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1993-11-24
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68000 Assembler
by Paul McKee
User's Manual
Table of Contents
1. Introduction ............................. 2
2. Source Code Format ....................... 3
2.1 Source Line Format....................... 3
2.1.1 Label Field............................ 3
2.1.2 Operation Field........................ 3
2.1.3 Operand Field.......................... 3
2.1.4 Comment Field.......................... 4
2.2 Symbols.................................. 4
2.3 Expressions.............................. 4
2.3.1 Operands in Expressions................ 4
2.3.1.1 Decimal Numbers...................... 4
2.3.1.2 Hexadecimal Numbers.................. 4
2.3.1.3 Binary Numbers....................... 5
2.3.1.4 Octal Numbers........................ 5
2.3.1.5 ASCII Constants...................... 5
2.3.2 Operators in Expressions............... 5
2.4 Addressing Mode Specifications........... 6
3. Assembly Details ......................... 7
3.1 Branch Instructions...................... 7
3.2 MOVEM Instruction........................ 7
3.3 Quick Instructions (MOVEQ, ADDQ, SUBQ)... 7
4. Assembler Directives ..................... 8
4.1 ORG - Set Origin......................... 8
4.2 Symbol Definition Directives............. 8
4.2.1 EQU - Equate Symbol.................... 8
4.2.2 SET - Set Symbol....................... 8
4.2.3 REG - Register List Symbol............. 9
4.3 Data Storage Directives.................. 9
4.3.1 DC - Define Constant................... 9
4.3.2 DCB - Define Constant Block............ 10
4.3.3 DS - Define Storage.................... 11
4.4 END - End of Source File................. 12
5. Usage .................................... 13
5.1 Command Line............................. 13
5.2 Listing File Format...................... 13
5.3 Object Code File Format.................. 14
1
2
1. Introduction
The program described here, 68000 Assembler, is a basic two-
pass assembler for the 68000 and 68010 microprocessors. It
supports the complete instruction set of both processors as well
as a modest but capable set of assembler directives. The program
produces formatted listing files as well as object code files in
S-record format.
The program was written in VAX-11 C by Paul McKee during the
fall semester, 1986. The program should be portable (with rea
sonable changes) to any C language implementation that supports
32-bit integers.
3
2. Source Code Format
2.1 Source Line Format
The input to the assembler is a file containing instruc
tions, assembler directives, and comments. Each line of the file
may be up to 256 characters long. It is recommended, however,
that the source lines be no longer that 80 characters, as this
will guarantee that the lines of the listing file do not exceed
132 characters in length. The assembler treats uppercase and
lowercase identically.
Each line of the source code consists of the following
fields:
LABEL OPERATION OPERAND,OPERAND,... COMMENT
For example,
LOOP MOVE.L (A0)+,(A1)+ Sample source line
The fields may be separated by any combination of spaces and
tabs. Except for the comment field and quoted strings, there must
be no spaces or tabs within a field.
2.1.1 Label Field
Legal labels follow the rules for forming symbol names
described in section 2.2. Labels may be distinguished in one of
two ways: (1) They may begin in column 1, or (2) they may end in
a colon, which does not become part of the label but simply
serves to mark its end. A line may consist of a label alone.
When a label is encountered in the source code, it is defined to
have a value equal to the current location counter. This symbol
may be used elsewhere is the program to refer to that location.
2.1.2 Operation Field
The operation field specifies the instruction that is to be
assembled or the assembler directive that is to be performed. A
size code (.B, .W, .L, or .S) may be appended to the operation
code if allowed, to specify Byte, Word, Long, or Short opera
tions, respectively. The operation field must not begin in the
column 1, because the operation would be confused with a label.
2.1.3 Operand Field
The operand field may or may not be required, depending on
the instruction or directive being used. If present, the field
consists of one or more comma-separated items with no intervening
spaces or tabs. (There may be spaces or tabs within an item, but
only within quoted strings.)
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2.1.4 Comment Field
The comment field usually consists of everything on a source
line after the operand field. No special character is needed to
introduce the comment, and it may contain any characters desired.
