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IBM Personal Computer (R) Virtual Teaching Language Interpreter Reference Manual July, 1983 COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 1 Table of Contents Introduction...........................................3 Installation of the Interpreter........................4 Features of the Interpreter............................6 Commands and Syntax....................................8 TYPE command......................................10 ACCEPT command....................................11 MATCH command.....................................12 JUMP command......................................13 USE subroutine command............................14 RETURN from subroutine command....................15 END command.......................................16 CALCULATE command.................................17 HARDCOPY command..................................18 OPEN file command.................................19 eXIT (close) file command.........................20 GET (read) from file command......................21 PUT (write) to file command.......................22 LINK (chain program) command......................23 INSERT (in comparison register) command...........24 FRAME (text display) command......................25 YES-IF (comparison) command.......................26 DO (execute file) command.........................27 WAIT (for keystroke) command......................28 ZERO (registers) command..........................29 Advanced Commands......................................30 Display modes.....................................30 VIDEO control commands............................31 VIDEO COLOR command...........................32 VIDEO LOCATE command..........................34 VIDEO SELECT command..........................35 SCREEN graphics commands..........................36 SCREEN DRAW command...........................38 SCREEN CIRCLE command.........................40 SCREEN SET and RESET commands.................41 SCREEN PAINT command..........................42 SCREEN LINE command...........................43 KEY (function key) command........................44 BEEP (sound) command..............................45 QUICKMOVE (graphics load/dump) command............47 NEWDISPLAY (adapter select) command...............48 Use of Interpreter and Programming Examples............49 Appendix A--Error Messages.............................57 Appendix B--Extended String Registers..................60 Appendix C--Intermixing Text and Graphics..............61 Index..................................................62 COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 2 Introduction This manual describes how to use the Virtual Teaching Language interpreter. The first section covers installation of the interpreter. The second section explores the features of the interpreter. The third section explains the commands available in this implementation of the PILOT language. The fourth section demonstrates some examples of how the commands may be used to create interactive courseware for computer-aided instruction (CAI). The manual assumes that the reader is at least slightly familiar with PC-DOS(R) and has learned how to use either the DOS line editor or a word-processing program to create text files. No other knowledge is directly required to use either this manual or the interpreter. Virtual Teaching Language (VTL) was written in DOS advanced BASIC, and is distributed in machine-readable form as the file VTL.EXE, except to those users who have purchased a source license. This manual, the interpreter, and source code are copyrighted material, under the protection of Federal Law. The program and manual are being distributed as 'share-ware' via computer clubs and public domain libraries. Anyone wishing to make use of VTL may do so for non-commercial purposes, and may copy and distribute the program without restrictions other than this. Persons wishing to distribute VTL for commercial uses (i.e. as part of a proprietary teaching package) may do so under license from the author for a nominal fee. Users are encouraged to register with the author. The registration fee is $25.00 US for non-commercial users. This money will be used to defray the costs of continued support and improvement of the VTL language. Registered users are entitled to support via written correspondence to the address listed below, and will also receive notice of improved releases of VTL (and other products from Nuclear Software. Unauthorized (e.g. commercial) copying or resale of the program or manual is a violation of Federal Law, and may be punishable by imprisonment for up to one year, and by a fine of up to $5000. VTL.EXE is a stand-alone executable file, and does not require the IBM BASIC Compiler (R) run-time system BASRUN.EXE. VTL.EXE requires at least 64K of RAM for safe operation, and may require 96K to execute programs with many long labels. VTL.EXE may be run safely on single-sided single-disk systems, and requires a lineprinter only for "HARDCOPY" commands, which may, however, cause an error if a lineprinter is not present or "on- line". This version of the interpreter is designed for users of either the IBM monochrome-printer adapter or the color display adapter in text or graphics mode. The interpreter recognizes which adapter is currently in use, and also tests for the presence of the other adapter. If both adapters are present, a Virtual Teaching Language program may use either via a built-in command, and can tell which is present. Users who are not familiar with the PILOT language may find the fourth section of this manual helpful as initial reading COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 3 before diving into the remainder of the material presented here. More experienced users may wish to begin by reading the reference section, and then refer to the last section for examples. COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 4 I. Installation of the Interpreter No specific procedure, apart from copying the material distributed, is required for installation of VTL.EXE. The interpreter is packaged with the manuals on one double- sided diskette. The diskette contains both manuals and program examples, in addition to two versions of the interpreter. The first version, VTL.EXE is for the IBM-PC/XT (R) and compatible computers; the other, VTLJR.EXE, is for the PC-Jr. (R). The user should immediately make at least one backup copy of the distributed material. To do so, follow this procedure: 1) Insert a DOS diskette containing the files DISKCOPY.COM and DISKCOMP.COM in disk drive A (the left-hand drive). 2) Type DISKCOPY A: B: and press ENTER. 3) Insert the distribution diskette in drive A, and a blank diskette in drive B (the right-hand drive). If you have only one drive, insert the distribution diskette. 4) Press any key. The copy procedure will begin. If you have only one drive, the DISKCOPY program will periodically instruct you to insert the blank diskette alternately with the distribution diskette. 5) When copying is complete, press the N key. Then replace the distribution disk in drive A with the disk holding the DISKCOMP program. Type DISKCOMP A: B: and press ENTER. 6) Replace the diskette in drive A with the distribution disk. Press any key. A comparison of the original and copied diskettes will be performed. If you have only one diskette drive, you will again be instructed to alternate diskettes. If the diskettes 'compare OK' then remove the previously blank diskette and label it. No special installation procedure is needed for the interpreter itself. Users of DOS V2.0 with the IBM fixed disk may copy the interpreter into a fixed disk partition. However, DOS V2.X users should be aware that the interpreter can access program and data files ONLY within the current directory. The interpreter and its related files should therefore be listed in the same directory on fixed disk, and are best listed in the root directory (\) on diskette, if possible. The user may wish to make additional copies of the manual, and may do so as follows: 1) Make sure that the lineprinter has an ample supply of paper. The manual is a LONG document! 2) Insert a diskette holding the manual in drive A. 3) Type COPY A:VTL.TXT LPT1: and press ENTER to print a COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 5 copy of the manual. The manual is distributed as ordinary text (in DOS form). The manual was written using WORDSTAR (R), and users with a source license may obtain a copy in WORDSTAR compatible form for modification. COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 6 II. Features of the Interpreter Virtual Teaching Language is a new dialect of the PILOT instructional language. Standard PILOT is designed to display information on a screen, ask questions, and get responses. The responses are matched against phrases defined by the programmer. Depending on whether or not a match is found, subsequent commands may or may not be executed. Most standard PILOT interpreters required that the program to be executed be loaded into main memory along with the inter- preter, thus constraining program size. Virtual Teaching Language reads the program line by line from disc, and thus, program size is limited only by disc capacity. In addition, Virtual Teaching Language permits one program to chain another while passing considerable amounts of data to the next program, thus removing even this constraint. This feature may cause brief pauses in execution, if a jump is made to a statement just preceding the current statement in a long program. However, an additional feature permits loading of screenfulls of data from a second file, which cuts program length considerably. Virtual Teaching Language also permits execution of other files as subroutines, permitting structuring of a program into a string of procedures. Multi-command lines are also supported. These advanced features permit development of terse, modular, almost fully structured program code, which may be entirely separate from the course material it is designed to present. When initiated, Virtual Teaching Language tries to execute a program named "autoexec.vtl". If this program is not present, the interpreter asks for the name of the program to be executed. A menu or instructional program may be named "autoexec.vtl", and will thus be executed automatically whenever the interpreter is run. In addition to the standard PILOT commands to print a line, ask a question, match phrases, jump to a new line, run a subrou- tine, return from a subroutine and end execution, and the frame- loading and chaining commands mentioned above, Virtual Teaching Language includes commands to perform limited numerical and text manipulations, read from and write to a disk file, specify a string to be matched against phrases, and print data on a lineprinter. All of these commands may make use of 'registers' built into the interpreter--temporary storage areas which may be used for calculations, and whose contents are automatically inserted into command lines where appropriate. Twenty-six registers for each of three data types--integer, real, and string--are provided. The use of registers permits indirect command addressing and indexed jumps, as well as embedded calculation of scores, execution checkpoints, and implementation of interruptable and resumable programs. An additional 26 extended