Assembly JLE jmp instruction example
Solution 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.
Solution 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
Solution 3
The x86 assembly uses a system of bit-flags that represent the result of comparisons. The conditional jump instructions use these flags when deciding whether to perform the jump or not.
In your case you'd use the following two instructions:
cmp ebx, 10 ; compare EBX and 10
jle label ; jump if EBX is "less than or equal" to 10
…
label:
…
Solution 4
JB - work with unsigned numbers (Jump Below) <
JL - work with signed numbers
mov bx,0 // BX := 0
cmp bx,FF // 0 < -1 or 0 < 255 (Jump Flag and Sign Flag will change)
jl butter // if you use JL jump will not occurs, cus 0 > -1
jb butter // if you use JB jump will occurs, cus 0 < 255
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Updated on July 09, 2022Comments
-
bobobobo almost 2 years
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 labelthere:
ifebx
is<= 10
.Specifically I'm interested in using the x86 jump family of instructions
-
Anderson Green over 11 yearsIn this example, does JLE jump when 10 is less than or equal to ebx, or does it jump when ebx is less than or equal to 10?
-
d0rmLife over 11 years@AndersonGreen It jumps when the ebx's contents are <= 10.
-
Peter Cordes over 3 years@AndersonGreen: Intel chose the mnemonics so
cmp x,y
/jcc
being taken lines up withx <= y
. felixcloutier.com/x86/jcc. The easiest one to remind yourself of is thatjb
is the same instruction asjc
, jump if CF=1. (This is one of the worst parts of AT&T syntax: the reversed operands tocmp
mean the semantic meanings for jcc mnemonics are backwards from the operand order in the source.) -
ecm about 2 yearsI don't think
test
is "Typically used for equality checks." The only typical cases I know are to test specific bits or masks, or to dotest
with the same register twice to check it for zero or check its sign bit. "Equality checks" sounds much more like a case forcmp
.