8086 Assembler Tutorial for Beginners (Part 9)

The Stack

Stack is an area of memory for keeping temporary data. Stack is used by CALL instruction to keep return address for procedure, RET instruction gets this value from the stack and returns to that offset. Quite the same thing happens when INT instruction calls an interrupt, it stores in stack flag register, code segment and offset. IRET instruction is used to return from interrupt call.

We can also use the stack to keep any other data,
there are two instructions that work with the stack:

PUSH - stores 16 bit value in the stack.

POP - gets 16 bit value from the stack.

Syntax for PUSH instruction:

PUSH REG
PUSH SREG
PUSH memory
PUSH immediate
REG: AX, BX, CX, DX, DI, SI, BP, SP.

SREG: DS, ES, SS, CS.

memory: [BX], [BX+SI+7], 16 bit variable, etc...

immediate: 5, -24, 3Fh, 10001101b, etc...


Syntax for POP instruction:

POP REG
POP SREG
POP memory
REG: AX, BX, CX, DX, DI, SI, BP, SP.

SREG: DS, ES, SS, (except CS).

memory: [BX], [BX+SI+7], 16 bit variable, etc...



Notes:
  • PUSH and POP work with 16 bit values only!

  • Note: PUSH immediate works only on 80186 CPU and later!


The stack uses LIFO (Last In First Out) algorithm,
this means that if we push these values one by one into the stack:
1, 2, 3, 4, 5
the first value that we will get on pop will be 5, then 4, 3, 2, and only then 1.



It is very important to do equal number of PUSHs and POPs, otherwise the stack maybe corrupted and it will be impossible to return to operating system. As you already know we use RET instruction to return to operating system, so when program starts there is a return address in stack (generally it's 0000h).

PUSH and POP instruction are especially useful because we don't have too much registers to operate with, so here is a trick:
  • Store original value of the register in stack (using PUSH).

  • Use the register for any purpose.

  • Restore the original value of the register from stack (using POP).

Here is an example: 


ORG    100h

MOV    AX, 1234h
PUSH   AX          ; store value of AX in stack.

MOV    AX, 5678h   ; modify the AX value.

POP    AX          ; restore the original value of AX.

RET

END


Another use of the stack is for exchanging the values,
here is an example:


ORG    100h

MOV    AX, 1212h   ; store 1212h in AX.
MOV    BX, 3434h   ; store 3434h in BX


PUSH   AX          ; store value of AX in stack.
PUSH   BX          ; store value of BX in stack.

POP    AX          ; set AX to original value of BX.
POP    BX          ; set BX to original value of AX.

RET

END

The exchange happens because stack uses LIFO (Last In First Out) algorithm, so when we push 1212h and then 3434h, on pop we will first get 3434h and only after it 1212h.


The stack memory area is set by SS (Stack Segment) register, and SP (Stack Pointer) register. Generally operating system sets values of these registers on program start.

"PUSH source" instruction does the following:
  • Subtract 2 from SP register.

  • Write the value of source to the address SS:SP.

"POP destination" instruction does the following:
  • Write the value at the address SS:SP to destination.

  • Add 2 to SP register.

The current address pointed by SS:SP is called the top of the stack.

For COM files stack segment is generally the code segment, and stack pointer is set to value of 0FFFEh. At the address SS:0FFFEh stored a return address for RET instruction that is executed in the end of the program.

You can visually see the stack operation by clicking on [Stack] button on emulator window. The top of the stack is marked with "<" sign.


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