The Runtime Stack

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A stack is a data structure for storing items which are to be accessed in last-in first-out order. The operations on a stack are

• to create a new stack,
• to “push” a new item onto the top of a stack, and
• to “pop” the top item off.

Error conditions are raised by attempting to pop an empty stack or to push an item onto a stack which has no room for further items (because of its implementation).

The runtime stack is a memory array managed directly by the CPU, and it is an essential part of the mechanism for calling and returning from procedures. The ESP (Extended Stack Pointer) register holds a 32-bit offset into some location on the stack. We rarely manipulate ESP directly; instead, it is indirectly modified by instructions such as CALL, RET, PUSH, and POP.

The following figures illustrate a series of stack operations:

Initial Contents   Offset   Stack      00001000 00000006 ESP 00000FFC     00000FF8     00000FF4     00000FF0

PUSH 000000A5   Offset   Stack      00001000 00000006   00000FFC 000000A5 ESP 00000FF8     00000FF4     00000FF0

PUSH 00000010   Offset   Stack      00001000 00000006   00000FFC 000000A5   00000FF8 00000010 ESP 00000FF4     00000FF0

POP eax   Offset   Stack      00001000 00000006   00000FFC 000000A5 ESP 00000FF8     00000FF4     00000FF0

PUSH 000001AF   Offset   Stack      00001000 00000006   00000FFC 000000A5   00000FF8 000001AF ESP 00000FF4     00000FF0

POP eax   Offset   Stack      00001000 00000006   00000FFC 000000A5 ESP 00000FF8     00000FF4     00000FF0