IA32 Assembly: lea instruction
I know that the sentence I am about to say is probably the best way to become very unpopular on StackOverflow very quickly. I'll say it anyway: Why doesn't this work (completly)?
I was trying to figure out what the lea/leal instruction does . The way I understand it, lea/leal finds out the memory address of the first operand and writes this address into the second operand (which might be a register or so).
This part seems to work. When I run the program below it says:
The memory address of var is 134518204.
However, right after this it says something like "memory access error". pushl (%eax) obviously doesn't work. Why not?
.data
var: .long 42
str1: .string "The memory address of var is %d.n"
str2: .string "At this memory address we find var's value: %d.n"
.text
.global main
main:
leal var, %eax # Copy the address of var into %eax
pushl %eax
pushl $str1
call printf # Print str1
pushl (%eax)
pushl $str2
call printf # Print str2
# Some stack cleaning should be here :)
movl $1, %eax
int $0x80
I am not even sure if I got right what lea/leal does. Help is appreciated. ;)
The way I understand it, lea/leal finds out the memory address of the first operand and writes this address into the second operand (which might be a register or so).
Sounds accurate enough. It's used to perform address arithmetic; which can be as simple as just loading an address, but you can also use it to perform multiplication by certain constants.
pushl (%eax) obviously doesn't work. Why not?
Your push instruction is not referencing the address you think it does: printf returns the number of characters written, and that value is returned in the %eax register, so %eax no longer contains the address of var .
lea is usually (ab)used for calculations: if you just want the address of a global in a register, movl $var, %eax is clearer (and a byte shorter) than the lea .
Also, to make it clear, the destination operand of lea must be a register.
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