINGS, whose parameters are all numbers. Each number is to be planted in a byte in the object file with a DFB directive: to make THINGS interesting, the number of parameters must be variable. Such a macro is fairly simple to write, but uses an ATV substitution technique that can, at first sight, be somewhat daunting. If the job were simply to plant the value of parameter 1, the line in the macro that does it would simply be DFB @P1 However, we need to access each parameter in turn: the Assembler must somehow be made to see "@P1" the first time round the loop, "@P2" in the second, and so on. Effectively, then, we need a substitution technique that lets us have a variable ATV name: i.e. one that first substitutes the number ("1", "2", "3" etc) then substitutes for the ATV name so formed. The Assembler can do this easily, since ATV substitution operates 48 USING THE 65C02 ASSEMBLER in a hierarchic fashion. For example, suppose that a source line contains @(P@COUNT) somewhere. On seeing the first "@" character, the Assembler prepares to substitute an ATV. It finds, though, that the next character is a "(", so that it now expects a bracketed expression rather than an ATV name. It notes where it has got to in the source line, and explores within the brackets. Now, though, it stores the characters it finds (rather than passing them on to be assembled), and expects to end up with a valid ATV name by the time it gets to the ")" character. It notes the "P", then finds the second "@". This makes it try again to substitute an ATV, and this time it finds a real ATV called COUNT, which we shall suppose contains the string "1". After "COUNT" it finds the ")" ending the bracketed expression; thus, within the brackets the string it has built is now "P1". Having ended the bracketed expression, the Assembler goes back to where it was.The "(P@COUNT)" has provided the string "P1", and this now is a valid ATV name. So it proceeds to substitute the value of ATV P1, the first macro parameter, as we intended. To see how we might use this technique, we can consider a definition of THINGS: THINGS MACRO COUNT MSET 1 %LOOP AIF '@(P@COUNT)'='',%ALL.DONE DFB @(P@COUNT) COUNT MSET @COUNT+1 AIF @COUNT<10,%LOOP %ALL.DONE ENDM To see how this works, we can examine each line in turn. Line 1 : This is the macro definition line. Line 2 : This line sets up a local ATV called COUNT, and gives it a string value of "1". Line 3 : This is a Sequence Symbol marking the top of the loop. Line 4 : Since the number of paameters must be variable, we need to test whether we've processed them all. You can see the substitution technique discussed above in use here 49 USING THE 65C02 ASSEMBLER to check if the next parameter is null - any parameters that you don't supply in a macro call are strings of zero size. If the parameter is null, the Assembler skips forwards in the macro until it gets to the Sequence Symbol %ALL.DONE. Line 5 : This is the DFB line that plants the current macro parameter as a byte. Line 6 : This line increments the value of COUNT, as in the previous example. Line 7 : This tests whether the Assembler is to loop round once more. The final macro parameter is P9, so once COUNT reaches 10 the macro is finished. Line 8 : This is the Sequence Symbol that line 4 skips to if it finds a null parameter. Thus, if the macro was called with a line THINGS 1,$FE,FRED-1 the loop would be traversed three times, and the lines actually assembled would be DFB 1 DFB $FE DFB FRED-1 The technique of hierarchical substitution, though most often used to access macro parameters in turn, can be used in many other applications: you can nest substitutions to as deep as you are likely to need, so that you might write something as horrendous looking as LDA #@(XX@(COUNT@PTR)B) if you really needed to. 10.3.2 Changing macro parameters Macro parameters are in fact local ATVs with names P1, P2, P3 and so on. This means that you can change them within a macro as you wish. One example of this might be to allow the use of default macro parameters (although section 10.3.3 below shows a n automatic way to do this). Suppose that a macro parameter should be a number, whose default value is 1. You could define it as: 50 USING THE 65C02 ASSEMBLER TEST MACRO AIF '@P1'#'',%NEXT P1 MSET 1 %NEXT LDA #@P1 ENDM Within this example, we have: Line 1 : The macro definition line. Line 2 : This tests if parameter 1 has been supplied. If so, it will not be a null string, so the Assembler skips to %NEXT. Line 3 : This line sets parameter 1 to the default value. Line 4 : This is the Sequence Symbol skipped to if the parameter is not defaulted. Line 5 : This line actually uses the parameter. It will assemble correctly even if the parameter was not given, since in that case line 3 will have set it up to be the default value. 