Assembly JLE jmp instruction example

assembly x86 mnemonics
bobobobo picture bobobobo · Dec 29, 2010 · Viewed 66.5k times · Source

How do you use the jump family of instructions?

This is what they've got:

JL label
"It" jumps if it is less than or if it is not larger than or equal to.

My question is what is it in this sentence? Say I have a variable in ebx and I want to jump to label there: if ebx is <= 10.

Specifically I'm interested in using the x86 jump family of instructions

Answer

CodesInChaos picture CodesInChaos · Dec 29, 2010

The jump itself checks the flags in the EFL register. These are usually set with TEST or CMP(or as a side effect of many other instructions).

CMP ebx,10
JLE there
  • CMP corresponds to calculating the difference of the operands, updating the flags and discarding the result. Typically used for greater/smaller checks
  • TEST corresponds to calculating the binary AND of the operands, updating the flags and discarding the result. Typically used for equality checks.

See also: The art of assembly language on CMP

As a sidenote: You should get the Intel reference manuals. In particular the two part "Intel® 64 and IA-32 Architectures Software Developer's Manual Volume 2: Instruction Set Reference" which describes all x86 instructions.

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