A comment may also be inserted in the source file in another
way: An asterisk ("*") at the beginning of the line or after the
label field will cause the rest of the line to be ignored, i.e.,
treated as a comment.
2.2 Symbols
Symbols appear in the source code as labels, constants, and
operands. The first character of a symbol must be either a
letter (A-Z) or a period ("."). The remaining characters may be
letters, dollar signs ("$"), periods ("."), or underscores("_").
A symbol may be of any length, but only the first 8 characters
are significant. Remember that capitalization is ignored, so
symbols which are capitalized differently are really the same.
2.3 Expressions
An expression may be used in the source program anywhere a
number is called for. An expression consists of one or more
operands (numbers or symbols), combined with unary or binary
operators. These components are described below. The value of
the expression and intermediate values are always computed to 32
bits, with no account being made of any overflow that may occur.
(Division by zero, however, will cause an error.)
2.3.1 Operands in Expressions
An operand in an expression is either a symbol or one of the
following sorts of constants.
2.3.1.1 Decimal Numbers
A decimal number consists of a sequence of decimal digits
(0-9) of any length. A warning will be generated if the value of
the number cannot be represented in 32 bits.
2.3.1.2 Hexadecimal Numbers
A hexadecimal number consists of a dollar sign ("$") fol
lowed by a sequence of hexadecimal digits (0-9 and A-F) of any
length. A warning will be generated if the value of the number
cannot be represented in 32 bits.
5
2.3.1.3 Binary Numbers
A binary number consists of a percent sign ("%") followed by
a sequence of binary digits (0 and 1) of any length. A warning
will be generated if the number consists of more that 32 digits.
2.3.1.4 Octal Numbers
An octal number consists of a commercial at sign ("@")
followed by a sequence of octal digits (0-7) of any length. A
warning will be generated if the value of the number cannot be
represented in 32 bits.
2.3.1.5 ASCII Constants
An ASCII constant consists of one to four ASCII characters
enclosed in single quote marks. If it is desired to put a single
quote mark inside an ASCII constant, then two consecutive single
quotes may be used to represent one such character.
If the ASCII constant consists of one character, then it
will be placed in the bottom byte of the 32 bit value; two
characters will be placed in the bottom word, with the first
character in the higher-order position. If four characters are
used, then all four bytes will contain characters, with the first
in the highest-order location. However, if three characters are
used, then they will be placed in the three highest-order bytes
of the 32-bit value, with 0 in the low byte (this is to accom
modate the high-byte-first addressing used on the 68000).
Note that ASCII constants in expressions are different from
strings in DC directives, as the latter may be of any length.
2.3.2 Operators in Expressions
The operators allowed in expressions are shown in the fol
lowing table, in order of decreasing precedence. Within each
group, the operators are evaluated in left-to-right order (except
for group 2, which is evaluated right-to-left).
Operators in Expressions
1. () Parenthesized subexpressions
2. - Unary minus (two's complement)
~ Bitwise not (one's complement)
3. << Shift left (x<<y produces x shifted left by
y bits and zero filled)
>> Shift right
4. & Bitwise and
! Bitwise or
5. * Multiplication
/ Integer division
\ Modulus (x\y produces the remainder when x
is divided by y)
6. + Addition
- Subtraction
6
2.4 Addressing Mode Specifications
The 68000 and 68010 provide 14 general addressing modes.