10.3.3 Setting default macro parameters The example above showed one way of establishing a default value of a macro parameter, but in fact the Assembler gives you an automatic and easy way of doing this with the DEFPARS directive. The effect of this directive, which you can issue at any time inside a macro, is to set the values of any of the macro parameters P1, P2, P3 and so on, unless they are already defined. For example, you might call a macro with a line FRED 1,,3 where you have defined parameters 1 and 3, but not parameter 2. If the macro now excutes, say, DEFPARS 100,200,300 the Assembler will check the parameters in turn. Parameters 1 and 3 are already defined, so the "100" and "300" values are not used. Parameter 2, though, is not yet defined, so it is set to "200" from this point on. If you wished, say, to establish a default for only some of the parameters of a macro, simply specify only thses parameters in the DEFPARS directive and default the others. Thus 51 USING THE 65C02 ASSEMBLER DEFPARS ,,,44 sets a default for parameter 4, but has no effect whatsoever on parameters 1, 2, 3, 5, 6, 7, 8 and 10. 10.3.4 Listing control for macros If you write complex macros with lots of loops, you will find that the Assembler actually executes a large number of lines that just contain the AIF, MSET directives, etc. This can swamp the listing, and make it hard to see the actual directives that plant data or code (as well as using up a large amount of paper). To overcome this, list level 2 will not list directives such as MSET, AIF, etc, unless they contain errors. In order to see all the lines of a macro expansion, use list level 3. (Note that outside macros, ASET and other similar directives will list at level 2). 10.3.5 Exiting a macro prematurely You may sometimes need to exit a macro as a result of a test. Depending on circumstances, you may be able to use AIF or AGO to skip to the physical end of the macro; however, you can also use the MEXIT directive. This exits the macro immediately, wherever in the macro body it is encountered. 52 USING THE 65C02 ASSEMBLER 10.4 System ATVs The Assembler provides a number of read-only ATVs that you can substitute. They provide information on the Assembler and the environment that you can use to control assembly. Each system ATV name starts with a "$" character: they are $CLST The current code listing level. $DEFCLST The default code listing level. $DEFLST The default listing level. $FLEVEL "1" if the current file is an INCLUDE file; "0" if it is not. $FS The number of the filing system in use, as returned by OSARGS with A=0, Y=0. The Acorn DFS returns "4", Econet returns "5" and ADFS returns "8". $LINE The number of the current source line. $LST The current listing level. $MC The macro call counter, used to generate unique labels within macros (see section 9.5) $MLEVEL The current macro nesting level. If not in a macro, the value is "0". $OBJECT The name of the current object file, which may include leading or trailing spaces. $OS The version of the MOS in use, as returned by OSBYTE with A=0. $OSHWM The primary OSHWM value of the machine running the Assembler. $SOURCE The name of the current source file, which may include leading or trailing spaces. $TIME The currently-set timestamp string, which may include leading or trailing spaces. $TTL The currently-set page title string, which may include leading or trailing spaces. $VERSION The version of the Assembler in use. This is returned as a numeric string, so that version 1.50 sets the string to be "150". 