assembly stack pointer

Question

Stack:

0xDDDD
0xCCCC
0xBBBB
0xAAAA

pop ax Means:

0xAAAA will be placed in ax
SS:SP will point now to 0xBBBB.

Now, I get this.

0xDDDD
0xCCCC
0xBBBB

I want to save ax's value.

push ax.

0xDDDD
0xCCCC
0xBBBB
0xAAAA

Here I go again.

mov ax, [ss:sp+2]

0xBBBB will be loaded in ax.

After my function is done, I will want to remove both 0xAAAA and 0xBBBB from the stack. First, I will restore ax.

pop ax. (Now loaded with 0xAAAA)

How do I get rid of 0xBBBB from my stack? Do I simply "pop" without arguments (Do this exist?) or do I add sp, 2 ?

Thank you.

Download CPU manuals from Intel or AMD. They will tell you whether operand-less pop exist, what add sp, 2 will do and whether there's any other error in the instructions from the question. — Alexey Frunze
– Alexey Frunze, Commented Mar 18, 2013 at 20:21
mov ax, [ss:sp+2] does not exist and won't assemble. pop without arguments also does not exist in x86 assembly and won't assemble, use add sp,2 for that. pop itself doesn't clear the values popped from the stack, but usually they get overwritten very soon by the next interrupt. — nrz
– nrz, Commented Mar 18, 2013 at 20:39

Eric Postpischil · Accepted Answer · 2013-03-18 20:26:44Z

1

Simply adjust the stack pointer to point earlier in the stack, as with add sp, $2. Syntax may vary depending on which assembler you use. $2 is intended to mean “the immediate value 2”.

You can download the Intel 64 and IA-32 Architectures Software Developer’s Manuals.

(Note that using 16-bit registers is rather old. Depending on which platform you are writing for, there might be requirements about maintaining stack alignment to be multiples of four bytes or more, particular when calling or returning from routines.)

answered Mar 18, 2013 at 20:26

Eric Postpischil

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