20/06/2015CSE1303 Part B lecture notes 1 Functions, part 2 Lecture B15 Lecture notes, section B15.

04/18/23 CSE1303 Part B lecture notes 1

Functions, part 2Functions, part 2

Lecture B15Lecture B15

Lecture notes, section B15Lecture notes, section B15


Last timeLast time

Function callingFunction calling jaljal and and jrjr instructions instructions

Calling conventionCalling convention for making a function callfor making a function call

Structure of stackStructure of stack stack framesstack frames


In this lectureIn this lecture

Accessing function parametersAccessing function parameters Returning from functionsReturning from functions RecursionRecursion


Stack framesStack frames

33

44

??????resultresult

expexp

basebase

0x7FFF039C0x7FFF039C

0x7FFF03980x7FFF0398


0x7FFF03A00x7FFF03A0



arg 2 (e)arg 2 (e)

arg 1 (b)arg 1 (b) 33

44




0x0040005C0x0040005C


saved $rasaved $ra

saved $fpsaved $fp$fp$fp = =

0x7FFF0384 0x7FFF0384

$sp$sp = = 0x7FFF0380 0x7FFF0380 resultresult 11

powerpower’s ’s stack framestack frame

mainmain’s ’s stack framestack frame


Local variablesLocal variables

33

44

??????resultresult

expexp

basebase







arg 2 (e)arg 2 (e)

arg 1 (b)arg 1 (b) 33

44




0x0040005C0x0040005C


saved $rasaved $ra


0x7FFF0384 0x7FFF0384


powerpower’s ’s local local

variables variables are are

accessed as accessed as mainmain’s were, ’s were,

with with negative negative

offsets from offsets from $fp$fp

resultresult is at is at -4($fp)-4($fp)


Function parametersFunction parameters

33

44

??????resultresult

expexp

basebase







arg 2 (e)arg 2 (e)

arg 1 (b)arg 1 (b) 33

44




0x0040005C0x0040005C


saved $rasaved $ra


0x7FFF0384 0x7FFF0384


bb and and ee are are respectively respectively accessible at accessible at +8($fp)+8($fp) and and +12($fp)+12($fp)

powerpower’s ’s parameters parameters (arguments) (arguments)

are are accessible accessible

with with positivepositive offset from offset from

$fp$fp


Example: calleeExample: callee/* A function that gets called. *//* A function that gets called. */

int power(int b, int e)int power(int b, int e){{ int result = 1;int result = 1;

/* Keep going while exponent/* Keep going while exponent is positive. */is positive. */ while (e > 0)while (e > 0) {{ /* Multiply by base. *//* Multiply by base. */ result = result * b;result = result * b;

/* less to multiply. *//* less to multiply. */ e--;e--; }}

/* Return the result/* Return the result to the caller. */to the caller. */ return result;return result;}}

# ... Continued from# ... Continued from# last lecture.# last lecture.loop: loop: # Stop if e <= 0.# Stop if e <= 0. lw $t0, 12($fp) lw $t0, 12($fp) # e# e ble $t0, 0, endble $t0, 0, end

# result = result * b# result = result * b lw $t0, -4($fp) lw $t0, -4($fp) # result# result lw $t1, 8($fp) lw $t1, 8($fp) # b# b mul $t0, $t0, $t1mul $t0, $t0, $t1 sw $t0, -4($fp) sw $t0, -4($fp) # result# result

# e--# e-- lw $t0, 12($fp) lw $t0, 12($fp) # e# e addi $t0, $t0, -1addi $t0, $t0, -1 sw $t0, 12($fp) sw $t0, 12($fp) # e# e

# Repeat loop.# Repeat loop. j loopj loop

end:end: # Now ready to return. # Now ready to return.# Continued ...# Continued ...







# ... Continued from# ... Continued from# last lecture.# last lecture.loop: loop: # Stop if e <= 0.# Stop if e <= 0. lw $t0, 12($fp) lw $t0, 12($fp) # e# e ble $t0, 0, endble $t0, 0, end

# result = result * b# result = result * b lw $t0, -4($fp) lw $t0, -4($fp) # result# result lw $t1, 8($fp) lw $t1, 8($fp) # b# b mul $t0, $t0, $t1mul $t0, $t0, $t1 sw $t0, -4($fp) sw $t0, -4($fp) # result# result

# e--# e-- lw $t0, 12($fp) lw $t0, 12($fp) # e# e addi $t0, $t0, -1addi $t0, $t0, -1 sw $t0, 12($fp) sw $t0, 12($fp) # e# e

# Repeat loop.# Repeat loop. j loopj loop

end:end: # Now ready to return. # Now ready to return.# Continued ...# Continued ...