53 USING THE 65C02 ASSEMBLER For example, the line ORG @$OSHWM could be used to set the base address of the code to the OSHWM value of the machine being used, without the need to know what that value was. 54 USING THE 65C02 ASSEMBLER Appendix 1 : OPCODES AND ADDRESSING MODES The Assembler supports all the opcodes of the 6502 and 65C02 processor families, using standard MOSTEK mnemonics. For descriptions of the opcodes, see for example "The Advanced User Guide for the BBC Micro" (for 6502-compatible opcodes) and the "Master Reference Manual Part 2" (both 6502- and 65C02-compatible opcodes, although it describes the BBC BASIC Assembler syntax which cannot be used with this Assembler.) The 6502-compatible opcode mnemonics available are: ADC AND ASL BCC BCS BEQ BIT BMI BNE BPL BRK BVC BVS CLC CLD CLI CLV CMP CPX CPY DEC DEX DEY EOR INC INX INY JMP JSR LDA LDX LDY LSR NOP ORA PHA PHP PLA PLP ROL ROR RTI RTS SBC SEC SED SEI STA STX STY TAX TAY TSX TXS TYA The 65C02-only mnemonics are: BRA DEA INA PHX PHY PLX PLY STZ TRB TSB The opcode CLR can be used as a synonym for STZ. The addressing modes common to both the 6502 and 65C02 processors are: Mode Syntax Implied op Accumulator op A or op Immediate op #expr8 Low byte op #>expr High byte op #<expr Zero page op expr8 Indexed by X op expr8,X Indexed by Y op expr8,Y Absolute op expr16 Indexed by X op expr16,X Indexed by Y op expr16,Y Indirect pre-indexed op (expr8,X) Indirect post-indexed op (expr8),Y Absolute indirect op (expr) The addressing modes usable with the 65C02 processor only are: Mode Syntax Zero-page indexed op (expr8) Absolute indirect pre-indexed op (expr16,X) In these definitions, "expr8" represents an 8 bit number; "expr16" 55 USING THE 65C02 ASSEMBLER a 16-bit number; and "expr" a value that is either 8 or 16 bits long. 56 USING THE 65C02 ASSEMBLER Appendix 2: ASSEMBLER DIRECTIVES The Assembler supports a large number of directives, or pseudo-ops. This section gives a detailed definition of each, arranged in alphabetical order. Directives follow the normal syntax of opcodes. All can be followed by comment fields started with a ";" character; however, not all directives may have labels. In the specification, items enclosed in "[]" brackets are optional, and can be omitted. 57 USING THE 65C02 ASSEMBLER AGO Skips to a Sequence Symbol. Syntax: AGO <sequence> Example: AGO %LOOP AIF Skips to a Sequence Symbol if a condition is true. Syntax: AIF <expr>,<sequence> or AIF <stringexpr>,<sequence> If <condition> is false assembly continues in the next line. Example: AIF 'FREDA'>'FRED',%LOOP AIFDEF Skips to a Sequence Symbol if a symbol has been defined. Syntax: AIFDEF <symbol>,<sequence> Example: AIFDEF RAM.CODE,%DOING.RAM AIFNDEF Skips to a Sequence Symbol if a symbol has not been defined. The syntax is as for AIFDEF. ALEN Sets a global ATV to the length of the operand string. Syntax: <label> ALEN <string> Example: SIZE ALEN 'This is a string' 58 USING THE 65C02 ASSEMBLER ASC Defines an ASCII string. Syntax: [<label>] ASC <string> Within <string> you can use the techniques of section 4.3.3 to plant control characters or characters with bit 7 set. If a label is specified, it is set to the address of the first byte of the string. Example: TITLE ASC 'Assembler 1.32' ASET Sets a global ATV to contain a string. Syntax: <label> ASET <string>[,<expr1>[,<expr2>]] or <label> ASET <expr> The final two parameters in the operand field allow the string parameter to be sliced. Both are expressions that must not involve forward references. <expr1> specifies the first byte to be sliced from the string (the first byte in the string is byte 1). <expr2> defines the size of the slice; if omitted, one byte is extracted. Example: COUNT ASET 100 ASTR Allocates space for a global ATV. Syntax: <label> ASTR <expr> <expr>, which must not contain forward references, defines the space needed. The value can be up to 255 bytes. Example: NAME ASTR 150 ASTRIP Sets a global ATV to a string value, removing leading and trailing spaces in the process. 59 USING THE 65C02 ASSEMBLER Syntax: <label> ASTRIP <string> Example: MIDDLE ASTRIP ' aardvark ' CASC This is similar in action to ASC, but will plant a byte containing the number of characters within the string before the string itself. CHN Closes the current source file and starts another. Syntax: [<label>] CHN <filename> The directive is not allowed in an INCLUDE file. Example: CHN :2.NEXTBIT CHR Defines the character used by the REP directive. Syntax: [<label>] CHR character The character is specified literally, and not as a character constant. Example: CHR # CLST Sets the code listing level. Syntax: [<label>] CLST [<expr>] <expr>, which must not contain forward references, gives the new listing level. If omitted, the level is reset to the default from the command line. The values allowed are: 0 List only the first 3 bytes produced by any line. 60 USING THE 65C02 ASSEMBLER 1 List all the bytes planted by any line except a CODE directive. 