Today's Topics

• The Stack
• Stack Implementation in Memory
• Preserving Registers with Push & Pop
• Direct Stack Access
• Procedures
• Recursion

The Stack

• Description
  • A temporary storage area
  • One point of Access (top)

  • Last-In-First-Out (LIFO)

  • Example

    

• Operations
  • PUSH - Place element on top of stack

  • POP - Remove element from top of stack

• Purpose
  • Temporary Register Storage
    • Push and pop registers to preserve their value
    • Example:

      PUSH AX ; Place AX on the stack PUSH BX ; Place BX on the stack ... modify contents of Registers AX & BX ... POP BX ; Restore original ; value of BX POP AX ; Restore original ; value of AX

    • Basis of the HP RPN calculator

  • Procedures
    • Stores Return Address of a procedure

    • Example:

      PrintRec: ... Print value of a record ... Ret main: ... Calculate Scores ... CALL PrintRec Continue Execution HERE ... CALL DOSXIT

    • Implicitly, The CALL statement stored the value of the Instruction Pointer (IP) on the stack.

    • Therefore, the computer knowns how to return to exactly the next opcode in your program!

Stack Implementation in Memory

• An area in memory

• Accessed by PUSH and POP opcodes

• Never moves data

  

• SS : Stack Segment

• SP always points to the top of the stack
  • SP initially points to top of the stack (high memory address).

  • SP Decreases as data is PUSHed
    • PUSH AX == SUB SP,2 ; MOV [SS:SP],AX

  • SP Increases as data is POPed
    • POP AX == MOV AX,[SS:SP] ; ADD SP,2

• BP can point to any element on the stack
  • Recall that BP also uses SS

In-class Example

• Given
  • SS=5000
  • SP=0100
  • Therefore: SS:SP=50100 (Segment+Offset)

• Code

  MOV AX,1 MOV BX,2 PUSH AX PUSH BX MOV AX,3 MOV BX,4 POP BX POP AX

• Analyze
  • Memory Operations
  • Final Values of AX,BX

Preserving Registers with PUSH & POP

• Procedures should save and restore registers that are modified in a subroutine.

  PrintRec: PUSH AX PUSH BX PUSH CX PUSH DX PUSH SI ; Code modifies { AX,BX,CX,DX,SI } POP SI POP DX POP CX POP BX POP AX RET

• LIB291 Routine to save ALL registers
  • RSAVE: Save ALL registers
    
  • RREST: Restore ALL registers

Direct Stack Access

• SP modified by PUSH/POP/CALL/RET

• BP (base pointer) modified by programmer

• Example

  PUSH AX PUSH BX PUSH CX MOV AX, [data] MOV BX, [data] MOV CX, [data]

• Fetch original value of AX

  MOV BP,SP ; Load BP with the current SP ADD BP,4 ; Up the the stack by two words MOV AX,[BP] ; Load AX with original value

• Utility
  • Handy way to pass values into a subroutine
    Before, you were limited to only being able to transfer a few arguments via the registers.

  • This mechanism is used to call ASM procedures from C.

Procedures

• Calling NEAR Subroutine
  • PUSH IP

  • JUMP My_Subroutine

• Calling FAR My_Subroutine
  • PUSH CS
    PUSH IP

  • JUMP Segment My_subroutine : My_Subroutine

• RET
  • POP IP (near routine)

  • POP IP
    POP CS (far routine)

Recursion

• Recurse: Procedures calls itself

• Base case: Termination

• Example

  Factorial: ; Input AX = CX = Value ; Output AX = Value ! DEC CX CMP CX,0 ; Test for JE FactDone ; base case IMUL CX Call Factorial ; Recurse FactDone: RET