CODEWITHSUNDEEP
assemblyx86stacknasm
Peter Chappy
Peter Chappy

Reputation: 1179

How do I create a second stack in NASM

I'm trying to do a two stack sort in assembly. I understand for the first stack you use the system stack; however have no idea how one would implement the second stack.

Upvotes: 4

Views: 1098

Answers (2)

ady
ady

Reputation: 173

Load everything into the stack and use pointers?

Mov eax,[point1-4] (top of stack down)

mov ebx,[point2-4] (limit of point 1 down)

you usually decrement by 4 because items are stored as dwords

if you mess with the stack start off by storing esp to a DW[stackstore]

mov dword [stak],esp

and return the stored value to esp at the end of your routine

mov esp,[stak]

ret

The stack can be a bit funny and crash a routine unless you keep your foot firmly on its throat

Upvotes: 0

ughoavgfhw
ughoavgfhw

Reputation: 39905

To solve this problem, you need to answer three questions: What operations do I need, what does each operation do, and how can I do that?

What operations do I need?

A stack has two operations: Push and pop.

What does each operation do?

Let's model our implementation after the system stack. Then this question becomes "what do the push and pop instructions do?". The push instruction decrements the stack pointer and stores its argument at the location it now points to. The pop instruction reads the value the stack pointer points to, increments the stack pointer, and returns the read value.

How can I do that?

Pushing was two steps: Decrementing the stack pointer and storing a value. There are already instructions for just this purpose. The push instruction uses esp as the stack pointer, but since we are writing our own, we can use whatever we want. We will use edx, and assume the value to store is in eax (we are storing 4 bytes).

sub edx, 4           ; Decrement the stack pointer one position (4 bytes)
mov dword [edx], eax ; Store the value at the new location

Popping was three steps: Getting the value, incrementing the stack, and returning the value. We will return the value simply by leaving it in eax.

mov eax, dword [edx] ; Load the value off of the stack
add edx, 4           ; Increment the stack pointer one position (4 bytes)
; Leave the result in eax to return it

Now that we have implementations for both operations, we can use them how we like. Perhaps we want to wrap them in a function by adding code to read the arguments into the registers (if needed) before, and a return after. Or, maybe we just want to insert these directly where we need them, changing registers to suit our needs.

Upvotes: 4

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