3. Saving Operands on the Stack

• The PUSH instruction stores its operand on the stack.

• The POP instruction retrieves the most recent pushed value.

• ESP register always points to the top of the stack.

• The PUSH and POP instructions use the ESP register to keep track of the current position.

• The operand can be a register 16, a register 32, a segment register, a word in memory, a doubleword in memory, an immediate byte, an immediate word, or an immediate doubleword.

• It is impossible to push an 8-bit item onto the stack.

• When an immediate byte-size operand is pushed, multiple bytes are pushed on the stack accordingly to the current mode:

  • In 16-bit modes stack items are WORD-size (2 bytes) at address SS:SP.

  • In 32-bit modes the top of the stack is the DWORD addressed by [ ESP ].

• Execution of PUSH AX instruction: