search for: rorig

...ortant 16 bit instruction is data movement, this instruction can move register pairs in a single cycle, doing something like this: mov r25, r23 mov r24, r22 into: movw r25:r24, r23:r22 The key point here is that the movw instruction can only move data of fixed pairs in this way. movw Rdest+1:Rdest, Rorig+1:Rorig, so movw R25:R23, R21:R18 is illegal because registers pairs aren't adjacent. Explaining this as if it was for x86 may make things more clear. Suppose we only have 8bit regs (ah, al, cl, ch, bl, bh, etc...) with 8 bit instructions and a few 16 bit instrs that only work with ax, bx, cx,...