The document discusses stacks, which are data structures that follow the LIFO (last in, first out) principle. It describes basic stack operations like push, pop, and peeking at the top element. Stacks can be implemented using arrays, vectors, or linked lists. In assembly language, the stack segment register SS and stack pointer SP are used to manage the call stack, temporarily storing return addresses and register values. PUSH adds an item to the top of the stack, while POP removes the top item.

2. Stacks
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3. Stack
• A stack is a data structure that stores data in such a way that the
last piece of data stored, is the first one retrieved i.e. Last in,
First out LIFO / First in Last out FILO
• Only access to the stack is the top element
• Anything added to the stack goes on the “top” of the stack
• Anything removed from the stack is taken from the “top” of the
stack
• Things are removed in the reverse order from that in which they
were inserted

5. Basic Stack Operations
• There are three Basic Stack Operations which are discussed below
• Push
• Pop
• Stack Top

6. Push
• The operation to place a new item at the top of the stack

7. Pop
• The operation to remove the next item from the top of the stack

9. Implementing a Stack
• At least three different ways to implement a stack
• array
• vector
• linked list

10. Stack in Assembly Language
• The stack segment register holds the starting address of the stack segment
in the memory. Majorly SS is used for the following purposes namely:
1.To hold the temporary results.
2.To hold the return address of the subroutine.
3.Finally to handle the interrupts
• SS (Stack Segment) Register: holds stack Segment Address
• SP (Stack Pointer) Register: initialized to the value specified with the .STACK
directive, represents empty stack position
• When stack is not empty, SP represents the top of the Stack

11. Stack (cont ..)
• In order to save the contents of a register during the execution, so that it
can be used later for purposes.
• To do so, the microprocessor has a area of memory where the contents of
specified registers or memory location can be saved temporarily, and is
called STACK.
• It is a top-down data structure whose elements are accessed using the
pointer that is implemented using the SP and SS registers.
• As we go on storing the data words onto the stack, the pointer goes on
decrementing.

12. PUSH And PUSHF
• PUSH: Used to add a new source (16-bit register or memory word) on stack
Syntax:
PUSH source
• Execution of PUSH causes:
SP is decreased by 2
A copy of source contents is moved to the address specified by SS:SP.
Source remains unchanged
• PUSHF has no operand and it pushes the contents of FLAG register onto
the stack.
• Usually this is done whenever the processor is interrupted
Syntax: PUSHF

14. POP And POPF
• POP: Used to remove top item from stack to destination (i.e. a 16-bit register
or memory word).
Syntax:
POP destination
• Execution of POP causes:
The contents of SS:SP (top of the stack) is moved to the destination.
SP is increased by 2
• POPF has no operand and pops the top of the stack into FLAG register.
• SP is incremented by 2 after executing this instant.
Syntax: POPF