The Stack

A procedure call is supported by a stack. So let's discuss stack in assembly. Stacks are 'Last In First Out' data structures and are used for storing return addresses of procedures and for parameter passing and saving the return value.

In 8086 microprocessor a stack is made in the stack segment. SS register stores the offset of stack segment whereas SP register stores the top of stack. A value is pushed in top of the stack or taken out (poped) from top of the stack. The stack segment may be initialized as below:   

STACK_ SEG SEGMENT STACK

            DW 100   DUP (0)

            TOS LABEL   WORD

STACK_SEG ENDS

CODE SEGMENT

ASSUME CS: CODE, SS: STACK_SEG

MOV   AX, STACK_SEG

MOV   SS, AX      ; initialise stack segment

LEA   SP, TOP      ; initialise stack pointer

CODE ENDS

END

The directive STACK_SEG SEGMENT STACK declares logical segment for stack segment. DW 100 DUP (0) assigns actual size of stack to 100 words. All locations of this stack are initialized to zero. Stacks are recognized by the stack top and so the Label Top of Stack (TOS) has been selected. Please consider that stack in 8086 is a WORD stack. Stack facilities include the use of indirect addressing by a special register, the stack pointer (SP). SP is decremented by itself as items are put on the stack and incremented as they are retrieved. Putting something on to stack is known as a PUSH and taking it off is known as a POP. The address of last element pushed on to stack is termed as the top of stack (TOS).