Function returnFunction return

When When returningreturning from a function, from a function, stack must be restoredstack must be restored to its initial to its initial statestate

Achieved by Achieved by undoing stepsundoing steps made made during calling of function, during calling of function, in in reverse orderreverse order


Function return Function return conventionconvention

On On function exitfunction exit, , calleecallee::5.5. chooses chooses return valuereturn value

by setting register by setting register $v0$v0, if necessary, if necessary

6.6. deallocates local deallocates local variablesvariables by popping by popping allocated spaceallocated space

7.7. restores restores $fp$fp by by popping its saved popping its saved value off stackvalue off stack

8.8. restores restores $ra$ra by by popping its saved popping its saved value off stackvalue off stack

9.9. returnsreturns with with jr $rajr $ra

On On returnreturn from from function, function, callercaller::4.4. clears function clears function

argumentsarguments by popping by popping allocated spaceallocated space

5.5. restores saved restores saved temporary registerstemporary registers by by popping their values popping their values off stackoff stack

6.6. usesuses the the return valuereturn value found in $v0, if found in $v0, if necessarynecessary


Example: calleeExample: callee

33

44

??????resultresult

expexp

basebase







arg 2 (e)arg 2 (e)

arg 1 (b)arg 1 (b) 33

00




0x0040005C0x0040005C


saved $rasaved $ra


0x7FFF0384 0x7FFF0384


callee step 5: put callee step 5: put return value in return value in

register register $v0$v0

no effect visible no effect visible on stackon stack

$v0 = 81$v0 = 81



33

44

??????resultresult

expexp

basebase







arg 2 (e)arg 2 (e)

arg 1 (b)arg 1 (b) 33

00




0x0040005C0x0040005C


saved $rasaved $ra


0x7FFF0384 0x7FFF0384


callee step 6: callee step 6: deallocate local deallocate local

variables by variables by popping allocated popping allocated

space off stackspace off stack

one local variable one local variable to deleteto delete



33

44

??????resultresult

expexp

basebase







arg 2 (e)arg 2 (e)

arg 1 (b)arg 1 (b) 33

00




0x0040005C0x0040005C


saved $rasaved $ra

saved $fpsaved $fp$fp$fp == $sp$sp ==


callee step 6: callee step 6: deallocate local deallocate local

variables by variables by popping allocated popping allocated

space off stackspace off stack

one local variable one local variable to deleteto delete

result is now “gone”result is now “gone”



33

44

??????resultresult

expexp

basebase







arg 2 (e)arg 2 (e)

arg 1 (b)arg 1 (b) 33

00




0x0040005C0x0040005C


saved $rasaved $ra

saved $fpsaved $fp$sp$sp = =

0x7FFF0384 0x7FFF0384

callee steps 7 and callee steps 7 and 8: restore saved 8: restore saved values of $fp and values of $fp and $ra by popping off $ra by popping off

stackstack

can do both these can do both these steps at oncesteps at once

$fp$fp = 0x7FFF03A0 = 0x7FFF03A0

$ra$ra = 0x0040005C = 0x0040005C



33

44

??????resultresult

expexp

basebase







arg 2 (e)arg 2 (e)

arg 1 (b)arg 1 (b) 33

00




callee steps 7 and callee steps 7 and 8: restore saved 8: restore saved values of $fp and values of $fp and $ra by popping off $ra by popping off

stackstack

$fp$fp = 0x7FFF03A0 = 0x7FFF03A0

$ra$ra = 0x0040005C = 0x0040005C

$sp$sp = = 0x7FFF038C 0x7FFF038C

$fp$fp = = 0x7FFF03A0 0x7FFF03A0

popping popping $fp$fp makes it point makes it point back to back to mainmain’s stack frame’s stack frame