string registers give the programmer access to the date and time, the current program name, the current line number, and several other useful quantities. VTL.EXE provides commands for performing comparisons between expressions, loading and display of Function Keys, timed pauses COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 7 until a key (or Function Key) is struck, control of character display modes for both the monochrome and color display systems, and directed location of the cursor on the screen. The interpreter also provides color graphics if a color adapter is present, as well as sound, loading of full graphics screens with a single command, and switching between monochrome and color display adapters, if both are present. These last features are extremely machine-dependent (i.e., they will work with the IBM Personal Computer, but may well not work with another 'compatible' machine). Virtual Teaching Language provides intelligible error messages, using error trapping for illegal file accesses, in addition to intercepting syntax and calculation errors. Unexpected system errors are also trapped and reported--a feature useful for users with source licenses who wish to modify the interpreter themselves. COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 8 III. Commands and Syntax Virtual Teaching Language programs are text files, resident on disc, which have been created by a text editor. Each line in such a file is read by the interpreter. If more than one command is present in a given line, each command is interpreted in turn. Multiple commands in a single line are separated by backslash (\) characters. A command may be treated as: 1) a comment, which begins with "!" (an exclamation point), and which is ignored; 2) a label, which begins with "*" (an asterisk), which must be the first command in a multi-command line, and which may be used as the target of a JUMP command; 3) a directive to be executed. A command has the following structure: [identifier][modifier][:][expression] The identifier is a single capital letter specifying the action to be taken. The modifier is either "Y" (meaning 'yes') or "N" (meaning 'no'), referring to the results of a preceding phrase match. If a phrase was matched, a command modified with "Y" will execute, and one modified with "N" will not. If a phrase was not matched, a command modified with "N" will execute and one modi- fied by "Y" will not. Unmodified commands will always execute. A modified command encountered before a phrase match is performed will not execute. Note that the "phrase match" condition may be set or reset under VTL by a comparison command or a wait-until- key-struck command, as well as by a phrase matching command. A command is delimited from the expression following it by a colon (":"), which must occur as the second or third character of the line. A colon occurring elsewhere in the line is ignored; one preceded by an unrecognized command and/or modifier is also ignored. An invalid or non-existant command is interpreted as a TYPE command, and the line is printed unmodified on the screen. The TYPE command is therefore the default command, which will be executed if no other command is encountered. The expression is a line of text, the contents of which may depend on the command preceding it. Excepting the ACCEPT, CALCULATE, and GET (disk file read) commands, references to a register embedded within the expression are replaced by the current contents of the register. The other commands use a register reference to specify the destination of data input by the user (ACCEPT), calculated (CALCULATE) or read (GET). A register reference consists of a two adjacent characters, the first of which specifies the register type: "%" -- integer; "#" -- real; "$" -- string. "@" -- extended string. COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 9 The second character is a capital letter from "A" to "Z", specifying which of the twenty-six registers of a given type is meant. For example, if string register "C" contains the string "help", and the line: T:Give me some $C!!! is executed, the expression: Give me some help!!! is printed on the screen. The contents of the extended string registers are set automatically by VTL.EXE, and may not be modified by ACCEPT, GET or CALCULATE commands. Not all of these registers are used by the current release of VTL.EXE. Those used, and their contents, are listed in Appendix B. A reference to an extended string register, with the exceptions above, is otherwise treated similarly to a reference to a standard string register. An additional register, the 'comparison register', which may not be addressed directly by a reference, is used to hold an expression to be used for phrase matching. The comparison register contents are changed by ACCEPT, WAIT, and INSERT commands as described below. The current contents of the comparison register, and their length in characters, may be obtained via extended register references. An expression may be further broken down into a subcommand folllowed by arguments. The subcommand is a single capital letter, possible values of which depend on the command. The subcommand and its arguments are separated by comments. The advanced (i.e. machine-dependent) commands having to do with video screen control, sound, and graphics use this form of expression. The individual commands' syntax is described below, using the following conventions: [modifier] is the optional modifier Y or N [register] is a register reference as described above [expression] is a line of text as described above [operator] is a special expression used for calculations, as described below [sub-cmd.] is a subcommand (a single letter used to modify the effect of a command, followed by a comma). [arg.] is an argument of a command (i.e. a subsection of an expression). Arguments are separated by commas. {} brackets enclose a quantity which may be omitted COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 10 TYPE command Form: T[modifier]:[expression] or [expression] This is the default command; a line of text without a command prefix (i.e. a colon in the first three characters) is displayed on the screen, unless the line begins with "*" (i.e. is a label) or "!" (i.e. is a comment). If a colon is found in the first three characters, and a command is not recognized, the line is typed unmodified; otherwise, the command prefix is stripped from the line to be displayed. If the expression ends with the character "&" (ampersand), the usual carriage return/line feed at the end of the line of text is suppressed. This permits conversational queries. Examples: TY:That is correct! ("That is correct!" is displayed if a phrase match preceded this statement.) This is a line of text. (This line is displayed.) !Another teaching program... (a comment--ignored) *12345 (a label--noted, as described below, but not displayed) Enter a number...& (The line, minus the "&", is displayed, and the cursor remains on the same line. A succeeding ACCEPT command may then be used to request input.) COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 11 ACCEPT command Form: A[modifier]: or A[modifier]:[register] This command is used to get a line of text input from the user. The text must be terminated by an 'enter' (<cr>). If a register is specified, the current contents of the register are replaced by the data input. If a string (i.e. non-numeric characters) is entered when a number is requested, the register contents are set to 0. If the most recent TYPE command expression ended with a "&", the ACCEPT command does not prompt input. If not, the ACCEPT command prompts for input with the character "<". The data input are, in any case, put in the comparison register for subsequent phrase matching. The data remain in the comparison register until the next ACCEPT, INSERT or timed WAIT command is executed. ACCEPT and INSERT commands cause the uppercase equivalent of whatever was entered or inserted to be placed in extended string register U (@U), the unmodified contents of the insertion register in extended string register J (@J) and the length in characters of whatever was put in the insertion register in extended string register I (@I). Examples: AY: (If a phrase match occurred, data input is accepted.) A:%B (Data input is accepted, and numeric integer result is inserted in integer register B.) COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 12 MATCH command Form: M[modifier]:[phrase],[phrase],[phrase]... The current contents of the comparison register are scanned for any occurrence of each of the phrases (separated by commas) included in the command, until either a match is found or all phrases are exhausted. If a match is found, all subsequent com- mands with the modifier "Y" will execute, and those with modifier "N" will not. If no match is found, the reverse is true. These conditions will hold until the next MATCH command is executed. Phrases are matched EXACTLY; upper case characters are treated as different from lower case. If the expression is preceded by a quotation mark ("), the entire expression, including any commas, is used for the phrase match. Examples: MY:yes,Yes (If the preceding MATCH command yielded a phrase match, this command matches for the phrases "yes" and "Yes".) M:a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z (If the comparison register contains any lower case character, a phrase match occurs.) M:"a,b,c,d (The entire phrase 'a,b,c,d' is used for the phrase match.) COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 13 JUMP command Form: J[modifier]:[expression] This command controls the flow of statement execution. The statement lines in a VTL program are executed one by one, in the order in which they are found in the program file. A JUMP command uses the expression as a label, and tries to find a matching label elsewhere in the program (excluding the "*" prefix used to designate a label to the TYPE command). If such a label is not found, an error message is printed and the program and interpreter terminate. If the label is found, the next line of text and subsequent lines are executed. As lines of program are executed, labels may be encountered as TYPE commands. These labels are noted by the interpreter, and their location is stored. A label may be any combination of characters (i.e. any valid expression). Up to 100 labels may be used in a single program. Labels passed during a jump are also noted. Although labels may be of any reasonable length (up to 254 characters), it is best to keep them short. If a JUMP command is executed, and the specified label has already been noted, control is passed as described above. If the label has not been noted, the remainder of the program is searched for it. Examples: JY:frame1 (If a phrase match has occurred, execution resumes at the line following the line "*frame1") C:$F=frame (inserts the string "frame" in string reg- ister F) J:$F1 (Jumps to the line following the label "frame1". The value "frame" is inserted in place of the register reference "$F".) COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 14 USE subroutine command Form: U[modifier]:[expression] This command executes a jump to a subroutine. The expression is interpreted as a label. The current line location is saved, and a jump to the specified label is performed. A RETURN command exits the subroutine and resumes execution of the program at the line following the USE command. Example: UN:mistake (If the last MATCH was unsuccessful, the subroutine beginning at the line starting with the label "*mistake" is executed.) COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 15 RETURN from subroutine Form: R[modifier]: If no subroutine is being executed, this command is ignored. Otherwise, a jump is executed to the command following the most recently executed USE command. COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 16 END command Form: E[modifier]: This command ends execution of the current program. The interpreter then asks the user for the name of the next program. If the current program was being executed as a subroutine, the originating program resumes. Example: EY: (If a phrase match has occurred, execution ends.) COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 17 CALCULATE command Form: C[modifier]:[register]=[expr.]