Ask HN: Favourite Assembly Instructions?

2 pointsRetroTechie17d ago2 comments

One for the low-level gurus among you: what is/are your favourite assembly-language instructions, accross the ages & instruction sets?

To get this started: for the good 'ol Z80, I'm torn between Decrement-and-Jump-if-Not-Zero (DJNZ), and Decimal Adjust Accumulator (DAA). Rarely used ComPare, Increment & Repeat (CPIR) could be another contender.

On RISC-V, I much like the Set-Less-Than (SLT..) instructions. Great for replacing conditional branches with branchless, sequential math.

Your favourites?

2 comments

gblargg17d ago

I always liked rlwimi on PowerPC. It rotates the source n bits, then writes any contiguous section of bits over the corresponding bits in the destination register. This allows copying any bitfield from any position in one register into another. Basically either of these:

  out = (out & ~mask) | (in << shift & mask)
  out = (out & ~mask) | (in >> shift & mask)

Z80's EXX to swap with the shadow registers was interesting (meant for fast interrupt response so you didn't have to save registers to memory).

absynth17d ago

HCF - Halt and Catch Fire.

j / k navigate · click thread line to collapse