The formats used to specify these modes in assembly language
programs are listed in the table below. The following symbols
are used to describe the operand formats:
Dn = Data Register
An = Address Register (SP may used instead of A7)
Xn = Data or Address register
.s = Index register size code (either .W or .L, .W will
be assumed if omitted)
<ex8> = Expression that evaluates to an 8-bit value (may be
empty, in which case 0 will be used)
<ex16> = Expression that evaluates to a 16-bit value (may be
empty, in which case 0 will be used)
<ex> = Any expression
PC = Program Counter
Addressing Mode Specifications
Mode Assembler Format
--------------------------------------------- ----------------
Data Register Direct Dn
Address Register Direct An
Address Register Indirect (An)
Address Register Indirect with Predecrement -(An)
Address Register Indirect with Postincrement (An)+
Address Register Indirect with Displacement <ex16>(An)
Address Register Indirect with Index <ex8>(An,Xn.s)
Absolute Short or Long (chosen by assembler) <ex>
Program Counter with Displacement <ex16>(PC)
Program Counter with Index <ex8>(PC,Xn.s)
Immediate #<ex>
In addition to the general addressing modes, the following
register names may be used as operands in certain instructions
(e.g., MOVEC or EORI to CCR):
SR = Status Register
CCR = Condition Code Register
USP = User Stack Pointer
VBR = Vector Base Register (68010)
SFC = Source Function Code Register (68010)
DFC = Destination Function Code Register (68010)
7
3. Assembly Details
3.1 Branch Instructions
The branch instructions (Bcc, BRA, and BSR) are unique in
that they can take a ".S" size code. This suffix directs the
assembler to assemble these as short branch instructions, i.e.,
one-word instructions with a range to -128 to +127 bytes. If the
".S" size code is used, and the destination is actually outside
this range, then the assembler will print an error message. If
the ".L" size code is used, the assembler will use a long branch,
which is a two-word instruction with a range of -32768 to +32767
bytes. If neither size code is specified, then the assembler
will use a short branch if possible (the branch destination must
be known on the first pass to be within the short branch range);
otherwise it will use long branch.
3.2 MOVEM Instruction
The MOVEM instruction, which is used for saving and restor
ing sets of registers, has one the following two forms:
MOVEM <register_list>,<effective_address>
MOVEM <effective_address>,<register_list>
The register list may be an explicit register list of the form
described in Section 4.2.3. On the other hand, if a particular
set of registers is to be saved and restored repeatedly, the REG
directive (Section 4.2.3) can be used to define a register list
symbol that specifies the registers. For example, if the regis
ter list symbol WORKSET is defined as follows:
WORKSET REG A0-A4/D1/D2
then the following instructions will perform the same function:
MOVEM.L WORKSET,-(SP)
MOVEM.L A0-A4/D1/D2,-(SP)
If a register list symbol is used, it must be defined before it
appears in any MOVEM instructions.
3.3 Quick Instructions (MOVEQ, ADDQ, SUBQ)
The MOVE, ADD, and SUB instructions have one-word "quick"
variations which can be used certain addressing modes and operand
values. The assembler will use these faster variations automat
ically when possible, or they may be specified explicitly by
writing the mnemonic as MOVEQ, ADDQ, or SUBQ.
The MOVEQ instruction may be used for moving an immediate
value in the range -128 to +127 into a data register. The assem
bler will assemble a MOVE.L #<value>,Dn as a MOVEQ if the value
is known on the first pass.
The ADDQ (SUBQ) instruction adds (subtracts) an immediate
value from 1 to 8 to (from) any alterable destination. The
assembler will use the quick form if the value is known on the
first pass to be in the range 1 to 8.
8
4. Assembler Directives
4.1 ORG - Set Origin
The assembler maintains a 32-bit location counter, whose
value is initially zero and which is incremented by some amount
whenever an instruction is assembled or a data storage directive
is carried out. The value of this location counter may be set
with the ORG directive. This is typically done at the start of a
program and at appropriate places within it. The format of the
ORG directive is
<label> ORG <expression>
where <expression> is an expression containing no forward refer
ences, i.e., its value must be known on the first pass at the
point where the ORG directive appears. An error will result if
an attempt is made to set the location counter to an odd value;
in this case the location counter will be set to the specified
value plus one. The <label> is optional and, if present, the
specified symbol will be set to the new value of the location
counter.
