i386 assembly question: why do I need to meddle with the stack pointer?
- by zneak
Hello everyone,
I decided it would be fun to learn x86 assembly during the summer break. So I started with a very simple hello world program, borrowing on free examples gcc -S could give me. I ended up with this:
HELLO:
.ascii "Hello, world!\12\0"
.text
.globl _main
_main:
pushl %ebp # 1. puts the base stack address on the stack
movl %esp, %ebp # 2. puts the base stack address in the stack address register
subl $20, %esp # 3. ???
pushl $HELLO # 4. push HELLO's address on the stack
call _puts # 5. call puts
xorl %eax, %eax # 6. zero %eax, probably not necessary since we didn't do anything with it
leave # 7. clean up
ret # 8. return
# PROFIT!
It compiles and even works! And I think I understand most of it.
Though, magic happens at step 3. Would I remove this line, my program would die between the call to puts and the xor from a misaligned stack error. And would I change $20 to another value, it'd crash too. So I came to the conclusion that this value is very important.
Problem is, I don't know what it does and why it's needed.
Can anyone explain me? (I'm on Mac OS, would it ever matter.)