{[operator][expr.]} This command is used to perform simple calculations, plac- ing the results in a register. Calculations may be numeric or string, depending on the type of the register specified as the results' destination. If no operator is specified, the value of the expression is inserted in the register. The operators permitted for numeric calculations are: + -- addition - -- subtraction * -- multiplication / -- division The two expressions separated by the operator are evaluated as real numbers. Exponential notation is permitted. Non-numeric expressions will be interpreted as 0. Division by 0 is permitted, but will cause an error message; the maximum positive real or integer value will be inserted in the destination register. String calculations may extract either the leftmost N characters of the first expression (operator "<<") or the rightmost N characters (operator ">>"), where N is the numeric value of the second expression. Examples: CN:$A=help me! (If the preceding MATCH was unsuccess- ful, the expression "help me!" is inserted in string register A.) C:#A=%C+1 (The current value of integer register C is inserted as the first expression; the second expression is "1". The value of integer register C is incremented by 1 and inserted in real regis- ter A. C:$B=abcdefg<<3 (The string "abc" is inserted in string register B.) COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 18 HARDCOPY command Form: H[modifier]:[expression] The expression is printed on the currently selected system lineprinter. If the lineprinter is off-line, or no lineprinter is present on the system, the computer will "hang". Example: HY:The result is #C. (The current value of real regis- ter C will be inserted in the expression, and the expression will be printed if a phrase match has occurred.) COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 19 OPEN a disk file command Form: O[modifier]:{[mode],}[expression] The expression must be a valid DOS filename. If no mode is specified, the file is initially opened for INPUT. If the file is not found, a new file is opened for OUTPUT. The file remains open until a file close command is executed. The access mode of the file may be changed, however, by read and write commands. The mode is a single letter, corresponding to the three possible modes of access to a sequential file: I -- input O -- output A -- append If the specified mode is input, the file is opened to be read only. If the file is not found, a new file with the same name will be opened for output. If the mode is output, a new file is opened; if the file already existed, its current contents are lost. If the mode is append, the file must already exist or an error will occur. If the file exists, all subsequent output will be appended to its current contents. Example: O:file.dat (The disk file "file.dat" is opened.) O:O,file.dat ("file.dat" is opened for output.) O:A,file.dat ("file.dat" is opened for append.) COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 20 eXIT (close) a disk file command Form: X[modifier]: A disk file previously opened is closed. If no file is open, an error message is printed, and the program and interpreter exit. Example: XY: (If a phrase match has occurred, the currently open disk file is closed.) COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 21 GET (read) from a disk file command Form: G[modifier]:[register] A line of input is read from the currently open disk file. If no file is open, an error message is printed and the program and interpreter terminate. If the file is currently open for OUTPUT or APPEND, the file is closed and reopened for INPUT. If the end of file is encountered on an attempt to read, an error message is printed and the program and interpreter terminate. If the destination register is numeric (real or integer), the line read is interpreted accordingly. If the register is a string register, the line replaces the register's current contents. Examples: GY:$A (If a phrase match has occurred, a line is read from the currently open file, and inserted in string register A.) G:%Y (A line is read from the currently open file converted to an integer, and the result inserted in integer register Y.) COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 22 PUT (write) to disk file command Form: P[modifier]:[expression] The expression is written to the currently open disk file. If the file is opened for INPUT, the file is closed and reopened for APPEND. The expression is then written to the end of the file. If no file is open, an error message is printed and the program and interpreter terminate. If an ampersand (&) is appended as the last character of the expression, the expression will be written to the file without a terminal carriage return and line feed. Otherwise, the a carriage return-line feed pair of characters will be added to the end of the expression written to disk. Example: PN:Test failed. (If the preceding MATCH was unsuccess- ful, the phrase "Test failed." is written to the currently open disk file.) COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 23 LINK (chain) to next program command Form: L[modifier]:[expression] The expression is a valid VTL program name (see below). The named program is executed. The contents of registers are left unchanged. Data may be passed to the new program via the registers. If the named program is not found, the interpreter displays an error message and exits. If the expression is omitted, the interpreter exits. The program name must ALWAYS be a full DOS filename with extension. No default file extension is assumed. A program being executed as a subroutine via a DO command may chain to another program; control will not be passed back to the original ('calling') program until an END command is encountered, or until the current line number exceeds the maximum line number specified in the DO command. Examples: L:prog2.vtl (The program prog2.vtl is executed.) LN: (If the preceding MATCH was unsuccessful, the interpreter exits to DOS.) COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 24 INSERT data into comparison register command Form: I[modifier]:[expression] The expression is inserted into the comparison register, and used for all subsequent MATCH commands until the comparison register is reloaded by another INSERT or an ACCEPT or timed WAIT command. Example: I:$Q (The current contents of string register Q are inserted in the comparison register). COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 25 FRAME load command Form: F[modifier]:[expression]{,[arg.1]{,...[arg.4]}} This command clears the display and then reads and displays up to 23 lines of text from a file, optionally beginning at a specified line of the file. The expression must be a valid DOS file name. Up to four arguments are optional. The first argument is interpreted as the number of the first line in the file to be displayed. The second argument is the number of lines to be displayed (range 1-23). If the first line to be read is out of range (i.e. EOF is encountered), an error message is printed and the program and interpreter exit. If EOF is encountered while loading a frame, the file is closed and execution proceeds. A FRAME load command with only the file name specified will load either the first 23 lines of the file or the entire file, if fewer than 23 lines are present. If only the first line is specified, that line and the next 22 lines (or fewer, if EOF is encountered) are displayed. The third argument specifies a starting line number on the display. This will be the line on which the first line read from the file is printed. The fourth argument specifies a starting column number on the display. All lines read from the file will be displayed beginning at this column number. Use of these two arguments permits changing the contents of a 'window' of the text display, without altering other contents outside the window. Examples: F:frame1.txt,11,5 (The display is loaded with lines 11-15 of the file "frame1.txt".) FN:frames.f1,20 (If the preceding MATCH was unsuccessful, line 20 and up to 22 succeeding lines of file "frames.f1" are displayed.) F:frame1.doc (Up to 23 lines from file "frame1.doc" are displayed, beginning at the first line.) F:frame1.txt,1,10,5,15 (The first 10 lines from file "frame1.txt" are displayed, beginning at line 5 of the display, and offset 14 columns. COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 26 YES-IF command Form: Y[modifier]:[sub-cmd.],[expr.]{[operator][expr.] This command either compares two expressions or compares a single expression with zero. If the comparison is true, the phrase-match condition flag is set to 'yes'. If not, the flag is set to 'no'. The sub-command is a single character which determines the mode of the comparison: % -- integer mode (expressions interpreted as integers) # -- real mode (expressions interpreted as real numbers) $ -- string mode (expressions interpreted as strings) A single expression is compared with zero (a null string in string mode). If the expression=0 (or a null string), the flag is set to 'no'; if not, the flag is set to 'yes'. In the two-expression form, the two expressions are separated by an operator: > -- greater than = -- equals < -- less than The two expressions are evaluated according to the mode, and compared according to the operator. If the condition thus expressed is true, the phrase match flag is set to 'yes'. If not, the flag is set to 'no'. This permits both structured decision-making within VTL programs, and exact string matching (obviating some of the difficulties in using the inexact matching of the M: command). In using this command, keep in mind that positive numbers placed in the extended string registers are preceded by a blank (' '). Examples: Y:%,%A>1 (the flag is set to 'yes' if the contents of integer register A are >1) YN:$,$A=hello (if the flag is already set to 'no', the contents of string register A are compared with the string "hello"; if the contents = "hello" then the flag is set to 'yes', otherwise to 'no') COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 27 DO command Form: D[modifier]:[filename]{,[first line]{,[last line]}} This command executes the named file, in effect, as a subroutine. Execution of the calling program is suspended; the context (i.e. all labels and pointers) of the calling program is saved. The registers' contents are left unchanged, however. The named file is executed beginning at the specified first line (or at line 1 if none is