4.2 Symbol Definition Directives
4.2.1 EQU - Equate Symbol
The equate directive is used to define symbols whose value
will not change within the program. The format of this directive
is
<label> EQU <expression>
where <expression> is an expression containing no forward refer
ences, i.e., its value must be known on the first pass at the
point where the EQU directive appears. The <label> must be speci
fied, since it tells what symbol is being defined. If <label> is
omitted, an error will result. If an attempt is made to redefine
a symbol that was defined with EQU, either as a label or using
any symbol definition directive, an error message will be printed.
4.2.2 SET - Set Symbol
The SET directive is similar in function and format to the
equate directive, with one important difference: symbols defined
using SET may be redefined later using another SET directive (but
not using a EQU or REG directive). The format of this directive
is
<label> SET <expression>
9
4.2.3 REG - Register Range
Register ranges consist of lists of registers separated by
slashes ("/"). Each register range may be either a single
register ("An" or "Dn") or a range of registers ("An-Am" or
"Dn-Dm"), which denotes all the registers between the two
registers listed (they may be given in either order). For exam
ple, the following register list specifies that D0, D1, D2, D3,
D7, A1, A2, and A3 are to be saved (or restored):
D3-D0/D7/A1-A3
The registers and ranges may be specified in any order. The same
format for register lists may be used with the MOVEM instruction
directly. In order to avoid confusion, it is best to avoid
specifying a range that includes both an address register and a
data register, although the assembler will not treat this as an
error.
4.3 Data Storage Directives
4.3.1 DC - Define Constant
The define constant directive is used to store strings and
lists of constants in memory. The format of a DC directive is
<label> DC.<size> <item>,<item>,...
The label will be defined to equal the address of the start of
the list of data. The size code specifies that a list of bytes
(.B), words (.W), or longwords (.L) is being defined; if omitted,
word size is used.
A list of items follows the directive; each item may be an
expression or a string. If an item is an expression, the expres
sion is evaluated and stored as the size indicated, i.e., a byte,
a word, or a longword. An error is generated if the value will
not fit as either a signed or unsigned value in the specified
size. If an item is a string, delimited by single quotes, then
the string will be stored in successive entities of the size
specified; if words or longwords are being generated, and the
string does not fit into an whole number of words or longwords,
then the string will be padded with zeros at the end to make a
whole number of words or longwords. Strings and expressions may
intermixed in a single DC directive.
If words (DC.W) or longwords (DC.L) are being generated,
then the start of the list of constants will be aligned on a word
boundary by increasing the location counter by one, if necessary.
This is not performed for DC.B directives, so that strings of
bytes may be contiguous in memory. If an instruction follows a
DC.B directive, the assembler will automatically adjust the loca
tion counter (if necessary) to place the instruction on a word
boundary.
10
An example of a DC directive that defines a null-terminated
string:
TEXT DC.B 'DC Example',$0D,$0A,0
This directive results in the following data at location TEXT:
44 43 20 45 78 61 6D 70 6C 65 0D 0A 00 (hexadecimal)
4.3.2 DCB - Define Constant Block
The define constant block directive generates a block of
bytes, words, or longwords that are all initialized to the same
value by the assembler. The format of the directive is
<label> DCB.<size> <length>,<value>
The label will be defined to equal the address of the start of
the block. The size code specifies that a block of bytes (.B),
words (.W), or longwords (.L) is being set up; if omitted, word
size is used.
The length argument is an expression that tells the number
of bytes, words, or longwords that are to be in the block. This
value must be known on the first pass at the point where the DCB
directive appears, and it must be non-negative. The value argu
ment is an expression whose value is to be placed in each data
item in the block; it needn't be known on the first pass. An
warning message will be printed if the value will not fit (as a
signed or unsigned number) in the data size selected.
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If word or longword size is selected, then the start of the
block will be placed on a word boundary by increasing the loca
tion counter by one, if necessary. If an instruction follows a
DCB.B directive, the assembler will automatically adjust the
location counter (if necessary) to place the instruction on a
word boundary.
4.3.3 DS - Define Storage
The define storage directive generates an uninitialized
block of bytes, words, or longwords. The format of the directive
is
<label> DS.<size> <length>
The label will be defined to equal the address of the start of
the block. The size code specifies that a block of bytes (.B),
words (.W), or longwords (.L) is being set up; if omitted, word
size is used.
