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Subroutine Call and Return

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• Stack and frame pointers: ESP and EBP
  • The 32 bit register ESP is the stack pointer

  • The 32 bit register EBP is typically used as the frame pointer

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• Push: passing parameters on the stack
  • The push instruction pushes a 2-bytes, 4-bytes or 8-bytes value on the stack.

  • Example:

    push ax ;; pushes 2 bytes push eax ;; pushes 4 bytes

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• Pop: passing values off the stack
  • The pop instruction pops a 2-bytes, 4-bytes or 8-bytes value on the stack.

  • Example:

    pop ax ;; pops 2 bytes pop eax ;; pops 4 bytes

  • If you just want to discard the values, you can just increase the stack pointer

    Example:

    add esp, 2 ;; pops and discard 2 bytes add esp, 4 ;; pops and discard 4 bytes

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• Subroutine call: call
  • The call instruction is used for subroutine call:

    call Label

  • It pushes the address of the next instruction on the stack and then jumps to Label (Just like M68000)

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• Subroutine return: ret
  • The ret instruction is to return from a subroutine call:

    ret

  • It pops the address value at the top of the stack into EIP (Just like M68000)

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• Example Programs

  • Factorial:                                                
    • Prog file: click here

  • Tower of Hanoi:                                                
    • Prog file: click here