specified). Execution of the named file proceeds until an END command is encountered, end of file is reached, or the number of the line to be executed exceeds the number of the last line, if this is specified. The named file cannot itself invoke another file via a DO command (attempting to execute a DO command causes the interpreter to halt, with an error message). Apart from this restriction, all other commands in the named file will execute normally. When the named file terminates execution, the calling program resumes at the next command. As with the Frame command, the file name specified must include the file extension (i.e. there is no default extension). Examples: DY:frame44.vtl,1,23 (file "frame44.vtl" is executed, beginning at line 1, and ending at line 23, if this line is reached prior to an L: or E: command is executed) C:%A=23\D:lesson.f%A (file "lesson.f23" is executed, beginning at line 1) COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 28 WAIT command Form: W:{[expression]} This command, if used without an expression, halts execution until a key is struck. The key value struck is discarded. If an expression is specified, the expression is evaluated as a number of seconds to wait for a key to be struck. If no key is pressed during this time period, the comparison register is erased (i.e. loaded with a null string), and the phrase match flag is set to 'no'. If a key (including a function key) is struck, the character(s) invoked by the keystroke replace the current contents of the comparison register, and the phrase match flag is set to `yes`. This command may be used to time replies (i.e. for a quiz). Note that if a function key is struck, the entire expression (not just the first letter) associated with that key is inserted in the comparison register. Examples: W: (execution halts until a key is struck) W:10 (a keystroke is awaited for 10 seconds) COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 29 ZERO command Form: Z[modifier]:{sub-cmd.} This command erases the contents of all standard registers, or all of a particular class of register. Numeric register contents are set to zero, and string register contents are set to a null string (i.e. one of zero length). If no subcommand is present, all registers are zeroed. A subcommand specifies the class of registers to be zeroed, and may consist of the single character: % -- integer registers # -- real registers $ -- string registers The specified class of registers is cleared. Examples: Z: (all registers are cleared.) Z:$ (all string registers are cleared.) COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 30 Advanced Commands (specific for the IBM (R) PC) These commands control function key access, video screen modes, graphics, sound, and switching between monochrome and color display adapters--functions which are dependent on the specific characteristics of the IBM (R) Personal Computer. Many of these commands have subcommands with multiple arguments. Depending on the subcommand, arguments in the same position in an argument list (a series of arguments separated by commas) may have different meanings. In some cases, therefore, different subcommand forms of the same command will be discussed on separate pages to avoid confusion. Display Modes The color graphics adapter may be used in one of 4 display modes. The selected mode determines the number of characters per line when text is displayed, and whether or not graphics (lines, shapes, etc.) in black and white or color may be displayed. It is noteworthy that text may be displayed in any of these modes, including all graphics modes, but that its appearance will be influenced by which mode is selected. The monochrome display adapter is always in 80-column text display mode, and permits only limited graphics using special characters (as described in the IBM BASIC Manual, pp. G-1 to G-5), which are also available in the color adapter's text modes. The color adapter's modes are: 0 -- text (40 or 80 columns--two text modes) 1 -- medium resolution (320 x 200) graphics (with 40 column text display) 2 -- high resolution (640 x 200) graphics (with 80 column text display) Either of the text modes, and medium resolution graphics, may be in either monochrome (black and white) or color. In VTL, the VIDEO WIDTH, VIDEO COLOR, and VIDEO SELECT commands are used to select display modes for the color adapter. The VIDEO COLOR command is also used to specify color combinations for text or graphics display, and is also used to vary character display modes (reverse, blinking, bright, underlined) for text for both adapters. At the cost of some complexity, the VIDEO commands give the courseware author complete command of either display adapter. The SCREEN commands are used to create graphics using the color adapter's graphics modes, which must first, however, be properly selected, or an error results. COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 31 VIDEO control command Form: V[modifier]:[sub-cmd.]{,[arg1,...,arg5]} This command provides control of character attributes, and cursor location, and also permits clearing the screen. The function desired is selected by the sub-command: Z -- clear screen C -- character attributes or selection of color L -- cursor location S -- selection of screen mode W -- selection of character display row width (in columns) Simple Forms The 'Z' subcommand requires no arguments, and causes the screen to clear. The 'W' subcommand requires only one argument, which is either '40' or '80', depending on the desired display width. The V:W command automatically changes the color video mode to character (i.e. non-graphics). This form of the VIDEO command should not be used with the monochrome adapter. If the argument is not specified, 40 columns is assumed. Examples: V:Z (clears the current screen) V:W,80 (clears the current screen and sets display mode to character, 80 columns) COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 32 VIDEO COLOR command Form: V[modifier]:C,arg1,arg2{,arg3} This command sets the display mode for all subsequently printed characters in character mode using the monochrome display adapter, and also sets character and background colors for either character mode or medium resolution graphics mode using the color adapter. The 'C' subcommand takes up to 3 arguments, which are in the same order and which may assume the same values as the arguments for the IBM advanced BASIC 'COLOR' command. The effects of different values of these parameters are discussed fully in the IBM BASIC manual, pp. 4-49 to 4-52. However, for the convenience of the user, an abridged discussion is included here. The three arguments, which should always be specified, represent 'foreground' (arg1), 'background' (arg2) and `border` (arg3). 'Border' has meaning only if the color adapter is being used. In text mode, arg1 may range from 0-31 (default 0), arg2 may range from 0-7, and arg3 may range from 0-15. If the monochrome adapter is in use, arg3 is ignored. If arg1=0 then the foreground intensity (that of all characters displayed) is black. If arg1=1 then the foreground intensity is low-intensity white, and all characters are displayed underlined. If arg1 is in the range 2-7, the foreground intensity is low- intensity white. If 8 is added to any of these values (except 0), the foreground intensity becomes high intensity white; if 16 is added, the characters blink. If arg2 (background) is 0, then the background intensity (surrounding displayed characters) is black. If arg2 is 1-7, the background intensity is low-intensity white. If the color adapter is in use, arg1 is the foreground color (one of the 16 possible colors in high or low intensity), and arg2 is the background color (one of the 8 possible low-intensity colors). Arg3 is one of the 16 possible colors in high or low intensity, which is assigned to the border of the screen, outside the area used for character display. Adding 16 to the desired color value for arg1 causes the characters to blink. If the color adapter is in medium resolution graphics mode, then arg1 becomes the background color (one of the 16 possible), and arg2 may be a number from 0-255. If the number is even, color palette '0' (green, red, brown) is used for the SCREEN graphics commands (see below). If the number is odd, color palette '1' (cyan, magenta, white) is used. Arg3 is ignored, and may be omitted. Examples: V:C,0,7,0 (this sets reverse video mode, with black characters on a white background) V:C,12,3,1 (this sets foreground=light red, background=cyan, border=blue in color mode) COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 33 Tables of colors and useful combinations of arg1,arg2,arg3 are found below: Color table: Low Intensity High Intensity 0 -- Black 8 -- dark grey 1 -- Blue 9 -- light blue 2 -- Green 10 -- light green 3 -- Cyan 11 -- light cyan 4 -- Red 12 -- light red 5 -- Magenta 13 -- light magenta 6 -- Brown 14 -- yellow 7 -- White 15 -- bright white Some Useful Monochrome Display Combinations 7,0,0 (standard) white characters on black background 0,7,0 black characters on white background 1,0,0 underlined white characters on black background 9,0,0 bright underlined white characters on black background 15,0,0 bright white characters on black background 16,7,0 blinking black characters on white background 17,0,0 blinking underlined white charac- ters on black background Note: the use of blinking characters is extremely distracting, and should be avoided except for EXTREME emphasis! COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 34 VIDEO LOCATE (cursor) command Form: V[modifier]:L,arg1,arg2,arg3{,arg4,arg5} This command is used in text mode to move the cursor to a specified position on the screen, turn the cursor on or off, and specify the cursor height and position. The 'L' subcommand takes up to 5 arguments, which are in the same order and which may assume the same values as the arguments for the IBM advanced BASIC 'LOCATE' command. The user is referred to the IBM BASIC manual pages 4-51 and 4-155, respectively, for further explanations. Non-specified (omitted or blank) arguments are interpreted as '0', and may have unintended results. Arg1 is the character display row (1-25). Arg2 is the display column (1-40 in width 40, 1-80 in width 80). Arg3 may be either 0 (cursor off) or 1 (cursor on). Arg4 and arg5 specify the beginning and ending dot row on the current line used to display the cursor. Fourteen dot rows (0-13) are present on a given line displayed by the monochrome adapter; 8 dot rows (0-7) are displayed on a line by the color adapter. If arg4 and arg5 are omitted, the current cursor (default 12,13 for monochrome and 6,7 for color) is preserved. Examples: V:L,23,1,0 (positions cursor to beginning of line 23 and turns it off) V:L,10,10,1,1,13 (positions cursor to line 10, column 10, displays it, and sets it to full character height) Note: all text displayed after the cursor is relocated is printed beginning at the current cursor location, even if the cursor itself is not displayed. COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 35 VIDEO SELECT screen mode command Form: V[modifier]:S,arg1,arg2,arg3,arg4 This command is used to select the display mode for the color graphics display adapter. Arg1 is the display mode--0 for text, 1 for medium resolution graphics, or 2 for high resolution graphics. Arg2 may be either 0 for color disabled (i.e. monochrome display) or 1 for color enabled. Arg3 and arg4 are meaningful only in text mode. In order to accomodate graphics, the color display adapter has memory capacity far in excess of that needed for even 25 row by 80 column text display. There is sufficient memory to hold four full pages of 25 x 80 display (numbered pages 0-3), or 8 full pages of 25 x 40 display (pages 0-7). The text display width, set by the VIDEO WIDTH command described above, determines how many possible pages of text may be specified. Arg3 is the 'active page'--that to which all subsequent text is printed, to be displayed either currently or later. Arg4 is the 'visual page'--i.e. the page currently being displayed. Use of these arguments permits concurrent loading of one page during display of another, as for animation. When changing pages, the current cursor location is made current for the new active page. It is therefore wise to specify cursor location with a VIDEO LOCATE command immediately after switching active pages, and to make note of the current cursor location on the old page, if intending to return to that location again. Under normal circumstances, page switching is an unnecessary complication, however, though experienced courseware designers may use it to stunning effect. Examples: V:S,0,0,1,1 (text mode selected, color disabled, active and visual pages both page 1) V:S,0,1,3,7 (text mode, color enabled, page 3 active, page 7 displayed--valid if width=40 columns ONLY!) V:S,1,1,0,0 (medium resolution graphics mode, color enabled) V:S,2,0,0,0 (high resolution graphics mode) COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 36 SCREEN graphics command Forms: S[modifier]:[sub-cmd.],[arg1,...arg8] S[modifier]:[sub-cmd.],[expression] This command is used to create graphic displays, mainly using the color/graphics adapter, though one subcommand may be used to produce a line of characters using either display system. An attempt to use a graphics subcommand applicable only to the color adapter while the monochrome adapter is in use will cause an error. The subcommands supported by the SCREEN command are: D -- draw lines, using turtle graphics C -- draw a circle S -- set a point to a given color R -- reset a point to background color P -- paint a region of the display a given color L -- display a line of characters Subcommand 'L' is the only one valid using the monochrome display adapter. In order to properly use the other subcommands, one must understand the coordinates used to specify locations on the screen in graphics modes. In medium-resolution graphics, the screen is divided into 200 rows of 320 columns of picture elements ('pels'), each of which may be colored one of four possible colors. In high-resolution graphics, the screen holds 200 rows of 640 columns of pels, each of which may be either black or white. The coordinates begin at 0,0 which is located at the upper left hand corner of the screen. The maximum coordinates (319,199 in medium, and 639,199 in high resolution) are of the point at the bottom right of the screen. When a graphics mode is selected, the screen is cleared, and the currently selected pel is that in the center of the screen (160,100 in medium, and 320,100 in high resolution). All turtle graphics commands then may reference points relative to this starting point. The term 'turtle graphics' refers to drawing lines on a graphics screen by moving an imaginary cursor (the 'turtle'), which leaves a trail of colored pels behind it. Movements may be specified either in terms of absolute coordinates (i.e. move turtle to a specified point on the screen) or in relative coordinates (move turtle n points vertical or horizontal from its current position). Although it is possible to draw any shape at all by moving the turtle, doing so requires considerable practice and forethought, as well as full knowledge of the command language used to move the turtle around the screen. Also, creating circles or painting in large areas of the screen would be very awkward using turtle graphics, and therefore, IBM BASIC provides special functions for these purposes. The shape of any figure drawn with the SCREEN graphics commands is influenced by the 'aspect ratio' of the graphics screen. The display area is rectangular, not square, and so each COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 37 pel is also a rectangle. Thus, a rectangle n pels wide and n pels long is not a square as one might expect, but a rectangle with its long axis in the y direction. Pels are more narrow in the x direction than in the y direction. The ratio of pel height to width is 1.2:1 in medium resolution graphics and 2.4:1 in high resolution graphics. Thus, a rectangle would have to be 24 pels wide and 20 pels high to appear as a square in medium resolution, and 48 pels wide and 20 pels high to appear as a square in high resolution. As might be expected, there is considerable correspondence between the SCREEN graphics commands and IBM BASIC graphics commands, as is explained below. COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 38 SCREEN DRAW command Form: S[modifier]:D,[expression] This command is used to draw lines on the color graphics screen, using turtle graphics; an attempt to use this command while in text mode will cause an error. The expression consists of a series of turtle graphics subcommands which may be separated by spaces, if desired, for readability. Each subcommand is a capital letter, which may be prefixed by another capital letter (to modify its effect), and which is followed by either one number or two numbers separated by a comma. These subcommands are fully explained in the IBM BASIC manual, pages 4-79 to 4-83. All of the subcommands supported by the IBM BASIC 'DRAW' statement are valid, except for the 'X' subcommand and the '={variable name' construction, both of which will cause errors if used. Improper turtle graphics subcommands may also cause an error, as well as unintended results! For the convenience of the user, the subcommands are summarized below. Small letter 'n' is used to represent an integer number; small letters 'x' and 'y' are used to represent coordinate values (either absolute or relative). Un -- move turtle up n pels (decreasing y coordinate) Dn -- move turtle down n pels (increasing y coordinate) Ln -- move turtle left n pels (decreasing x coordinate) Rn -- move turtle right n pels (increasing x coordinate) En -- move diagonally up and right n pels Fn -- move diagonally down and right n pels Gn -- move diagonally down and left n pels Hn -- move diagonally up and left n pels These subcommands move the turtle relative to its current position. Mx,y -- move the turtle to the point with coordinates x,y M+x,y -- move the turtle x pels horizontal, y pels vertical relative to current position M-x,y -- move turtle -x pels horizontal (i.e. left) and y pels vertical relative to current position This subcommand is used to move the turtle either to a particular point (absolute coordinates x,y) or in an x,y displacement relative to the current position. Any of these subcommands may be prefixed with 'B', in which case the turtle moves but does not draw a line; or with 'N', in which case the turtle moves, then returns to its previous position. COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 39 The additional subcommands below may be used to modify the effect of the movement commands. An -- rotate all subsequent displacements n increments of 90 degrees (n in the range 0-3) Cn -- draw all subsequent lines in color n (n in the range 0-3). The default color is 3. The actual color selected depends on which palette is in use--which may be set using the VIDEO COLOR command. Sn -- all subsequent relative displacements are multiplied by n/4 (as a scale factor) before they are executed In VTL, very long turtle graphics commands may be composed by assigning a string of subcommands to a string register, and then referencing the register multiple times within the [expression] of a SCREEN DRAW command. The compiled form of the VTL interpreter permits strings of length up to 65,535 characters (!); however, the interpreter will surely run out of memory before it runs out of string length, especially on a computer with only 64K of main memory. The interpreted form of the interpreter permits a maximum string length of only 255 characters. In any case, command constructions using multiple string register references should be used with caution! Examples: S:D,BM25,25 U30R40D30L40 (draws a square with its lower left corner at 25,25) C:$A=U30R40D30L40\S:D,$ABR25$A (draws two squares sep- arated by 25 pels) COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 40 SCREEN CIRCLE command Forms: S[modifier]:C,arg1,arg2,arg3 S[modifier]:C,arg1,arg2,arg3,arg4 S[modifier]:C,arg1,arg2,arg3,arg4,arg5,arg6,arg7 This command is used to draw a circle, or part of one, in graphics mode using the color adapter. An attempt to use this command in text mode will cause an error. Arg1,arg2 are the coordinates of the center of the circle. Arg3 is the radius of the circle. These arguments must always be specified. Arg4 is the color of the circle (default 3, range 0-3). Arg5 and arg6 are starting and ending angles in radians. If either is negative, the ends of the segment of circle specified will be connected by lines to the center point. These angles are specified using standard mathematical notation (i.e. moving counterclockwise with increasing angle). Arg7 is the aspect ratio for the circle, which may be varied to draw an ellipse with its long axis in either the x or y direction. The effect of different values of these parameters is further explained in the IBM BASIC manual, pages 4-41 to 4-43. Examples: S:C,100,100,40 (draws a circle, radius 40, center at 100,100) S:C,100,100,40,1 (draws similar circle, in color 1) S:C,100,100,40,1,3.14,6.28 (draws lower half of same circle) S:C,100,100,40,1,3.14,6.28,2 (draws lower half of ellipse with aspect ratio 2) COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 41 SCREEN SET command SCREEN RESET command Forms: S[modifier]:S,arg1,arg2{,arg3} S[modifier]:R,arg1,arg2{,arg3} These commands are used to change the color of an individual point on the graphics screen. Arg1,arg2 is the coordinate of the point. Arg3 need not be specified, and defaults to 3 for the SET command and 0 for the RESET command. If arg3 is specified, it is the color the point is to be set to (range 0-3 in medium, and 0-1 in high resolution), and the two commands are equivalent. Examples: S:S,100,100 (pel with coordinates 100,100 intensified with color 3) S:R,100,100 (same pel intensified with color 0) S:R,100,100,2 (same pel intensified with color 2) COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 42 SCREEN PAINT command Form: S[modifier]:P,arg1,arg2,arg3,arg4 This command is used to fill in an area of the screen with a given color. The area is defined by picking a point within it, with coordinates arg1,arg2. Arg3 is the color to be painted. Arg4 is the color of all points forming the boundary of the area to be filled in. All of these arguments must be specified. Example: S:D,BM25,25U20R24D20L24\S:P,7,27,3,3 (draws a square in color 3, and fills it in with color 3) COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 43 SCREEN LINE command Form: S[modifier]:L,arg1,arg2,arg3 This command may be used in either text or graphics mode to place a line of characters