33

44

??????resultresult

expexp

basebase







arg 2 (e)arg 2 (e)

arg 1 (b)arg 1 (b) 33

00





callee step 9: callee step 9: return by return by

executing executing jr $rajr $ra

nothing visible on nothing visible on stackstack








# ... Continue# ... Continuedd

# Return result in $v0.# Return result in $v0. lw $v0, -4($fp) lw $v0, -4($fp) # result# result

# Remove local var.# Remove local var. addu $sp, $sp, 4addu $sp, $sp, 4

# Restore $fp & $ra# Restore $fp & $ra lw $fp, 0($sp)lw $fp, 0($sp) lw $ra, 4($sp)lw $ra, 4($sp) addu $sp, $sp, 8addu $sp, $sp, 8

# Return to caller.# Return to caller. jr $rajr $ra







# ... Continue# ... Continuedd

# Return result in $v0.# Return result in $v0. lw $v0, -4($fp) lw $v0, -4($fp) # result# result

# Remove local var.# Remove local var. addu $sp, $sp, 4addu $sp, $sp, 4

# Restore $fp & $ra# Restore $fp & $ra lw $fp, 0($sp)lw $fp, 0($sp) lw $ra, 4($sp)lw $ra, 4($sp) addu $sp, $sp, 8addu $sp, $sp, 8

# Return to caller.# Return to caller. jr $rajr $ra


Example: callerExample: caller/* C program which calls a/* C program which calls a function. */function. */

#include <stdio.h>#include <stdio.h>#include <stdlib.h>#include <stdlib.h>

int power(int b, int e);int power(int b, int e);

int main()int main(){{ int base;int base; int exp;int exp; int result;int result;

scanf("%d", &base);scanf("%d", &base); scanf("%d", &exp);scanf("%d", &exp);

result = result = power(base, exp)power(base, exp);;

printf("%d", result);printf("%d", result);

exit(0);exit(0);}}

now back in caller, about to now back in caller, about to assign return value of assign return value of

function to function to mainmain’s local ’s local variable variable resultresult


Example: callerExample: caller

caller step 4: clear caller step 4: clear function function

arguments by arguments by popping them off popping them off

stackstack

bb and and ee are no are no longer needed, longer needed, destroy themdestroy them

33

44

??????resultresult

expexp

basebase









arg 2 (e)arg 2 (e)

arg 1 (b)arg 1 (b) 33

00



bb and and ee are no are no longer needed, longer needed, destroy themdestroy them

33

44

??????resultresult

expexp

basebase








$sp$sp = = 0x7FFF0394 0x7FFF0394

caller step 4: clear caller step 4: clear function function

arguments by arguments by popping them off popping them off

stackstack



caller step 5: caller step 5: restore saved restore saved

temporary temporary registers by registers by

popping their popping their values off stackvalues off stack

we didn’t save we didn’t save any, so nothing to any, so nothing to

do heredo here

33

44

??????resultresult

expexp

basebase








$sp$sp = = 0x7FFF0394 0x7FFF0394



caller step 6: use caller step 6: use return value, return value,

found in register found in register $v0$v0

store returned store returned value into a value into a

variablevariable

33

44

??????resultresult

expexp

basebase








$sp$sp = = 0x7FFF0394 0x7FFF0394

$v0 = 81$v0 = 81

main stores main stores return value return value

into local into local variable variable resultresult



caller step 6: use caller step 6: use return value, return value,

found in register found in register $v0$v0

store returned store returned value into a value into a

variablevariable

33

44

8181resultresult

expexp

basebase








$sp$sp = = 0x7FFF0394 0x7FFF0394

$v0 = 81$v0 = 81

main stores main stores return value return value

into local into local variable variable resultresult



Done!Done!

after the end of after the end of the function call the function call

the stack has the stack has been returned to been returned to its original stateits original state

33

44

8181resultresult

expexp

basebase






$sp$sp = = 0x7FFF0394 0x7FFF0394







result = power(base, exp);result = power(base, exp);


exit(0);exit(0);}}

# ... main function, continued# ... main function, continued # Last lecture, we # Last lecture, we # finished with this ...# finished with this ... ## jal power jal power

# Remove arguments, they# Remove arguments, they # are no longer needed.# are no longer needed. # 2 * 4 = 8 bytes.# 2 * 4 = 8 bytes. addu $sp, $sp, 8addu $sp, $sp, 8

