9.27.2 Instruction expansion

When as encounters an instruction with an operand that is either not known or does not fit the operand size of the instruction, as (and ld) will expand the instruction into a sequence of instructions semantically equivalent to the operand fitting the instruction. Expansion will take place for the following instructions:

‘GETA’

Expands to a sequence of four instructions: SETL, INCML, INCMH and INCH. The operand must be a multiple of four.

Conditional branches

A branch instruction is turned into a branch with the complemented condition and prediction bit over five instructions; four instructions setting $255 to the operand value, which like with GETA must be a multiple of four, and a final GO $255,$255,0.

‘PUSHJ’

Similar to expansion for conditional branches; four instructions set $255 to the operand value, followed by a PUSHGO $255,$255,0.

‘JMP’

Similar to conditional branches and PUSHJ. The final instruction is GO $255,$255,0.

The linker ld is expected to shrink these expansions for code assembled with ‘--relax’ (though not currently implemented).