VAX Dependent Features

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VAX dependent features

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

- Command line options for VAX
- Floating point for VAX
- Machine directives for VAX
- Opcodes for VAX
- Branch improvement for VAX
- Operands for VAX
- Unsupported features for VAX

Command line options for VAX

The VAX version of as accepts any of the following options, gives a warning message that the option was ignored and proceeds. These options are for compatibility with scripts designed for other people’s assemblers.

-D (Debug)
-S (Symbol Table)
-T (Token Trace)

These are obsolete options used to debug old assemblers.

-d (Displacement size for JUMPs)

This option expects a number following the -d. Like options that expect filenames, the number may immediately follow the -d (old standard) or constitute the whole of the command line argument that follows -d (GNU standard).

-V (Virtualize Interpass Temporary File)

Some other assemblers use a temporary file. This option commanded them to keep the information in active memory rather than in a disk file. as always does this, so this option is redundant.

-J (JUMPify Longer Branches)

Many 32-bit computers permit a variety of branch instructions to do the same job. Some of these instructions are short (and fast) but have a limited range; others are long (and slow) but can branch anywhere in virtual memory. Often there are 3 flavors of branch: short, medium and long. Some other assemblers would emit short and medium branches, unless told by this option to emit short and long branches.

-t (Temporary File Directory)

Some other assemblers may use a temporary file, and this option takes a filename being the directory to site the temporary file. Since as does not use a temporary disk file, this option makes no difference. -t needs exactly one filename.

The VAX version of the assembler accepts two options when compiled for VMS. They are -h, and -+. The -h option prevents as from modifying the symbol-table entries for symbols that contain lowercase characters (I think). The -+ option causes as to print warning messages if the FILENAME part of the object file, or any symbol name is larger than 31 characters. The -+ option also inserts some code following the _main symbol so that the object file is compatible with VAX-11 "C".

Floating point for VAX

Conversion of flonums to floating point is correct, and compatible with previous assemblers. Rounding is towards zero if the remainder is exactly half the least significant bit. D, F, G and H floating point formats are understood.

Immediate floating literals (e.g., S‘$6.9) are rendered correctly. Again, rounding is towards zero in the boundary case.

The .float directive produces f format numbers.

The .double directive produces d format numbers.

Machine directives for VAX

The VAX version of the assembler supports four directives for generating VAX floating point constants. They are described in the following.

.dfloat

This expects zero or more flonums, separated by commas, and assembles VAX d format 64-bit floating point constants.

.ffloat

This expects zero or more flonums, separated by commas, and assembles VAX f format 32-bit floating point constants.

.gfloat

This expects zero or more flonums, separated by commas, and assembles VAX g format 64-bit floating point constants.

.hfloat

This expects zero or more flonums, separated by commas, and assembles VAX h format 128-bit floating point constants.

Opcodes for VAX

All DEC mnemonics are supported. Beware that case... instructions have exactly 3 operands. The dispatch table that follows the case... instruction should be made with .word statements. As far as we know, this is compatible with all Unix assemblers.

Branch improvement for VAX

Certain pseudo opcodes are permitted. They are for branch instructions. They expand to the shortest branch instruction that reaches the target.

Generally these mnemonics are made by substituting j for b at the start of a DEC mnemonic. This feature is included both for compatibility and to help compilers. If you do not need this feature, avoid these opcodes. What follows are the mnemonics, and the code into which they can expand.

jbsb

Jsb is already an instruction mnemonic, so we chose jbsb.

(byte displacement)
bsbb...
(word displacement)
bsbw...
(long displacement)
jsb...

jbr
jr

Unconditional branch.

(byte displacement)
brb...
(word displacement)
brw...
(long displacement)
jmp...

jCOND

COND may be any one of the conditional branches neq, nequ, eql, eqlu, gtr, geq, lss, gtru, lequ, vc, vs, gequ, cc, lssu, cs. COND may also be one of the bit tests bs, bc, bss, bcs, bsc, bcc, bssi, bcci, lbs, lbc. NOTCOND is the opposite condition to COND.

(byte displacement)
bCOND...
(word displacement)
bNOTCOND foo ; brw...; foo:
(long displacement)
bNOTCOND foo ; jmp...; foo:

jacbX

X may be one of b d f g h l w.

(word displacement)
OPCODE...
(long displacement)
OPCODE..., foo ;
brb bar ; foo: jmp... ;
bar:

jaobYYY

YYY may be one of lss leq.

jsobZZZ

ZZZ may be one of geq gtr.

(byte displacement)
OPCODE...
(word displacement)
OPCODE... , foo ;
brb bar ;
foo: brw destination ;
bar:
(long displacement)
OPCODE..., foo ;
brb bar ;
foo: jmp destination ;
bar:

aobleq
aoblss
sobgeq
sobgtr

(byte displacement)
OPCODE...
(word displacement)
OPCODE... , foo ;
brb bar ;
foo: brw destination ;
bar:
(long displacement)
OPCODE..., foo ;
brb bar ;
foo: jmp destination ;
bar:

Operands for VAX

The immediate character is $ for Unix compatibility, not # as DEC writes it.

The indirect character is * for Unix compatibility, not @ as DEC writes it.

The displacement sizing character is ‘ (an accent grave) for Unix compatibility, not ˆ (a circumflex) as DEC writes it. The letter preceding ‘ may have either case. G is not understood, but all other letters (b, i, l, s, w) are understood.

Register names understood are r0 r1 r2...r15 ap fp sp pc. Upper and lower case letters are equivalent. For instance, tstb *w‘$4(r5).

Any expression is permitted in an operand. Operands are comma separated.

Unsupported VAX features

VAX bit fields can not be assembled with as. Someone can add the required code if they really need it.

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