Computer Systems Lecture 12 Subroutines CS 230 - Spring 2020 2-1 - PowerPoint PPT Presentation

CS 230 – Introduction to Computers and Computer Systems Lecture 12 – Subroutines CS 230 - Spring 2020 2-1

Subroutines  Also called functions, methods, or procedures  all the same thing in assembly language  examples from C: sqrt() or printf()  Important technique to modularize programs  separation of concerns, smaller coding units  Challenges  call/return – how to redirect execution?  how do I get back to where I came from?  arguments (parameters) and result(s) passing CS 230 - Spring 2020 2-2

Calling jal x  J ump A nd L ink  copy (current PC + 4) into register $31  set PC to x (where x is a label)  labels mark the start of functions same type of labels we use for beq and bne   x can also be an immediate (don’t do this) CS 230 - Spring 2020 2-3

Indirect Calling jalr $q  J ump A nd L ink R egister  copy (current PC + 4) into register $31  set PC to contents of register $q  load address of label into $q with lis  value in $q is called a function pointer  allows functions to be passed as parameters CS 230 - Spring 2020 2-4

Returning jr $s  J ump R egister  set PC to $s  usually jr $31  register $31 holds return address  register $31 begins with the outer return address  jr $31 to this address ends the program CS 230 - Spring 2020 2-5

Calling and Returning Example function addTwoNumbers: add $2, $4, $5 jr $31 Call that function jal addTwoNumbers Call that function indirectly lis $1 .word addTwoNumbers jalr $1 Need more than this (wait for slide 9) CS 230 - Spring 2020 2-6

Execution Context  Subroutines can call subroutines  what happens to outer subroutine’s parameters?  what happens to old value of register $31?  How about registers that are in use?  who gets to use which registers?  Save/restore current execution context when a function is called  set of registers following the register conventions  save to the stack CS 230 - Spring 2020 2-7

Register Conventions  $2, $3 – function return values  $4 - $7 – function arguments (parameters)  $1, $8 - $15, $24, $25 – unsaved temporary  $16 - $23 – saved temporary  $26 - $29 – reserved for operating system  $30 – stack pointer  $31 – return address (always save on stack) CS 230 - Spring 2020 2-8

Calling Expanded Calling a function addi $30, $30, -4 sw $31, 0($30) jal addTwoNumbers lw $31, 0($30) addi $30, $30, 4 Calling a function indirectly lis $1 .word addTwoNumbers addi $30, $30, -4 sw $31, 0($30) jalr $1 lw $31, 0($30) addi $30, $30, 4 Might want to store other registers on stack too CS 230 - Spring 2020 2-9

Arguments and Return  Arguments are in registers $4-$7  in order as needed  example: addTwoNumbers would use $4 and $5  Return values (results) in registers $2 and $3  used in order as needed  functions may have zero, one, or two results  example: addTwoNumbers would place sum in $2 CS 230 - Spring 2020 2-10

Unsaved Temporaries  $1, $8 - $15, $24, $25  “if I call a function, I must assume it overwrites these registers”  “I am free to modify these registers at any time”  “if I call a function but I want to preserve a value from one of these registers, I must store that value on the stack”  “or in a saved temporary”  $2, $3, $4-$7 (function results and arguments) can be treated as unsaved temporaries with extra functionality CS 230 - Spring 2020 2-11

Saved Temporaries  $16 - $23  “if I call a function I can assume it will not modify these registers”  “if I modify one of these registers, I must save the old value on the stack first and then restore that value after I am done using the register” CS 230 - Spring 2020 2-12

Preserving the Stack  A function must preserve the value of the stack pointer  Always add back any value you subtracted from the stack pointer before ending a function  The stack pointer should have the same value at the begging and end of a function CS 230 - Spring 2020 2-13