The length argument is an expression that tells the number
of bytes, words, or longwords that are to be in the block. This
value must be known on the first pass at the point where the DCB
directive appears, and it must be non-negative. The effect of
the DS directive is basically to increase the value of the loca
tion counter by <length> times one (if DS.B is used), two (if
DS.W is used), or four (if DS.L is used)
If word or longword size is selected, then the start of the
block will be placed on a word boundary by increasing the loca
tion counter by one, if necessary. Thus, DS.W 0 can be used to
force the location counter to be aligned on a word boundary
without allocating any space. However, if an instruction follows
a DS.B directive, the assembler will automatically adjust the
location counter (if necessary) to align the instruction on a
word boundary.
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4.4 END - End of Source File
The end directive is used to mark the end of the source
file. It is purely optional. The format is simply
END
The assembler will ignore anything in the source file after the
END directive.
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5. Usage
5.1 Command Line
The 68000 Assembler is run by typing a command line of the
following form:
!68kAsm <options> Asm.<filename>
The options are a string of letters, preceded by a dash,
which alter the behavior of the assembler. The following option
letters are allowed:
C = Show all the Constants produced by DC directives
(see Section 5.2)
L = Produce a Listing file
N = Produce No object code file
If these options are not specified, the defaults are to show only
one line of data from a DC directive, to produce no listing, and
to produce an object file.
The filename is the name, including directory specifica
tions, of the file to be assembled. No default file extension is
applied. The names of the listing and object code files, if
generated, are constructed by using the source file name; the
output files are always placed in the .h68 & .lis of the currently
selected directory.
The program will print "68000 Assembler by Paul..." etc when
it begins work. If any errors occur, the program will print "ERROR
in line X" or "WARNING in line X" (this information is also
placed in the listing file). Upon conclusion, it will print the
number of errors encountered and warnings generated during the
assembly.
If there is an error in the command line, e.g., if no file
name is specified, then the assembler will print a brief usage
summary and abort.
5.2 Listing File Format
The assembler produces a listing file which shows the source
code alongside the the object code produced by the assembler. A
typical listing file line might appear as follows (not to scale):
0000102E 22D8 200 LOOP MOVE.L (A0)+,(A1)+ Sample
The eight digit hexadecimal number at the left is the assembler's
location counter; the generated instruction word, $22D8, is
placed at that address. The next number is the source file line
number, and the remainder of the line simply repeats the source
line. Remember that if the source lines are no longer than 80
columns, then the listing file lines will not exceed 132 columns.
14
If an error is encountered or a warning is generated for a
given source line, then in the listing that line will be followed
by a line that describes the error. At the end of the listing,
the program prints the total number of errors and warnings.
There is only limited space to list the object code in this
format. There is sufficient space for the longest possible
instruction, but the DC directive poses a problem, since it may
generate, e.g., dozens of longwords from a single source line.
The assembler's -C command line option controls the assembler's
actions when the object code exceeds the space available on one
line. If -C is not specified, then the assembler will print only
one listing line with an ellipsis ("...") at the end of the
object code field, indicating that some of the data produced by
the directive was omitted from the listing. If -C is included on
the command line, then the assembler will use as many source
lines as are needed to print all the data produced by the direc
tive. Each line after the first will contain only the location
counter and the object code field (the source line is not re
peated).
5.3 Object Code File Format
The 68000 Assembler produces an object code output file in
S-record format. The object file name is the source file name,
with the extension changed to ".H68". The object file and the
listing file are always placed in the user's default directory.
The S-record format is designed to allow files containing
any data to be interchanged in text file format with checksum
error detection. The format of these files will not be described
here, but the following technical information will be provided:
The first line of the object file is an S0 (header) record and
the last line is an S9 (termination) record. The lines in be
tween are S1, S2, or S3 records, depending on the whether the
address of the first byte in a record requires 2, 3, or 4 bytes
to be represented. No record is longer than 80 characters.