on the screen, horizontally, vertically or diagonally. Arg1 is the ASCII code for the character to be used, specified as a decimal integer. Arg2 is the number of characters to be displayed (i.e. the number of times the character is to be repeated). Arg3 is the direction of the line, relative to the current cursor position, as displayed below: 6 7 8 5 C 1 4 3 2 where C is the current cursor position. As shown, if arg3=1, the line is drawn to the right; if arg3=4, the line is drawn diagonally down and left. ASCII codes for all characters are found in Appendix G of the IBM BASIC manual. Example: S:L,65,10,6 (displays a row of 10 'A's diagonally up and left) COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 44 KEY control command Forms: K[modifier]:[expression],[expression] K[modifier]:[subcommand] This command permits the user to assign a string expression of up to 16 characters to any of the ten function keys (F1-F10), turn the function key display on line 25 of the screen on or off, and list the assigned values, if any, of the function keys. The first form of this command is used to assign a value to a function key. The first expression is evaluated as a number between 1 and 10, specifying the key; the second expression (which is automatically truncated to 16 characters if too long) is assigned to the specified key. If the second expression ends in an ampersand (`&`), a carriage return will be appended to the expression if its length is 15 characters or less. The second form of the command performs other functions according to the subcommand: ON -- turns on the function key display OFF -- turns off the function key display LIST -- lists all function key values on the screen The effects of these subcommands are those of the IBM BASIC 'KEY ON', 'KEY OFF', and 'KEY LIST' commands (see Manual, pp. 4-131 to 4-133). Initially, VTL assigns no values to the function keys. Note WELL: If the soft key display is ON, and a VIDEO LOCATE command is used to attempt to move the cursor to line 25, an error will result. Examples: K:1,Valley Forge (the string expression "Valley Forge" is assigned to function key F1) K:LIST (all function key values are displayed) COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 45 BEEP (sound) command Forms: B[modifier]:[sub-cmd.],arg1{,arg2} B[modifier]:[sub-cmd.],[expression] This command is used to activate the built-in speaker of the PC, to create single tones, or to play a melody. Three subcommands specify the action desired: B -- beep (at standard frequency) S -- sound a specified frequency for a specified interval P -- play a melody using the IBM BASIC tone definition language For subcommand 'B', arg1 specifies the number of times a 'beep' should be repeated. The repeated beep sounds as a continuous tone; thus arg1 specifies the length of the beep. For subcommand 'S', arg1 represents the frequency in Hz, and ranges between 37 and 32767. Arg2 represents the duration of the sound in system clock 'ticks' (18.2 ticks in one second). For subcommand 'P', the second form of the BEEP command is used. The expression is a string of commands in the IBM BASIC tone definition language, used by the PLAY statement (manual, pp. 4-209 to 4-212). For the user's convenience, the commands, which may be separated by spaces for readability, are summarized below. All commands listed in the manual except 'X' are permitted. A to G -- plays the indicated note in the current octave, sustaining the note for the current length. A '+' or '#' as a suffix (i.e. 'G#' or 'G+') indicates a sharp (if valid); a '-' as a suffix (i.e. 'G-') indicates a flat Nn -- n ranges between 0 (rest) and 84 (C'''), and specifies a note (an alternate method besides A-G and accidentals) On -- n ranges between 0 and 6. Selects the current octave. Octave 3 starts with middle C; octave 4 is the default. Ln -- n ranges between 1 and 64. Selects the current note length (1/nth note). As an alternate method, if using A-G and accidental notation, a note may be followed by n to specify the length of only that note (i.e. B-16 is a B-flat held as a 16th note) Pn -- n ranges between 1 and 64. Specifies a rest of value 1/nth. Tn -- n ranges between 32 and 255. Sets the tempo as n quarter notes in a minute. The default is 120. MF -- Music Foreground. The VTL program does not continue until the specified melody has been completed. This is the default state. MB -- Music Background. The VTL program continues while the melody is playing. Use this with caution, as the music may be distracting, and melodies of >256 notes to be played in background may cause an error. COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 46 MN -- Music Normal. Each note plays for 7/8 of the specified length. This is the default state. ML -- Music Legato. Each note plays for the full length. MS -- Music Staccato. Each note plays for 3/4 of the specified length. Examples: BY:B,10 (10 concatenated beeps) B:S,55,36 (2 seconds of 55 Hz) B:P,CDEFGAB O5 C (plays a scale in quarter notes) Note: complex melodies may be formed by assigning phrases to string registers, then using register references to insert them into a 'B:P' command. This feature should be used cautiously since very long expressions may cause an error by exhausting memory, exceeding maximum string length, or, if music is played in the background, exceeding maximum background note storage space. COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 47 QUICKMOVE command Form: Q[modifier]:[sub-cmd.],[filename]{,arg1} This command is used to load an entire screen of text or graphics. In text mode, not only the characters are loaded into the screen, but all character attributes (i.e. colors or modes) are loaded too. For the color adapter, arg1 specifies the page of text memory (0-3 for 80 column, 0-7 for 40 column) to be loaded. In graphics mode, the entire screen is loaded for either high or medium resolution, and arg1 is omitted. The argument is ignored if the monochrome adapter is in use. Similarly, this command may be used to 'dump' the contents of the specified text page, or the graphics screen, to a disk file. The subcommands are: L -- load screen from specified file D -- dump screen from specified file The action taken depends on the CURRENT display mode, set either by default or by a VIDEO SELECT command. For subcommand 'L', the specified file is checked for appropriate length (2000 bytes for a 40-column text screen, 4000 bytes for an 80-column text screen, and 16000 bytes for a graphics screen). If the file length does not match the current display mode, an error occurs. For subcommand 'D', the current display mode and text screen width determines the size of the file created, and which data are transferred. For either subcommand, the specified file name must be a valid DOS filename, with extension. Examples: V:S,1,1,0,0\V:C,1,0\Q:L,video.dmp (medium-resolution graphics mode is selected; palette 0 is selected, with color 1 as background; and a graphics screen previously saved in file 'video.dmp' is loaded) V:W,40\V:S,0,0,0,1\Q:L,char.dmp,1 (40-column text mode is selected with page 1 visualized; page 1 is loaded with the previously saved contents of file 'char.dmp') Q:D,char.dmp,3 (assuming text mode, the current contents of page 3--2000 bytes if in 40 column width, 4000 bytes if in 80 column width--is dumped into file 'char.dmp') Q:D,video.dmp (assuming graphics mode, the contents of the screen are dumped into file 'video.dmp'. If the monochrome display is in use, the contents of the current display are saved.) COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 48 NEWDISPLAY command Form: N[modifier]:[sub-cmd.] This command is valid if and only if both monochrome and color/graphics adapters are present. Otherwise, use of this command will result in an error. The subcommands are: C -- switch to color adapter M -- switch to monochrome adapter If the specified adapter is already in use, the command is ignored. When a switch is made to the color adapter, 40-column text mode is selected as default. Default mode for the monochrome adapter is 80-column text. Examples: N:C (switch to color adapter) N:M (switch to monochrome adapter) Note: Extended register V (referenced by @V) contains a single capital letter specifying which adapter(s) VTL found present. This may be 'M' (monochrome adapter only), 'C' (color adapter only), or 'B' (both present). The command, 'Y:@V=B' may be used to test for the presence of both adapters. COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 49 III. Use of the Interpreter and Virtual Teaching Language The interpreter is invoked with the command "VTL<cr>". As described above, VTL first searches for the file, "autoexec.vtl" as a default program to execute. If this file is not found, the interpreter asks: program name: The user then specifies the name of the program to be run. The file extension ".vtl" need not be entered, and is assumed. Files with another extension may be executed as programs. The PILOT teaching language was developed in the late 1960's by Dr. John Starkweather, as a specialized computer language for development of 'courseware'--programs used for computer-aided instruction (CAI). The original language was quite simple, with only a few basic commands (TYPE, ACCEPT, MATCH, JUMP, END). As computers became less expensive and more widespread, the original language was extended to incorporate more commands, and was further modified to fit the capabilities of different computer. The result was chaos (as it was with many other computer languages), and a standard (PILOT '73) was established--a standard which this interpreter ignores (!). As usual, when a standard is ignored, there are good reasons for adding to the PILOT chaos. The 1973 standard includes a number of multi- character commands (e.g. AS: for ACCEPT SINGLE) which add confusion to the syntax, and therefore make the language less usable by the novices it was supposedly designed for. The present version uses only single-letter commands, most of which require only one expression following the colon (:), and some of which require no modification at all. Twenty-six (A: to Z:) commands comprise the entire command set of the language. The arrangement and form of arguments (expressions following the colon) in the more complex commands has been made consistent. Also, arguments for the graphics and sound commands have been kept consistent with those used by corresponding IBM BASIC statements: VTL IBM BASIC V:L LOCATE V:C COLOR V:S SCREEN V:W WIDTH S:D DRAW (excluding "X") S:C CIRCLE S:S PSET S:R PRESET S:P PAINT B:S SOUND B:P PLAY (excluding "X") COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 50 The following paragraphs explore the ways this new PILOT dialect may be used to create courseware, as well as presentations, adaptive tests, and other uses for the fun and profit of the programmer. Creating a Program VTL programs are PC-DOS text files. A file is a collection of data on disk. A text file is a collection of text--letters, numbers, etc. arranged in sentences, stored on disk. Text files are created using a computer program designed to enter, edit and store text, using the keyboard and video display to enter and edit, and the disk drive to store. The simple line editor EDLIN.COM which is supplied with PC-DOS is more than adequate for entering VTL programs. Instructions for EDLIN are found in Chapter 4 of the DOS manual. Users who have more sophisticated editors, such as the IBM Personal Editor (R), or word-processing programs such as Wordstar (R), may wish to use these programs to create VTL files. Word- processing programs must be used with caution, however, since they may insert peculiar characters into the text to effect formatting of printed output. Wordstar, for example, must be used in its 'non-document' mode to create programs. Presenting Text with VTL The simplest possible VTL program would be an existing text file, such as a chapter from a book. One would only have to rename the file containing the text, giving the file the new extension ".plt" (e.g. chapter1.txt would become chapter1.plt). Such a file would be treated by the interpreter as a long series of TYPE commands. All of the text would be displayed on the screen, but would scroll past the reader too quickly to be read. One could modify the file by inserting a line consisting of: \Press any key to continue...