Example addi $4, $0, 15 addi $5, $0, 13 addi $30, $30, -4 sw $31, 0($30) jal addTwoNumbers lw $31, 0($30) addi $30, $30, 4 jr $31 addTwoNumbers: add $2, $4, $5 jr $31 What’s the value in register $2 at the end of the program? CS 230 - Spring 2020 2-14

Example addi $4, $0, 15 ? 0x64 addi $5, $0, 13 0x68 ? 0x6C ? addi $30, $30, -4 0x70 ? sw $31, 0($30) 0x74 ? jal addTwoNumbers 0x78 ? lw $31, 0($30) 0x7C ? addi $30, $30, 4 0x80 ? SP = jr $31 0x84 ? 0x88 ? addTwoNumbers: add $2, $4, $5 PC = 0x1C 10 jr $31 $0 $1 $2 $3 $4 $5 ... $30 $31 What’s the value in register $2 at the ? … 0x80 0 ? ? ? ? end of the program? CS 230 - Spring 2020 2-15

Example addi $4, $0, 15 ? 0x64 addi $5, $0, 13 0x68 ? 0x6C ? addi $30, $30, -4 0x70 ? sw $31, 0($30) 0x74 ? jal addTwoNumbers 0x78 ? lw $31, 0($30) 0x7C ? addi $30, $30, 4 0x80 ? SP = jr $31 0x84 ? 0x88 ? addTwoNumbers: add $2, $4, $5 PC = 0x20 10 jr $31 $0 $1 $2 $3 $4 $5 ... $30 $31 What’s the value in register $2 at the ? 15 ? … 0x80 0 ? ? end of the program? CS 230 - Spring 2020 2-16

Example addi $4, $0, 15 ? 0x64 addi $5, $0, 13 0x68 ? 0x6C ? addi $30, $30, -4 0x70 ? sw $31, 0($30) 0x74 ? jal addTwoNumbers 0x78 ? lw $31, 0($30) 0x7C ? addi $30, $30, 4 0x80 ? SP = jr $31 0x84 ? 0x88 ? addTwoNumbers: add $2, $4, $5 PC = 0x24 10 jr $31 $0 $1 $2 $3 $4 $5 ... $30 $31 What’s the value in register $2 at the ? 15 13 … 0x80 0 ? ? end of the program? CS 230 - Spring 2020 2-17

Example addi $4, $0, 15 ? 0x64 addi $5, $0, 13 0x68 ? 0x6C ? addi $30, $30, -4 0x70 ? sw $31, 0($30) 0x74 ? jal addTwoNumbers 0x78 ? lw $31, 0($30) 0x7C ? SP = addi $30, $30, 4 0x80 ? jr $31 0x84 ? 0x88 ? addTwoNumbers: add $2, $4, $5 PC = 0x28 10 jr $31 $0 $1 $2 $3 $4 $5 ... $30 $31 What’s the value in register $2 at the ? 15 13 … 0x7C 0 ? ? end of the program? CS 230 - Spring 2020 2-18

Example addi $4, $0, 15 ? 0x64 addi $5, $0, 13 0x68 ? 0x6C ? addi $30, $30, -4 0x70 ? sw $31, 0($30) 0x74 ? jal addTwoNumbers 0x78 ? lw $31, 0($30) 0x7C SP = addi $30, $30, 4 0x80 ? jr $31 0x84 ? 0x88 ? addTwoNumbers: add $2, $4, $5 PC = 0x2C 10 jr $31 $0 $1 $2 $3 $4 $5 ... $30 $31 What’s the value in register $2 at the ? 15 13 … 0x7C 0 ? ? end of the program? CS 230 - Spring 2020 2-19

Example addi $4, $0, 15 ? 0x64 addi $5, $0, 13 0x68 ? 0x6C ? addi $30, $30, -4 0x70 ? sw $31, 0($30) 0x74 ? jal addTwoNumbers 0x78 ? lw $31, 0($30) 0x7C SP = addi $30, $30, 4 0x80 ? jr $31 0x84 ? 0x88 ? addTwoNumbers: add $2, $4, $5 PC = 0x3C 10 jr $31 $0 $1 $2 $3 $4 $5 ... $30 $31 What’s the value in register $2 at the ? 15 13 … 0x7C 0x30 0 ? ? end of the program? CS 230 - Spring 2020 2-20