# Store return value# Store return value # in result.# in result. sw $v0, -4($fp) sw $v0, -4($fp) # result# result

# Print result.# Print result. li $v0, 1li $v0, 1 lw $a0, -4($fp) lw $a0, -4($fp) # result# result syscallsyscall

# deallocate main’s locals# deallocate main’s locals addu $sp, $sp, 12addu $sp, $sp, 12 li $v0, 10 li $v0, 10 # exit# exit syscallsyscall







result = power(base, exp);result = power(base, exp);


exit(0);exit(0);}}

# ... main function, continued# ... main function, continued # Last lecture, we # Last lecture, we # finished with this ...# finished with this ... ## jal power jal power

# Remove arguments, they# Remove arguments, they # are no longer needed.# are no longer needed. # 2 * 4 = 8 bytes.# 2 * 4 = 8 bytes. addu $sp, $sp, 8addu $sp, $sp, 8

# Store return value# Store return value # in result.# in result. sw $v0, -4($fp) sw $v0, -4($fp) # result# result

# Print result.# Print result. li $v0, 1li $v0, 1 lw $a0, -4($fp) lw $a0, -4($fp) # result# result syscallsyscall



Function calling Function calling conventionconvention

In summary, In summary, callercaller::1.1. saves temporary registerssaves temporary registers by pushing their by pushing their

values on stackvalues on stack2.2. pushes pushes argumentsarguments on stack on stack3.3. callscalls the function with the function with jaljal instruction instruction ((function runs until it returnsfunction runs until it returns, then:), then:)4.4. clears function argumentsclears function arguments by popping by popping

allocated spaceallocated space5.5. restores saved temporary registersrestores saved temporary registers by by

popping their values off the stackpopping their values off the stack6.6. usesuses the the return valuereturn value found in $v0 found in $v0


Function calling Function calling conventionconvention

In summary, In summary, calleecallee::1.1. saves saves $ra$ra by pushing its value on stack by pushing its value on stack2.2. saves saves $fp$fp by pushing its value on stack by pushing its value on stack3.3. copies copies $sp$sp to to $fp$fp4.4. allocates allocates local variableslocal variables ((body of function goes herebody of function goes here, then:), then:)5.5. chooses chooses return valuereturn value by setting register by setting register $v0$v06.6. deallocates local variablesdeallocates local variables by popping by popping

allocated spaceallocated space7.7. restores restores $fp$fp by popping its saved value by popping its saved value8.8. restores restores $ra$ra by popping its saved value by popping its saved value9.9. returnsreturns with with jr $rajr $ra


RecursionRecursion

Function calling convention works Function calling convention works exactly the exactly the samesame for for recursive recursive functionsfunctions don’t need to do anything specialdon’t need to do anything special

Each invocationEach invocation of the function has of the function has its its own stack frameown stack frame local variables and parameterslocal variables and parameters

• with their current valueswith their current values return addressreturn address

• where to return towhere to return to


Recursion exampleRecursion example/* Main program to call/* Main program to call factorial function. */factorial function. */

#include <stdlib.h>#include <stdlib.h>#include <stdio.h>#include <stdio.h>

int factorial(int p);int factorial(int p);

int main()int main(){{ int n;int n;

scanf("%d", &n);scanf("%d", &n);

printf("%d", factorial(n));printf("%d", factorial(n));

exit(0);exit(0);}}









0x7FFEFFFC0x7FFEFFFC

0x7FFEFFF80x7FFEFFF8



0x7FFEFFEC0x7FFEFFEC

0x7FFEFFE80x7FFEFFE8

nn$fp$fp = 0x7FFF001C = 0x7FFF001C

$sp$sp = 0x7FFF0018 = 0x7FFF0018


Recursion exampleRecursion example/* Main program to call/* Main program to call factorial function. */factorial function. */





printf("%d", printf("%d", factorial(n)factorial(n)););

exit(0);exit(0);}}















nn$fp$fp = 0x7FFF001C = 0x7FFF001C

$sp$sp = 0x7FFF0018 = 0x7FFF0018 33arg 1 (p)arg 1 (p)argument is at argument is at

0($sp)0($sp)


