Assembly JLE jmp instruction example

Question

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 1

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.

Answer 2

JLE instruction conducts two tests:

• Signed Flag (SF) != Overflow Flag (OF)
• Zero flag (ZF) == 1

If Zero flags is 1 and Signed Flag and Overflow Flag are not equal, then the short relative jump will be executed.

Maybe just a word how CMP instruction works. CMP instruction is like SUB (subtract), but the destination register will not be updated after exsecution. So the following code will perform the same result like CMP ebx, 10. CMP and SUB instruction affect to flags: Carry, Parity, Auxiliary, Zero, Sign and Overflow flags.

push   ebx            //store ebx value to stack
sub    ebx, 10
pop    ebx            //restore ebx value from stack