M680x0 Dependent Features

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M68K dependent features

See the following documentation for the Motorola M68K architecture’s features and options for the assembler.

- Options for M68K
- Syntax for M68K
- Floating point for M68K
- Machine directives for M68K
- Branch improvement for M68K
- Special characters for M68K

Options for M68K

The Motorola 68K version of as has a few machine dependent options.

You can use the -l option to shorten the size of references to undefined symbols. If you do not use the -l option, references to undefined symbols are wide enough for a full long (32 bits). (Since as cannot know where these symbols end up, as can only allocate space for the linker to fill in later. Since as does not know how far away these symbols are, it allocates as much space as it can.) If you use this option, the references are only one word wide (16 bits). This may be useful if you want the object file to be as small as possible, and you know that the relevant symbols are always less than 17 bits away. For some configurations, especially those where the compiler normally does not prepend an underscore to the names of user variables, the assembler requires a % before any use of a register name. This is intended to let the assembler distinguish between C variables and functions named a0 through a7, and so on. The % is always accepted, but is not required for certain configurations, notably sun3. The --register-prefix-optional option may be used to permit omitting the % even for configurations for which it is normally required. If this is done, it will generally be impossible to refer to C variables and functions with the same names as register names.

as can assemble code for several different members of the Motorola 68K family. The default depends upon how as was configured when it was built; normally, the default is to assemble code for the 68020 microprocessor. The following options may be used to change the default. These options control which instructions and addressing modes are permitted. The members of the 68K family are very similar. For detailed information about the differences, see the Motorola manuals.

-m68000
-m68008
-m68302

Assemble for the 68000. -m68008 and -m68302 are synonyms for -m68000, since the chips are the same from the point of view of the assembler.

-m68010

Assemble for the 68010.

-m68020

Assemble for the 68020. This is normally the default.

-m68030

Assemble for the 68030.

-m68040

Assemble for the 68040.

-m68060

Assemble for the 68060.

-mcpu32
-m68331
-m68332
-m68333
-m68340
-m68360

Assemble for the CPU32 family of chips.

-m68881
-m68882

Assemble 68881 floating point instructions. This is the default for the 68020, 68030, and the CPU32. The 68040 and 68060 always support floating point instructions.

-mno-68881

Do not assemble 68881 floating point instructions. This is the default for 68000 and the 68010. The 68040 and 68060 always support floating point instructions, even if this option is used.

-m68851

Assemble 68851 MMU instructions. This is the default for the 68020, 68030, and 68060. The 68040 accepts a somewhat different set of MMU instructions; -m68851 and -m68040 should not be used together.

-mno-68851

Do not assemble 68851 MMU instructions. This is the default for the 68000, 68010, and the CPU32. The 68040 accepts a somewhat different set of MMU instructions.

Syntax for M68K

This syntax for the Motorola 68K was developed at MIT.

The 68K version of as uses instructions names and syntax compatible with the Sun assembler. Intervening periods are ignored; for example, movl is equivalent to mov.l.

In the following, note that apc stands for any of the address registers (%a0 through %a7), the program counter (%pc), the zero-address relative to the program counter (%zpc), a suppressed address register (%za0 through %za7), or it may be omitted entirely.

The use of size means one of w or l, and it may be omitted, along with the leading colon, unless a scale is also specified. The use of scale means one of 1, 2, 4, or 8, and it may always be omitted along with the leading colon.

The following addressing modes are understood.

- Immediate
#number

- Data Register
%d0 through %d7

- Address Register
%a0 through %a7
%a7 is also known as %sp; i.e., the Stack Pointer.
%a6 is also known as %fp; i.e., the Frame Pointer.

- Address Register Indirect
%a0@ through %a7@

- Address Register Postincrement
%a0@+ through %a7@+

- Address Register Predecrement
%a0@- through %a7@-

- Indirect Plus Offset
apc@(number)

- Index
apc@(number,register:size:scale)
The number may be omitted.

- Postindex
apc@(number)@(onumber,register:size:scale)
The onumber or the register, but not both, may be omitted.

- Preindex
apc@(number,register:size:scale)@(onumber)
The number may be omitted. Omitting the register produces the Postindex addressing mode.

- Absolute
symbol, or digits, optionally followed by :b, :w, or :l.

The standard Motorola syntax for this chip differs from the syntax already discussed. as can accept Motorola syntax for operands, even if MIT syntax is used for other operands in the same instruction. The two kinds of syntax are fully compatible. In the following, apc stands for any of the address registers (%a0 through %a7), the program counter (%pc), the zero-address relative to the program counter (%zpc), or a suppressed address register (%za0 through %za7). The use of size means one of w or l, and it may always be omitted along with the leading dot. The use of scale means one of 1, 2, 4, or 8, and it may always be omitted along with the leading asterisk. The following additional addressing modes are understood.

- Address Register Indirect
%a0 through %a7
%a7 is also known as %sp; i.e., the Stack Pointer.
%a6 is also known as %fp; i.e., the Frame Pointer.

- Address Register Postincrement
(%a0)+ through (%a7)+

- Address Register Predecrement
-(%a0) through -(%a7)

- Indirect Plus Offset
number(%a0) through number(%a7), or number(%pc).
The number may also appear within the parentheses, as in (number,%a0). When used with the pc, the number may be omitted (with an address register, omitting the number produces Address Register Indirect mode).

- Index
number(apc,register.size*scale)
The number may be omitted, or it may appear within the parentheses. The apc may be omitted. The register and the apc may appear in either order. If both apc and register are address registers, and the size and scale are omitted, then the first register is taken as the base register, and the second as the index register.

- Postindex
([number,apc],register.size*scale,onumber)
The onumber, or the register, or both, may be omitted. Either the number or the apc may be omitted, but not both.

- Preindex
([number,apc,register.size*scale],onumber)
The number, or the apc, or the register, or any two of them, may be omitted. The onumber may be omitted. The register and the apc may appear in either order. If both apc and register are address registers, and the size and scale are omitted, then the first register is taken as the base register, and the second as the index register.

Floating point for M68K

Packed decimal (P) format floating literals are not supported. Feel free to add the code!

The floating point formats generated by directives are the following.

.float

Single precision floating point constants.

.double

Double precision floating point constants.

.extend
.ldouble

Extended precision (long double) floating point constants.

Machine directives for M68K

In order to be compatible with the Sun assembler, the 68K assembler understands the following directives.

.data1

Similar to .data 1 directive.

.data2

Similar to a .data 2 directive.

.even

A special case of the .align directive; it aligns the output to an even byte boundary.

.skip

Similar to a .space directive.

Branch improvement for M68K

Certain pseudo opcodes are permitted for branch instructions. They expand to the shortest branch instruction that reach the target. Generally these mnemonics are made by substituting j for b at the start of a Motorola mnemonic. The following summarizes the pseudo-operations.

Displacement
+--------------------------------------------------
| 68020 68000/10
Pseudo-Op |BYTE WORD LONG LONG non-PC relative
+--------------------------------------------------
jbsr |bsrs bsr bsrl jsr jsr
jra |bras bra bral jmp jmp
* jXX |bXXs bXX bXXl bNXs;jmpl bNXs;jmp
* dbXX |dbXX dbXX dbXX;bra;jmpl
* fjXX |fbXXw fbXXw fbXXl fbNXw;jmp

XX: condition
NX: negative of condition XX

* See the following description.

jbsr
jra

These are the simplest jump pseudo-operations; they always map to one particular machine instruction, depending on the displacement to the branch target.

JXX

Here, jXX stands for an entire family of pseudo-operations, where XX is a conditional branch or condition-code test. The full list of pseudo-ops in this family is the following:

jhi jls jcc jcs jne jeq jvc
jvs jpl jmi jge jlt jgt jle

For the cases of non-PC relative displacements and long displacements on the 68000 or 68010, as issues a longer code fragment in terms of NX, the opposite condition to XX.

jXX foo

For the non-PC relative case in the previous example , as gives the following.

bNXs oof
jmp foo
oof:

dbXX

The full family of pseudo-operations meant by dbXX is the following.

dbhi dbls dbcc dbcs dbne dbeq dbvc
dbvs dbpl dbmi dbge dblt dbgt dble
dbf dbra dbt

Other than for word and byte displacements, when the source reads dbXX foo, as emits the following output.

dbXX oo1
bra oo2
oo1: jmpl foo
oo2:

fjXX

This family includes the following.

fjne fjeq fjge fjlt fjgt fjle fjf
fjt fjgl fjgle fjnge fjngl fjngle fjngt
fjnle fjnlt fjoge fjogl fjogt fjole fjolt
fjor fjseq fjsf fjsne fjst fjueq fjuge
fjugt fjule fjult fjun

For branch targets that are not PC relative, as emits the following when it encounters fjXX foo.

fbNX oof
jmp foo
oof:

Special characters for M68K

The immediate character is # for Sun compatibility. The line-comment character is |. If a # appears at the beginning of a line, it is treated as a comment unless it looks like # line file, in which case it is treated normally.

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