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MIPS R3000 Subroutine Calls and Stack

MIPS R3000 Subroutine Calls and Stack. CS 286 – Project #2. Department of Computer Science Southern Illinois University Edwardsville Fall, 2019 Dr. Hiroshi Fujinoki E-mail: hfujino@siue.edu. Subcall/001. Subroutine Calls in MIPS R3000. CS 286 – Project #2.

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MIPS R3000 Subroutine Calls and Stack

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  1. MIPS R3000 Subroutine Calls and Stack CS 286 – Project #2 Department of Computer Science Southern Illinois University Edwardsville Fall, 2019 Dr. Hiroshi Fujinoki E-mail: hfujino@siue.edu Subcall/001

  2. Subroutine Calls in MIPS R3000 CS 286 – Project #2 • This PPT presentations describe how to write an assembly program for MIPS R3000 processor that uses subroutine calls • The concept of “Stack Frame” in MIPS R3000 processor • Implementation of recursive subroutine calls in MIPS R3000 assembly program Subcall/002

  3. Subroutine Calls in MIPS R3000 main: the first address in your memory Low Address High Address the last address in your memory CS 286 – Project #2 • In this presentation, it is assumed that the low address appears at the top • Each program executes from the top to the bottom jr $31 Subcall/003

  4. Subroutine Calls in MIPS R3000 main:    “jal” instruction Jump to “xyz” “jr $ra” instruction xyz: Go back to the instruction right after the “jar” instruction CS 286 – Project #2 • In MIPS R3000 assembly language: jal xyz (a) Calling a subroutine (b) Returning to a calling routine jr $31 jr $ra Subcall/004

  5. Subroutine Calls in MIPS R3000 main:    jal xyz xyz: jr $31 jr $ra CS 286 – Project #2 • “jr $ra” instruction makes sure that the program execution flow goes back to “the right return point” jal xyz    Subcall/005

  6. Subroutine Calls in MIPS R3000 li $s0, 10 Destroying register contents during a subroutine call LOOP: S0 register as a loop counter    add $s0, $s0, 5 main:    slt $s0, $t3, $zero Processor xyz: jal xyz    We have one processor jal xyz    jr $ra jr $31 sub $s0, $s0, 1 jne $s0, $zero, LOOP CS 286 – Project #2 •  a danger in using subroutine calls in MIPS assembly programming li $s0, 200 Subcall/006

  7. Subroutine Calls in MIPS R3000 main: “recursive subroutine call” li $s0, 10 li $s0, 10 li $s0, 10 jal xyz    add $s0, ... add $s0, ... add $s0, ... jr $ra jr $ra jr $ra xyz: xyz: xyz: Why are recursive subroutine calls problematic in using processor registers? It’s guaranteed that the same registers will be used in every recursive call ... CS 286 – Project #2 jal xyz    jal xyz jal xyz jr $31 What should we do this to solve this problem? Subcall/007

  8. Concept of stack (1) li $s0, 10 xyz: LOOP:    add $s0, $s0, 5    slt $s0, $t3, $zero • restore all registers I modified • release my stack space jr $ra sub $s0, $s0, 1 Stack Area jne $s0, $zero, LOOP $s0 CS 286 – Project #2 main: jal xyz • reserve my stack space • save all registers I modify    li $s0, 200 jal xyz    jr $31 xyz: Subcall/008

  9. Concept of stack (2) main: “recursive subroutine call” save $s0 save $s0 save $s0 li $s0, 10 li $s0, 10 li $s0, 10 jal xyz jal xyz jal xyz add $s0, ... add $s0, ... add $s0, ...    xyz: xyz: xyz: restore $s0 restore $s0 restore $s0 jr $ra jr $ra jr $ra Stack Each called instance of a subroutine reserves its stack space and saves the register it modifies $s0 $s0 $s0 $s0 CS 286 – Project #2 jal xyz    jal xyz    jr $31 Subcall/009

  10. Subroutine Calls in MIPS R3000  “jr $ra” jumps to the address in $ra register $ra $ra Resume to “main” main: xyz: xyz: jal xyz (next instruction) jal xyz    next inst. xyz: jr $ra jr $ra jr $ra Stack CS 286 – Project #2 jal xyz jal xyz jr $31  Load the address of the next instruction to “$ra” register  Jump (set PC register) to the first instruction in the called subroutine Subcall/010

  11. Subroutine Calls in MIPS R3000 $ra $ra $ra main: xyz: xyz: jal xyz (next instruction) next inst. next inst. xyz: jr $ra jr $ra Stack $s0 $s0 $s0 $ra $ra $ra CS 286 – Project #2 This structure is a necessary condition for a recursive structure save $s0 save $s0 save $s0 save $ra save $ra save $ra jal xyz jal xyz restore $ra restore $ra restore $ra restore $s0 restore $s0 restore $s0 jr $31 jr $ra Subcall/011

  12. Subroutine Calls in MIPS R3000 main: xyz: xyz: jal xyz jal xyz    xyz: jr $ra jr $ra jr $ra Stack Stack Frame for “main” Stack Frame for “xyz(1)” Stack Frame for “xyz(2)” Stack Frame for “xyz(3)” CS 286 – Project #2 jal xyz jal xyz jr $31 Subcall/012

  13. Example (3) Set up the stack frame and save the registers CS 286 – Project #2 xyz: # create stack frame subu $sp, $sp, 32 # save $raregister sw $ra, 0($sp) # save other GPR’s sw $s0, 4($sp) sw $t0, 8($sp)        Subcall/013

  14. Example (4) Restore the registers and release the stack frame CS 286 – Project #2 xyz:    # restore registers lw $s0, 4($sp) lw $t0, 8($sp) # restore $ra for the caller lw $ra, 0($sp) # restore the caller's stack pointer addu $sp, $sp, 32 jr $ra Subcall/014

  15. Subroutine Calls in MIPS R3000 Low Address +28 -32 $fp $sp $fp $sp Stack Frame 8 registers High Address 4 bytes CS 286 – Project #2 $sp = $sp-32 $fp = $sp+28 Subcall/015

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