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Question 35

What is the difference between ja/jb and jg/jl?

Answer

  • jl/jg are used to perform jumps after signed comparison.
mov rax, 42
mov rcx, -1
cmp rax, rcx
jl rcx    ; this jump _will not_ be performed
  • ja / jb are used to perform jumps after unsigned comparison:
mov rax, 42
mov rcx, -1
cmp rax, rcx
jb rcx    ; this jump _will_ be performed

The reason why 42 < -1 is that -1 is interpreted as an unsigned integer. Its hexadecimal representation is 0xFFFFFFFFFFFFFFFF (all 64 bits are set). If we look at it as if it were an unsigned integer, it will be the greatest unsigned integer that is representable in 64-bit format (that is, $2^{64}-1$). Apparently, $42 &lt; 2^{64}-1$

$42 &lt; -1$ is false, so the jump will not beperformed.

  • You can also memorize the flags that are tested by these instructions. If these flags are set, the jump occurs. However, from my experience, it is useless. If you are in need, refer to the Intel software developer manual.

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