Example addi $4, $0, 15 ? 0x64 addi $5, $0, 13 0x68 ? 0x6C ? addi $30, $30, -4 0x70 ? sw $31, 0($30) 0x74 ? jal addTwoNumbers 0x78 ? lw $31, 0($30) 0x7C SP = addi $30, $30, 4 0x80 ? jr $31 0x84 ? 0x88 ? addTwoNumbers: add $2, $4, $5 PC = 0x40 10 jr $31 $0 $1 $2 $3 $4 $5 ... $30 $31 What’s the value in register $2 at the ? 28 ? 15 13 … 0x7C 0x30 0 end of the program? CS 230 - Spring 2020 2-21

Example addi $4, $0, 15 ? 0x64 addi $5, $0, 13 0x68 ? 0x6C ? addi $30, $30, -4 0x70 ? sw $31, 0($30) 0x74 ? jal addTwoNumbers 0x78 ? lw $31, 0($30) 0x7C SP = addi $30, $30, 4 0x80 ? jr $31 0x84 ? 0x88 ? addTwoNumbers: add $2, $4, $5 PC = 0x30 10 jr $31 $0 $1 $2 $3 $4 $5 ... $30 $31 What’s the value in register $2 at the ? 28 ? 15 13 … 0x7C 0x30 0 end of the program? CS 230 - Spring 2020 2-22

Example addi $4, $0, 15 ? 0x64 addi $5, $0, 13 0x68 ? 0x6C ? addi $30, $30, -4 0x70 ? sw $31, 0($30) 0x74 ? jal addTwoNumbers 0x78 ? lw $31, 0($30) 0x7C SP = addi $30, $30, 4 0x80 ? jr $31 0x84 ? 0x88 ? addTwoNumbers: add $2, $4, $5 PC = 0x34 10 jr $31 $0 $1 $2 $3 $4 $5 ... $30 $31 What’s the value in register $2 at the ? 28 ? 15 13 … 0x7C 0 end of the program? CS 230 - Spring 2020 2-23

Example addi $4, $0, 15 ? 0x64 addi $5, $0, 13 0x68 ? 0x6C ? addi $30, $30, -4 0x70 ? sw $31, 0($30) 0x74 ? jal addTwoNumbers 0x78 ? lw $31, 0($30) 0x7C addi $30, $30, 4 0x80 ? SP = jr $31 0x84 ? 0x88 ? addTwoNumbers: add $2, $4, $5 PC = 0x38 10 jr $31 $0 $1 $2 $3 $4 $5 ... $30 $31 What’s the value in register $2 at the ? 28 ? 15 13 … 0x80 0 end of the program? CS 230 - Spring 2020 2-24

Example addi $4, $0, 15 ? 0x64 addi $5, $0, 13 0x68 ? 0x6C ? addi $30, $30, -4 0x70 ? sw $31, 0($30) 0x74 ? jal addTwoNumbers 0x78 ? lw $31, 0($30) 0x7C addi $30, $30, 4 0x80 ? SP = jr $31 0x84 ? 0x88 ? addTwoNumbers: add $2, $4, $5 PC = 0x3C 10 jr $31 $0 $1 $2 $3 $4 $5 ... $30 $31 What’s the value in register $2 at the ? 28 ? 15 13 … 0x80 0 end of the program? $2 = 28 10 CS 230 - Spring 2020 2-25

Example Assume there exists a subroutine fact which takes one parameter x returns x! ( x factorial). Write a MIPS assembly language program that uses that subroutine and adds the factorials of the value in register $1 and the value in register $2 together and places the result in register $2. Follow the register conventions. CS 230 - Spring 2020 2-26