Question

I am having trouble understanding the difference between ja and jg for assembly language. I have a section of code:

cmp  dh, dl
j--  hit

and am asked which conditional jump to hit (that replaces j– hit) will be taken with the hex value of DX = 0680.

This would make dl = 06 and dh = 80, so when comparing, 80 > 06. I know that jg fits this as we can directly compare results, but how should I approach solving if ja fits (or in this case, does not fit) this code?

Answer 1

1. If dx is 0x0680, then dh is 0x06 and dl is 0x80.
2. 0x80 is interpreted as 128 in unsigned mode, and -128 in signed mode.
3. Thus, you have to use jg, since 6 > -128, but 6 < 128. jg does signed comparison; ja does unsigned comparison.