\W: every 22 lines. As an alternative, one could write and execute the following program: Z:%\!this clears all the integer registers C:%A=1\!set the initial line number to 1 *loop\F:chapter1.txt,%A\C:%A=%A+23 \Press any key to continue...\W:\J:loop This program would display every 23 lines as a frame, then add 23 to the starting line number, and display the next 23 lines. This program incorporates an 'endless loop' in line 3, which would continue to display frames until the FRAME command would run out of lines in the file, at which time a "frame eof" error would result. As an alternative, one could count the number of frames in the chapter, and modify the program thus: COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 51 Z:%\C:%A=1\C:%F=1\!use %F for frame count *loop\F:chapter1.txt,%A\\Press any key to continue... W:\Y:%,%F=9\JY:end\!test if at 9th frame--last frame C:%A=%A+23\C:%F=%F+1\J:loop *end\E: The FRAME command is extremely powerful, and is best used to separate the text of a lesson or a presentation from the statements controlling the flow of a program. The FRAME command can also be used to set up part of a screen as an illustration (using graphics characters built into the IBM PC character set), and then present text on another part of the screen. For example, the lines: F:illus23.grp,1,12 F:text23.txt,1,12,12,1 would read 12 lines from file 'illus23.grp' to the top 12 lines of the display, and read 12 lines from the file 'text23.txt' to the next 12 lines of the display. This is 'vertical windowing', which is the easiest form of using sections of the display screen as 'windows' for different sorts of information. Using column offsets (the 5th argument of the FRAME command), it is possible to do horizontal windowing as well, but this is much trickier. Presenting Graphics There are several ways to present graphics using VTL. The first--creating a figure using graphics characters in a text file--was alluded to in the preceding section. Turtle graphics may be used to create pictures if you have a color/graphics adapter and display which is IBM BASIC compatible. Using turtle graphics is slightly more complicated, however. First, the display must be set to graphics mode--medium or high resolution. The color palette must then be selected. Finally, the image is created using a series of graphics commands. For example, this sequence: V:S,1,1,0,0\!this selects graphics mode, medium resolution V:C,7,0,0\!this selects white as foreground color, !palette 0, and a black border to the screen S:D,BM50,100U30R40D30L40\!this creates a white box with !its lower left corner at coordinates 50,100 on the screen A third technique for presenting graphics is to use a graphics editing program to create, and save an image. The picture is saved as a disk file, which, if it is in the proper form, may be loaded using a QUICKMOVE LOAD command. Of course, the display mode and color must be set before the QUICKMOVE command can be executed. The author recommends PCCRAYON (R) as a cheap, effective graphics editor, whose DRAW (.drw) files can be loaded by VTL. For example: COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 52 V:S,1,1\V:C,7,0,0\Q:L,illus23.drw would load the contents of file illus23.drw into the color display in medium resolution graphics mode. Note that a high- resolution graphics file would also be loaded (erroneously!) by this command sequence--it is up to the programmer to see that graphics modes are correct (but this is simple). Any graphics editor program which saves the contents of color adapter video memory in BLOAD form can be used to create screens for VTL. The disk file holding a screen must be 16512 bytes in length to be loaded by VTL. (This statistic may be checked using the DOS 'DIR' command.) Testing--Adaptive and Otherwise After presenting material, one may wish to test the comprehension of the information by the user of your program. The easiest way to do so is to ask a question on the screen, have the user type in a reply, and test the reply for correctness. For example: How many bytes must a graphics screen file hold to be valid for loading by a QUICKMOVE command? &\A:\!get reply M:16512\TY:Correct!!! TN:Nope--16512 bytes is the correct answer. This example uses a MATCH command to test the user's response, which is automatically entered into the 'comparison register'--a temporary storage area which holds the most recently typed response from an ACCEPT or WAIT command. One need not have the user type a response directly; for multiple choice questions, the function keys can be assigned values, and then pressed by the user to respond, as below: K:1,true\K:2,false *1\V:Z\\\\\\ True or false--Nuclear War can be hazardous to your health. F1 -- true F2 -- false \W:\M:true\TY:You said a mouthful, buddy!! M:false\TY:You must not have read the question!!!\JN:1 Note that the WAIT command will return ALL characters transmitted by pressing a key (including a function key). The program above tests also for pressing an invalid key, and repeats the question if one is pressed. Using these and other facilities of VTL, one may develop many varieties of test question. Individual questions can then be compiled into examinations. User responses can be recorded, either in print (via a HARDCOPY command) or on disk (via sequential file operations). For example: COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 53 Your response: &\A:$A\O:A,response.txt\P:$A\X: This records the response of the user in a file (already created) named 'response.txt'. The user input in this example is appended to the existing contents of the file by using the Append file mode for the OPEN command. (The Output file mode would be used initially to create the file and erase a previous file with the same name, if any existed.) A commonly used application of computer-aided instruction is 'adaptive testing'. For this technique, an examination with multiple questions is compiled. The order in which the questions are presented to the user is determined in part by the user's responses. Thus, the examination 'adapts' to the user's apparent level of knowledge. In VTL, this can be done by using conditional JUMP or DO commands, where the condition is tested for as described above. If responses are to be timed, the timed WAIT command can be used for user responses: F1 -- true F2 -- false\\W:10\JN:noresponse\M:true\!etc. etc.... The timed wait can also be used to time presentation of material. The number of seconds should be set so that the slowest reader anticipated can finish the material (unless you wish to be tricky). A faster reader can omit the pause by pressing the space bar, or any other key. Program Flow and Segmentation Discussing adaptive testing brings us to consider how a teaching program should be designed in its entirety. Since VTL reads program lines one at a time from disk, the best way to write a teaching program in this dialect is to separate the course material from the actual program as much as possible. This keeps the main program concise, and keeps the pause involved in JUMPing to a statement earlier in the program as brief as possible. Different parts of the main program can be kept in short program files which chain to each other via the LINK command. Subprograms invoked by DO commands can be used to create graphics, play music, present text, or execute an examination question. In any case, the designer of a program MUST have a firm plan of the material to be presented, the methods to be used to present it, and the strategy to be used to test the student's comprehension. Use of RAMDISK Electronic disk, or 'RAMDISK' refers to the use of extended computer main memory ('RAM') to emulate a disk drive. This technique is very popular with users of the IBM PC, which can address up to 640K bytes (roughly 655,000 characters) of data in main memory, but whose programs typically require only about 128K bytes (roughly 131,000 characters) for operation. Memory in COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 54 excess of 128K bytes can be used as temporary 'disk' storage by running a program which convinces DOS that a third diskette drive is present, whose capacity equals the amount of excess memory. Needless to say, random-access main memory will transfer data to the computer much faster than diskette. If the computer is running VTL, and the program the interpreter is executing resides in 'RAMDISK', the program will execute somewhat faster than from diskette. FRAME and QUICKMOVE commands will also load the screen faster. However, if only a small amount of excess memory is available for RAMDISK, it is best to store only the main program and subroutines in electronic disk. The interpreter itself need not reside there. A DOS batch file can be used to set up the electronic disk, copy the requisite files to it, and begin execution (if the initial program segment is named 'AUTOEXEC.VTL') as in this example, which assumes that the program diskette is in drive A and the RAMDISK is drive C: DOS Command Effect RAMDISK/160 sets up 160K byte electronic disk COPY *.PLT C: copies all program segments COPY *.TXT C: copies all frame files C: sets default drive to C A:VTL starts interpreter If the program only is to be copied to disk C (i.e. all frame files remain on disk A), the program must specify the disk drive to be searched for frame files, for example: F:a:frame1.txt instead of F:frame1.txt As an alternative, drive assignments can be entered by the user: Program drive (A:, B:, C:): &\A:$P Text drive (A:, B:, C:): &\A:$T ... L:$Pprog44.plt ... F:$Tframe44.txt or these assignments can be read from a disk file. Since electronic disk can be so useful for VTL, not to mention most other applications, the author enthusiastically recommends this technique to all IBM PC users. A typical electronic disk program retails for $50-100, a small price to pay for the performance improvement, if you already have more than 128K bytes of main memory. If you don't, 64K byte RAM chip sets currently retail for under $50 each.... Envoi The author hopes that the information in this section will help the reader become an effective and enthusiastic user of the VTL interpreter and language. The additional demonstration programs included with the interpreter provide some other COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 55 examples of the uses of VTL--the extent of which may be limited only by your imagination! COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 