2 List all the bytes planted by all lines. Example: CLST 2 CODE Includes a file (such as a screen dump) directly into the object file. Syntax: [<label>] CODE <filename> The specified file is copied, with no interpretation, directly to the object file. If <label> is specified it is set the address of the first byte copied. The address count is increased by the size of the file. Example: CODE SCRDUMP CSTR Similar to STR, but this directive plants a byte containing the number of characters in the string before the string itself. The count includes the $0D byte that is automatically added at the end of the string. DB This is identical to DFB. DDB Defines a number of two-byte values, output in high-low order. Syntax: [<label>] DDB <expr>[,<expr>...] Each expression in the operand field is evaluated and stored as a two-byte value. If <label> is present, it is set to the address of the high-order byte of the first expression. Each expression can be preceded by a repeat count as decribed in section 4.2.3. Example: TABLE DDB 28,32105,[10]$FFFF 61 USING THE 65C02 ASSEMBLER DEFPARS Sets up default parameters for a macro. See section 9.3.3 for full details of this directive. DEND Terminates a DSECT. Syntax: [<label>] DEND The address that was saved when the DSECT directive was encountered is reset. Example: DEND DFB Defines a number of single-byte values. Syntax: [<label>] DFB <expr>[,<expr>...] Each expression in the operand field is evaluated and stored as a byte value. If <label> is present, it is set to the address of the first expression. Each expression can be preceded by a repeat count as decribed in section 4.2.3. Example: TABLE DFB 0,[27]1,FRED+2 DFDB This is identical to DDB. DFS This is identical to DS. DFW This is identical to DW. DISP Displays a message on both pass 1 and pass 2. Syntax: [<label>] DISP <string> <string> may contain references to variables: for details see section 7.1. Example: 62 USING THE 65C02 ASSEMBLER DISP 'Program size is %D(*-$8000)' DISP1 As for DISP, except that the message is displayed only on pass 1. DISP2 As for DISP, except that the mesage is displayed only on pass 2. DO A synonym for IF. DS Reserves space in the object file, setting the bytes so affected to zero. Syntax: [<label>] DS <expr> <expr>, which must not include forward references, defines the amount of space to be reserved. If <label> is present it is set to the address of the first byte reserved. Example: DS $C000-PROG.TOP.ADDRESS DSECT Opens a "dummy section" that allows an area of memory to be defined without generating any object code. Syntax: [<label>] DSECT When the DSECT directive is processed, the current address is noted by the Assembler, and the value is reset to that at the end of the previous dummy section (or 0, if this is the first). An ORG directive can be used to change the value if required. When the dummy section is terminated by a DEND directive the old value of the address is restored. Example: DSECT DW Defines a number of two-byte values output in low-high order. 63 USING THE 65C02 ASSEMBLER Syntax: [<label>] DW <expr>[,<expr>...] Each expression in the operand field is evaluated and stored as a two-byte value. If <label> is present, it is set to the address of the low-order byte of the first exression. Each expression can be preceded by a repeat count as decribed in section 4.2.3. Example: TABLE DW FRED,BERT ELSE Marks the end of the TRUE branch of a conditional assembly block begun by IF, IFDEF and IFNDEF. For details see section 7. ENDM Terminates the definition of a macro begun by the MACRO directive. Syntax: ENDM EQU Assigns a value to a symbol. Syntax: <label> EQU <expr> <expr> must not contain forward references. Example: OSWRCH EQU $FFEE EXEC Specifies the bottom 16 bits of the 32 bit execution address of the object code. Syntax: [<label>] EXEC <expr> The catalogue entry of the object file will be updated to show the specified execution address. <expr> can only be a 16-bit quantity: the value of the two top bytes of the address are set by the MSW directive. 