Recursion example: callerRecursion example: caller/* Main program to call/* Main program to call factorial function. */factorial function. */






exit(0);exit(0);}}

.text.textmain:main: # 1 * 4 = 4 bytes local. # 1 * 4 = 4 bytes local. move $fp, $spmove $fp, $sp subu $sp, $sp, 4subu $sp, $sp, 4

li $v0, 5li $v0, 5 syscallsyscall sw $v0, -4($fp) sw $v0, -4($fp) # n# n

# Call factorial.# Call factorial. # 1 * 4 = 4 bytes arg.# 1 * 4 = 4 bytes arg. subu $sp, $sp, 4subu $sp, $sp, 4 lw $t0, -4($fp) lw $t0, -4($fp) # n# n sw $t0, 0($sp) sw $t0, 0($sp) # arg 1# arg 1 jal factorialjal factorial # Clear argument.# Clear argument. addu $sp, $sp, 4addu $sp, $sp, 4 move $a0, $v0move $a0, $v0 # returned # returned li $v0, 1 li $v0, 1 # print int# print int syscallsyscall



Recursion example: callerRecursion example: caller/* Main program to call/* Main program to call factorial function. */factorial function. */






exit(0);exit(0);}}

.text.textmain:main: # 1 * 4 = 4 bytes local. # 1 * 4 = 4 bytes local. move $fp, $spmove $fp, $sp subu $sp, $sp, 4subu $sp, $sp, 4

li $v0, 5li $v0, 5 syscallsyscall sw $v0, -4($fp) sw $v0, -4($fp) # n# n

# Call factorial.# Call factorial. # 1 * 4 = 4 bytes arg.# 1 * 4 = 4 bytes arg. subu $sp, $sp, 4subu $sp, $sp, 4 lw $t0, -4($fp) lw $t0, -4($fp) # n# n sw $t0, 0($sp) sw $t0, 0($sp) # arg 1# arg 1 jal factorialjal factorial # Clear argument.# Clear argument. addu $sp, $sp, 4addu $sp, $sp, 4 move $a0, $v0move $a0, $v0 # returned # returned li $v0, 1 li $v0, 1 # print int# print int syscallsyscall

# deallocate main’s locals# deallocate main’s locals add $sp, $sp, 4add $sp, $sp, 4 li $v0, 10 li $v0, 10 # exit# exit syscallsyscall


Recursion example: calleeRecursion example: callee/* The factorial function. *//* The factorial function. */

int factorial(int p)int factorial(int p){{ int result;int result;

if (p <= 1)if (p <= 1) {{ /* Base case. *//* Base case. */ result = 1;result = 1; }} elseelse {{ /* Recursive case. *//* Recursive case. */ result =result = factorial(p-1) * p;factorial(p-1) * p; }}

return result;return result;}}




......















nn

$fp$fp = 0x7FFF000C = 0x7FFF000C

$sp$sp = 0x7FFF0008 = 0x7FFF0008

33pp

0x004000480x00400048saved $rasaved $ra

0x7FFF001C0x7FFF001Csaved $fpsaved $fp??????resultresult

resultresult is at is at -4($fp)-4($fp)

pp is at is at 8($fp)8($fp)






factorial: factorial: # Function entry.# Function entry. subu $sp, $sp, 8subu $sp, $sp, 8 sw $ra, 4($sp)sw $ra, 4($sp) sw $fp, 0($sp)sw $fp, 0($sp) move $fp, $spmove $fp, $sp # 1 * 4 = 4 bytes local.# 1 * 4 = 4 bytes local. subu $sp, $sp, 4subu $sp, $sp, 4

# if (p <= 1) ...# if (p <= 1) ... lw $t0, 8($fp) # plw $t0, 8($fp) # p bgt $t0, 1, recbgt $t0, 1, rec

# result = 1# result = 1 li $t0, 1li $t0, 1 sw $t0, -4($fp) sw $t0, -4($fp) # result# result j endj end

# Continued ...# Continued ...






factorial: factorial: # Function entry.# Function entry. subu $sp, $sp, 8subu $sp, $sp, 8 sw $ra, 4($sp)sw $ra, 4($sp) sw $fp, 0($sp)sw $fp, 0($sp) move $fp, $spmove $fp, $sp # 1 * 4 = 4 bytes local.# 1 * 4 = 4 bytes local. subu $sp, $sp, 4subu $sp, $sp, 4

