9.49.5 VAX Branch Improvement

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. Here are the mnemonics, and the code they can expand into.

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: