MMIX-Expand (Using as) (original) (raw)

9.29.2 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 <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mn>255</mn><mo separator="true">,</mo></mrow><annotation encoding="application/x-tex">255,</annotation></semantics></math>255,255,0.

‘PUSHJ’

Similar to expansion for conditional branches; four instructions set$255 to the operand value, followed by a PUSHGO <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mn>255</mn><mo separator="true">,</mo></mrow><annotation encoding="application/x-tex">255,</annotation></semantics></math>255,255,0.

‘JMP’

Similar to conditional branches and PUSHJ. The final instruction is GO <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mn>255</mn><mo separator="true">,</mo></mrow><annotation encoding="application/x-tex">255,</annotation></semantics></math>255,255,0.

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