# if (p <= 1) ...# if (p <= 1) ... lw $t0, 8($fp) # plw $t0, 8($fp) # p bgt $t0, 1, recbgt $t0, 1, rec

# result = 1# result = 1 li $t0, 1li $t0, 1 sw $t0, -4($fp) sw $t0, -4($fp) # result# result j endj end







# # ... Continued... Continuedrec: rec: # Recursive call.# Recursive call. # 1 * 4 = 4 bytes arg.# 1 * 4 = 4 bytes arg. subu $sp, $sp, 4subu $sp, $sp, 4

# argument 1 = p-1# argument 1 = p-1 lw $t0, 8($fp) lw $t0, 8($fp) # p# p sub $t0, $t0, 1 sub $t0, $t0, 1 # p-1# p-1 sw $t0, 0($sp) sw $t0, 0($sp) # arg 1# arg 1

jal factorialjal factorial

# Clean up argument.# Clean up argument. addu $sp, $sp, 4addu $sp, $sp, 4

# Multiply by p.# Multiply by p. lw $t0, 8($fp) lw $t0, 8($fp) # p# p mul $t0, $v0, $t0mul $t0, $v0, $t0 # Store result.# Store result. sw $t0, -4($fp) sw $t0, -4($fp) # result# result







# # ... Continued... Continuedrec: rec: # Recursive call.# Recursive call. # 1 * 4 = 4 bytes arg.# 1 * 4 = 4 bytes arg. subu $sp, $sp, 4subu $sp, $sp, 4

# argument 1 = p-1# argument 1 = p-1 lw $t0, 8($fp) lw $t0, 8($fp) # p# p sub $t0, $t0, 1 sub $t0, $t0, 1 # p-1# p-1 sw $t0, 0($sp) sw $t0, 0($sp) # arg 1# arg 1

jal factorialjal factorial

# Clean up argument.# Clean up argument. addu $sp, $sp, 4addu $sp, $sp, 4

# Multiply by p.# Multiply by p. lw $t0, 8($fp) lw $t0, 8($fp) # p# p mul $t0, $v0, $t0mul $t0, $v0, $t0 # Store result.# Store result. sw $t0, -4($fp) sw $t0, -4($fp) # result# result







# # ... Continued again... Continued again

end: # return resultend: # return result lw $v0, -4($fp) # resultlw $v0, -4($fp) # result

# Destroy local variable# Destroy local variable addu $sp, $sp, 4addu $sp, $sp, 4

# Function exit.# Function exit. lw $fp, 0($sp)lw $fp, 0($sp) lw $ra, 4($lw $ra, 4($ssp)p) addu $sp, $sp, 8addu $sp, $sp, 8 jr $rajr $ra






# # ... Continued again... Continued again

end: # return resultend: # return result lw $v0, -4($fp) # resultlw $v0, -4($fp) # result

# Destroy local variable# Destroy local variable addu $sp, $sp, 4addu $sp, $sp, 4

# Function exit.# Function exit. lw $fp, 0($sp)lw $fp, 0($sp) lw $ra, 4($lw $ra, 4($ssp)p) addu $sp, $sp, 8addu $sp, $sp, 8 jr $rajr $ra


RecursionRecursion
















$sp$sp = 0x7FFF0018 = 0x7FFF0018 stack frame stack frame of of mainmain

during during mainmain, about , about to call to call

factorial(3)factorial(3)

33nn


RecursionRecursion

33















arg 1 (p)arg 1 (p)


$sp$sp = 0x7FFF0014 = 0x7FFF0014

stack frame stack frame of of mainmain

passing argument passing argument to to factorial(3)factorial(3), ,

just before just before jaljal

33nn


RecursionRecursion

33















arg 1 (p)arg 1 (p)


stack stack frame of frame of mainmain

after entry to after entry to factorial(3)factorial(3)

33nn


0x7FFF001C0x7FFF001Csaved $fpsaved $fp??????resultresult$sp$sp = 0x7FFF0008 = 0x7FFF0008

stack frame of stack frame of factorial(3)factorial(3)

pp > 1, so must > 1, so must recurserecurse


RecursionRecursion

33















arg 1 (p)arg 1 (p)



about to call about to call factorial(2)factorial(2)

