The Motorola 680x0 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.
Normally the character `|' is treated as a comment character, which means that it can not be used in expressions. The `--bitwise-or' option turns `|' into a normal character. In this mode, you must either use C style comments, or start comments with a `#' character at the beginning of a line.
If you use an addressing mode with a base register without specifying
the size, as will normally use the full 32 bit value.
For example, the addressing mode `%a0@(%d0)' is equivalent to
`%a0@(%d0:l)'. You may use the `--base-size-default-16'
option to tell as to default to using the 16 bit value.
In this case, `%a0@(%d0)' is equivalent to `%a0@(%d0:w)'.
You may use the `--base-size-default-32' option to restore the
default behaviour.
If you use an addressing mode with a displacement, and the value of the
displacement is not known, as will normally assume that
the value is 32 bits. For example, if the symbol `disp' has not
been defined, as will assemble the addressing mode
`%a0@(disp,%d0)' as though `disp' is a 32 bit value. You may
use the `--disp-size-default-16' option to tell as
to instead assume that the displacement is 16 bits. In this case,
as will assemble `%a0@(disp,%d0)' as though
`disp' is a 16 bit value. You may use the
`--disp-size-default-32' option to restore the default behaviour.
as can assemble code for several different members of the
Motorola 680x0 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 680x0 family are
very similar. For detailed information about the differences, see the
Motorola manuals.
This syntax for the Motorola 680x0 was developed at MIT.
The 680x0 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 table 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:
%a6
is also known as `%fp', the Frame Pointer.
The standard Motorola syntax for this chip differs from the syntax
already discussed (see section Syntax). 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 table 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:
%a6
is also known as `%fp', the Frame Pointer.
Packed decimal (P) format floating literals are not supported. Feel free to add the code!
The floating point formats generated by directives are these.
.float
Single precision floating point constants.
.double
Double precision floating point constants.
.extend
.ldouble
Extended precision (long double) floating point constants.
In order to be compatible with the Sun assembler the 680x0 assembler understands the following directives.
.data1
.data 1 directive.
.data2
.data 2 directive.
.even
.align directive; it
aligns the output to an even byte boundary.
.skip
.space directive.
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 table summarizes the pseudo-operations. A * flags
cases that are more fully described after the table:
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 full description below
jbsr
jra
jXX
jhi jls jcc jcs jne jeq jvc jvs jpl jmi jge jlt jgt jleFor 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. For example, for the
non-PC relative case:
jXX foo
gives
bNXs oof
jmp foo
oof:
dbXX
dbhi dbls dbcc dbcs dbne dbeq dbvc dbvs dbpl dbmi dbge dblt dbgt dble dbf dbra dbtOther than for word and byte displacements, when the source reads `dbXX foo',
as emits
dbXX oo1
bra oo2
oo1:jmpl foo
oo2:
fjXX
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 fjunFor branch targets that are not PC relative,
as emits
fbNX oof
jmp foo
oof:
when it encounters `fjXX foo'.
The immediate character is `#' for Sun compatibility. The line-comment character is `|' (unless the `--bitwise-or' option is used). 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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