64 USING THE 65C02 ASSEMBLER If this directive is not used, the execution address is set to the address defined by the first ORG that is not in a dummy section. Example: EXEC START+3 FI Terminates a conditional assembly block begun by IF, IFDEF or IFNDEF. For details, see section 7. FIN This is identical to FI. HEX Defines a series of bytes in the object program, the bytes being specified as a hexadecimal string. Syntax: [<label>] HEX <string> <string> must contain hexadecimal digits (i.e. '0'..'9' or 'A'..'F'), and each pair of digits is output as one byte. If <label> is present it is set to the address of the first byte from the string. Example: BYTE.LIST HEX 'AB34FF2E7E' IF Begins a conditional assembly block. Syntax: [<label>] IF <expr> If <expr>, which cannot contain forward references, is non-zero, the condition is TRUE, otherwise it is FALSE. For details, see section 4.1. IFDEF Begins a conditional assembly block. Syntax: [<label>] IFDEF <symbol> If <symbol> has been defined, the condition is TRUE, otherwise it is FALSE. For details, see section 7.2 65 USING THE 65C02 ASSEMBLER IFNDEF Begins a conditional assembly block. Syntax: [<label>] IFNDEF <symbol> If <symbol> has not been defined, the condition is TRUE, otherwise it is FALSE. For details, see section 7.2. INCLUDE Begins assembly from a new source file, afterwards resuming with the current file. Syntax: [<label>] INCLUDE <filename> The Assembler will process lines from the specified file. Lines in the listing file will begin with an "I" to indicate their origin. At end-of-file, it resumes after the INCLUDE directive. The directive may not be used within an INCLUDE file or a macro. Example: INCLUDE :3.DEFINES INFO This is identical to DISP2. LFCOND Specifies that lines skipped in a conditional are to be listed. Syntax: [<label>] LFCOND For details, see section 4.4. LOAD Specifies the bottom 16 bits of the 32 bit load address of the object code. Syntax: [<label>] LOAD <expr> The catalogue entry of the object file will be updated to show the specified load address. <expr> can only be a 16-bit 66 USING THE 65C02 ASSEMBLER quantity: the value of the two top bytes of the address are set by the MSW directive. If this directive is not used, the load address is set to the address defined by the first ORG that is not in a dummy section. Example: LOAD $1902 LST Sets the source listing level. Syntax: [<label>] LST [<expr>] <expr>, which must not contain forward references, give the new level. If omitted, the level is reset to the default specified in the command line. The allowed values are: 0 No lines are listed. 1 List lines from files, but not from macros. 2 List all lines, but not directives such as AIF within macros. 3 List all lines. Lines containing errors are always listed. MACRO Defines a macro. Syntax: <label> MACRO The following lines are stored as the definition of the macro. No ATV substitution is performed on the lines. The definition is terminated by an ENDM directive. The macro name may not be more than 8 characters long. The MACRO directive may not be used within a macro. Example: THING MACRO 67 USING THE 65C02 ASSEMBLER MEXIT Causes a macro to terminate immediately. Syntax: MEXIT MLEN As for ALEN, but this can be used only within a macro and creates a local ATV rather than a global one. MSET As for ASET, but this can be used only within a macro and creates a local ATV rather than a global one. MSTR As for ASTR, but this can be used only within a macro and creates a local ATV rather than a global one. MSTRIP As for ASTRIP, but this can be used only within a macro and creates a local ATV rather than a global one. MSW Sets the two top bytes of the load and execution addresses. Syntax: [<label>] MSW <expr> By default the bytes used are $FFFF, but if you are assembling code to run on a second processor you will need to change them to the appropriate value. Example: MSW 0 ORG Sets the value of the address counter. Syntax: [<label>] ORG <expr> <expr>, which must not contain forward references, gives the new value. The first ORG directive that is not in a DSECT..DEND block also defines the default load and execution addresses. The object file is not affected in any way; thus if you use ORG to advance the address value, the Assembler will not plant any bytes in the object file to fill the gap. To accomplish this effect, you can use the DS directive. 68 USING THE 65C02 ASSEMBLER Example: ORG $1900 PAGE Defines the page length and width of the listing. Syntax: [<label>] PAGE <expr1>[,<expr2>] <expr1> is the physical number of lines on the paper (default 66). The Assembler leaves a small gap at the end of each page, to avoid the perforations. <expr2> is the number of characters to print on each line. If omitted, the value is set to 80. PAGE has no effect on the screen display. Example: PAGE 88,132 QUERY Displays a question and reads in a reply, which is evaluated as an expression and used to define a label. Syntax: <label> QUERY <string> For details, see section 8.3 REP Outputs a comment line to the listing to mark a source division. Syntax: [<label>] REP <expr> <expr> must not exceed 132 and must not contain forward references. For details, see section 3.5.4 SFCOND Specifies that lines skipped in a conditional should not be listed. 