56 Appendix A: Error Messages VTL is designed to detect and report most of the possible errors in command syntax, file access, and IBM BASIC command access (i.e. for graphics) which a courseware designer or programmer may accidentally include in his programs. Of course, despite careful design by the author, not every possible error can be provided for. If an error occurs which is not trapped or internally detected, it is reported by the message: IBM BASIC error N at PILOT line M where N is the BASIC error code and M is the VTL program line number at which the error occurred. If such an error occurs, the courseware author should check the error code against the list in Appendix A of the IBM BASIC manual. Some errors, such as 'Out of Memory', may be self-explanatory. An error code of 5 (invalid function call) usually signifies an error in tone-definition language or turtle graphics command construction. In any case, errors of this sort, if not correctable by simple courseware modifications should be reported to your software dealer or directly to the publisher. VTL error codes are displayed using the form: ** [error explanation] at line N of program [file]** where N is the line of the VTL program in which the error occurred. If the error occurred while another file is being executed by a DO command, an additional message is printed stating the name and line of the program file from which the DO command was executed. The following error messages are listed by error explanation. bad chain file -- the filename specified for a LINK command is invalid, or the file does not exist bad draw syntax -- an error in construction of a turtle graphics expression has been made bad exec. file -- the filename specified for a DO command is invalid, or the file does not exist bad frame file -- the filename specified for a FRAME command is invalid, or the file does not exist bad graphics syntax -- illegal arguments have been provided for a SCREEN command other than DRAW bad play syntax -- an error in construction of a tone-definition language expression has been made bad quickmove file -- the filename specified for a QUICKMOVE command is invalid, or a load file does not exist duplicate label = [label] -- a second occurrence of the same label in the current program has been detected end of file -- a GET command encounters end of file (EOF) on an attempted read. exec. file eof -- the end of a file named in a DO command was reached before the specified first line was found COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 57 file already open -- a file has already been opened for access prior to the current OPEN command. file not open -- a file close, GET or PUT is attempted prior to opening a file. frame eof -- the specified first line to be displayed of a frame file is out of range. illegal beep command -- invalid construction of a BEEP command has been detected illegal beep subcommand -- a non-existant or invalid subcommand has been specified for a BEEP command illegal calculation -- either no "=" or an invalid register reference encountered in a CALCULATE command. illegal comp. type -- the subcommand for a YES-IF command was incorrect (i.e. not %, # or $) illegal do command -- invalid construction of a DO command has been detected illegal filename -- a filename given to an OPEN command is invalid. illegal key directive -- the directive for a KEY command of the form K[modifier]:[directive] is invalid (i.e. not ON, OFF or LIST). illegal key # -- the function key number specified for a KEY command of the form K[modifier]:[key # expr.],[expr.] is outside the range of 1-10. illegal quickmove command -- invalid construction of a QUICKMOVE command has been detected illegal quickmove subcommand -- a non-existant or invalid subcommand has been specified for a QUICKMOVE command illegal read register -- an invalid register reference was supplied as the destination for a GET command. illegal register destination -- an invalid register reference was supplied as the input destination for an ACCEPT command illegal screen command -- invalid construction of a SCREEN command has been detected illegal screen subcommand -- an illegal or non-existant subcommand has been specified for a SCREEN command illegal video command -- invalid construction of a VIDEO command has been detected illegal video subcmd. -- the subcommand for a VIDEO command is invalid (i.e. not L, C, or Z). illegal zero command -- an illegal register type has been specified for a ZERO command invalid newdisplay command -- either both display adapters are not present, or an invalid subcommand has been specified for a NEWDISPLAY command label not found -- a JUMP command cannot find the specified label. nested Do -- a file being executed by a DO command itself contains a Do command. printer error -- a printer hardware error (not ready or paper out) occurs with a HARDCOPY command quickmove mode mismatch -- the length of a specified load file does not match that expected for the current display mode COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 58 recursive register reference -- this error may occur when a string register reference occurs within a line of input from a file or the keyboard, which is assigned to the same string register referenced in the line of input. The next reference to that register will cause infinite expansion of the reference, with either an 'out of memory' or `illegal string length' error specified line out of range -- the first line of a file specified to a DO command does not exist in the file subr. stack overflow -- more than 30 subroutines were nested, causing this error message on the 31st subroutine call. too many labels -- the current program contains more than 100 labels; the 101st has just been encountered. unspecified error -- an error has been trapped for which VTL has no handling routine. This message precedes the 'IBM BASIC error' message. COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 59 Appendix B: Extended String Registers As noted in Section II, there are 26 extended string registers maintained by VTL, the contents of which may not be changed directly by the user, but may be inserted into expressions when referenced. A reference to an extended string register consists of the character '@' followed by a capital letter from A-Z. The contents of the specified register will be inserted in the expression containing the reference, replacing the reference (as for the other types of register). Not all 26 of the extended string registers are used in the current release; therefore, references to unused registers will be replaced with a null string. The registers used by the current release, and their contents are: @A -- date VTL execution was begun (set on ini- tialization) @B -- time VTL execution was begun (set on initialization) @C -- current command line (set when line read) @D -- date of current program execution (set when program file first opened) @I -- length of contents of comparison register (set whenever contents changed) @J -- contents of comparison register (set whenever contents changed) @K -- returned value of numeric register from ACCEPT @L -- current program line number (set when line read) @P -- name of current program (set when program file first opened or when new file chained) @S -- number of seconds elapsed before key struck on execution of most recent WAIT command (set if key struck) @T -- current time (set whenever a command is executed) @U -- uppercase equivalent of text ACCEPTed or INSERTed @V -- display adapters present: M, C or B @X -- filename of file most recently executed by a DO command (set when file opened for execution) COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 60 Appendix C: Intermixing Text and Graphics Although the techniques for doing so are not well explained in the IBM BASIC manual, it is possible to place text on the screen while using the color/graphics adapter in either medium or high resolution graphics mode, even though a cursor is not displayed while in graphics mode. This is done by specifying a cursor location (via the VIDEO LOCATE command), and then using a TYPE command to print text at the specified location. Relating the position of text to graphics coordinates is not difficult. Characters are displayed in an 8 x 8 pel matrix; thus, 200 rows of pels permit 25 lines; 320 columns of pels (in medium resolution) permit 40 columns; and 640 columns of pels (in high resolution) permit 80 columns. The upper left hand corner of a character matrix will begin at the coordinate (8*(R-1),8*(C-1)) where R is the row number and C is the column number of the character. Characters printed will be displayed in the current foreground color. COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 61 INDEX A ACCEPT command 8,9,10,11,24,52,53 conversational form 11 Arguments (for commands) 9 Automatic program execution 6,54 AUTOEXEC.VTL 6,54 B BASIC compiler dialect 3 BASIC run time errors 57 BEEP command 45 C CALCULATE command 8,9,13,17,27,39,50,51 Chaining VTL programs 6,23 Closing a disk file 20 Color, setting 33 Command syntax 10-48 ACCEPT 11 BEEP 45 CALCULATE 17 DO 27 END 16 FRAME 25 GET 21 HARDCOPY 18 INSERT 24 JUMP 13 KEY 44 LINK 23 MATCH 12 NEWDISPLAY 48 OPEN 19 PUT 22 QUICKMOVE 47 RETURN 15 SCREEN 36-43 TYPE 10 USE 14 VIDEO 31-35 WAIT 28 XIT 20 YES-IF 26 ZERO 29 Comments 8 COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 62 D Disk copying 4 DO command 23,27,53 E END command 16,23,51 Error messages 56-58 F File copying 4,5 File operations 20-23 open 20 close 21 read 22 write 23 FRAME command 25,50,51,54 G GET command 8,9,21 H HARDCOPY command 3,18 I INSERT command 9,24 Installation 4-5 Interpreter features 6-7 installation 4-5 J JUMP command 13,50,52,53 K KEY command 44 L Labels 8,10,13,14 LINK command 23,53,54 M Manual, printing 4 MATCH command 12,52 Modifier 8,9 COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 63 N NEWDISPLAY command O OPEN (a disk file) command 19,53 modes 19,21 Operators 9,17,26 P Phrase match 8,12,16 Q QUICKMOVE command 47,52,54 R Ramdisk (electronic disk) 53,54 Registers 6,8-9 comparison 9,12 extended string 9,59 integer 6,8,11,17,21,26,27 naming 8-9 real 6,8,17,18 string 6,8,9,13,17,21,24,26,39 RETURN command 14,15 S SCREEN command 30,36-43,51 CIRCLE subcommand 36,40,49 DRAW subcommand 36,38-39,49,51 LINE subcommand 36,43,49 PAINT subcommand 36,42,49 RESET subcommand 36,41,49 SET subcommand 36,41,49 Subcommands 9,30 T Turtle graphics 36 TYPE command 10,51,52 U USE (subroutine) command 14 V COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 64 VIDEO command 30,31-35,49,51,52,60 COLOR subcommand 30,31,32-33,49,51,52 LOCATE subcommand 31,34,49,60 SELECT subcommand 30,31,35,49,51,52 WIDTH subcommand 30,31,35,49 ZERO subcommand 31,49,52 Video display modes 30,32,35 selection 35 VTL interpreter running 49 language design 49 VTL programs creation 8,50 chaining 23 naming 50 W WAIT command 9,24,28,50,51,52,53 X XIT (close disk file) command 20 Y YES-IF command 26,51 Z ZERO command 29,50,51 COPYRIGHT STUART A. JONES 1982, 1983, 1984 Page 65 To print the manual, use the 'docu.bat' batch file. The self-running demonstration programs may be run by specifying program 'srun' to the interpreter. Requests for information, support, and (of course) registrations should be addressed to: Stuart A. Jones C/O Nuclear Software 1045 Treeline Drive Allentown, Pa. 18103