33nn



$sp$sp = 0x7FFF0004 = 0x7FFF0004


22arg 1 (p)arg 1 (p)


RecursionRecursion

33















arg 1 (p)arg 1 (p)

$fp$fp = 0x7FFEFFFC = 0x7FFEFFFC



33nn



$sp$sp = 0x7FFEFFF8 = 0x7FFEFFF8



0x004000B40x004000B4saved $rasaved $ra

0x7FFF000C0x7FFF000Csaved $fpsaved $fp

??????resultresult


pp > 1, so > 1, so must must

recurserecurse


RecursionRecursion

33















arg 1 (p)arg 1 (p)



about to call about to call factorial(1)factorial(1)

33nn








??????resultresult




RecursionRecursion

33















arg 1 (p)arg 1 (p)

$fp$fp = 0x7FFEFFEC = 0x7FFEFFEC



33nn



$sp$sp = 0x7FFEFFE8 = 0x7FFEFFE8





??????resultresult




0x7FFEFFFC0x7FFEFFFCsaved $fpsaved $fp??????resultresult


pp <= 1, so <= 1, so resultresult = 1 = 1


RecursionRecursion

33















arg 1 (p)arg 1 (p)

$fp$fp = 0x7FFEFFEC = 0x7FFEFFEC


about to about to return from return from


33nn



$sp$sp = 0x7FFEFFE8 = 0x7FFEFFE8





??????resultresult




0x7FFEFFFC0x7FFEFFFCsaved $fpsaved $fp11resultresult


return return result result (1) (1)

in in $v0$v0

$v0$v0 = 1 = 1


RecursionRecursion

33















arg 1 (p)arg 1 (p)



returned back returned back to to


33nn







22resultresult


resultresult = = return value return value (1) × (1) × pp (2) (2)

$v0$v0 = 1 = 1



RecursionRecursion

33















arg 1 (p)arg 1 (p)



about to return about to return from from


33nn








22resultresult


$v0$v0 = 2 = 2

return return result result (2) (2)

in in $v0$v0


RecursionRecursion

33















arg 1 (p)arg 1 (p)

$fp$fp = 0x7FF = 0x7FFF000CF000C


returned back returned back to to


33nn


0x7FFF001C0x7FFF001Csaved $fpsaved $fp66resultresult


$v0$v0 = 2 = 2

resultresult = = return value return value (2) × (2) × pp (3) (3)

$sp$sp = 0x7FF = 0x7FFF0008F0008


RecursionRecursion

33















arg 1 (p)arg 1 (p)



about to return about to return fromfrom

factorial(factorial(33))

33nn


0x7FFF001C0x7FFF001Csaved $fpsaved $fp66resultresult$sp$sp = 0x7FF = 0x7FFF0008F0008


$v0$v0 = = 66

return return resultresult in in

$v0$v0


RecursionRecursion
















33nn

$v0$v0 = = 66

returned back to returned back to mainmain


$sp$sp = 0x7FF = 0x7FFF0018F0018

print out return print out return value in value in $v0$v0 (6) (6)


RecursionRecursion
















33nn$sp$sp = 0x7FF = 0x7FFF0018F0018

$v0$v0 = = 66

print out return print out return value in value in $v0$v0 (6) (6)



Covered in this lectureCovered in this lecture

Accessing function parametersAccessing function parameters Returning from functionsReturning from functions RecursionRecursion


Going furtherGoing further

Official MIPS stack frame Official MIPS stack frame conventionconvention doesn’t use doesn’t use $fp$fp at all! at all! slightly more efficient than CSE1303 slightly more efficient than CSE1303

conventionconvention can be generated by compilerscan be generated by compilers hard for humans to write/understandhard for humans to write/understand


Next timeNext time

Analysis of translationAnalysis of translation efficiencyefficiency

How to write better C codeHow to write better C code

Reading:Reading:

Lecture notes section B16Lecture notes section B16

20/06/2015CSE1303 Part B lecture notes 1 Functions, part 2 Lecture B15 Lecture notes, section B15.

Documents

b result

return result return

int result

fp result

b lecture notes

e 20062015cse1303

int powerint b

0x7fff0380 result1 b