69 USING THE 65C02 ASSEMBLER Syntax: [<label>] SFCOND For details, see section 7.4 SKP Leaves a gap in the listing or starts a new page. Syntax: [<label>] SKP <expr> or [<label>] SKP H In the first form, <expr> gives the number of lines to be left blank. In the second, a new page is started. The SKP directive itself is not listed. Example: SKP 5 STOP Aborts the assembly on pass 1, outputting a message. Syntax: [<label>] STOP <string> The Assembler will display <string>, then abort with the error message "Stopped". Example: STOP 'Too much in page zero' STR This is similar to ASC, except that the Assembler will automatically add a $0D byte (Carriage Return) to the bytes generated. SYSCLI Issues a MOS command, allowing you, for example, to change the default disc drive. Syntax: [<label>] SYSCLI <string> You need not begin the string to be issued as a MOS command with a "*", but it doesn't matter if you do. Note that care is needed over the commands you try to execute. Anything that 70 USING THE 65C02 ASSEMBLER uses memory will almost certainly corrupt the Assembler's workspace and cause assembly to go horriby wrong. Example: SYSCLI 'DRIVE 2' SYSFX This directive has no effect. SYSVDU Outputs one or more bytes to the screen. Syntax: [<label>] SYSVDU <expr>[,<expr>...] The directive is similar to the BASIC VDU command. The low-byte of each <expr> is written with a call to OSWRCH. This gives you a means of sending control codes to a printer to set page formats, etc. Normally bytes are written to the screen only; to send them to a printer you need to output a code of "2" to turn the printer on, and, if you wish, to precede each value with a "1" byte to send it only to the printer. SYSVDU operates on both passes of the Assembler. Example: SYSVDU 2,1,27,1,"E",3 TTL Defines the title string on the top of each listing page. Syntax: [<label>] TTL <string> The Assembler uses the first 20 bytes of the string as the page title, and also starts a new page. Example: TTL 'Screen Dumper' TIME Defines the timestamp string output on each listing page. Syntax: [<label>] TIME <string> The Assembler uses the first 25 bytes of the string for the 71 USING THE 65C02 ASSEMBLER new timestamp. The value need not be a time; you can use it as a subsidiary page title. On a Master 128 the directive is ignored, as the timestamp is read from the real-time clock. Example: TIME '25-August-1986 13:00' WAIT This displays a string (see under DISP), then pauses the assembly until you press a key. WAIT1 As for WAIT, but operates only on pass 1. WAIT2 As for WAIT, but operates only on pass 2. 72 USING THE 65C02 ASSEMBLER Appendix 3. DIFFERENCES FROM THE ADE ASSEMBLER The 65C02 Assembler contains a number of similar directives to the ADE Assembler, and in almost all cases it will be possible to assemble programs written for ADE with it. A few directives, though, have some minor differences, and there are also some small points on source format that must be considered. - Only single- and double-quote characters may delimit strings. - The second delimiter must not be omitted from character constants. - Character constants obey the same rule as strings in the ASC and STR directives for control characters and those with bit 7 set. Thus the "|" and "^" characters need to be doubled if they are being defined as constants. - Strings containing commas or leading or trailing spaces are specified as macro parameters within normal string delimiters rather than in [] brackets. - Macro parameters must always be separated by commas. - The SYSFX directive is ignored: to achieve the desired effect you should use the SYSCLI directive to issue a *FX command. - Macro libraries are not implemented. - Local labels within macros are not implemented. The system ATV $MC should be used to construct unique labels as described in section 9.5. - Expressions are evaluated fully hierarchically rather than left-to-right. - The "#" arithmetic operator is used for "inequality", returning -1 if unequal and 0 if equal, rather than "modulus". - The LST directive has a numeric operand rather than the strings OFF, ON and FULL. For ADE compatibility, define symbols OFF, ON and FULL with the EQU directive to have values 0, 1 and 2. List level 3 has no equivalent in ADE. - The load and execution addresses are set by the first ORG directive only. To set them to different values use the LOAD and EXEC directives. 73