Lecture 8. MIPS Instructions #4 – Branch Instructions #2

Prof. Taeweon SuhComputer Science Education

Korea University

2010 R&E Computer System Education & Research

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Procedure (Function)

• Programmers use a procedure (or function) to structure programs To make the program modular and easy to understand To allow code to be re-used

• Procedures allow the programmer to focus on just one portion of the task at a time

• Parameters (arguments) act as an interface between the procedure and the rest of the program

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Procedure Calls

Definitions• Caller: calling procedure (in this case, main)• Callee: called procedure (in this case, sum)

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High-level code example

void main(){ int y; y = sum(42, 7); ...}

int sum(int a, int b){ return (a + b);}

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MIPS Instructions for Accessing Procedures

• Procedure call instruction:

jal ProcedureAddress #jump and link# $ra <- pc + 4

# pc <- jump target

Saves PC+4 in register $ra to have a link to the next instruction for the procedure return

Instruction format (J format):

4

26-bit address0x03

High-level code

int main() { simple(); a = b + c;}

void simple() { return;}

MIPS assembly code

0x00400200 main: jal simple

0x00400204 add $s0, $s1, $s2

...

0x00401020 simple: jr $ra

void means that simple doesn’t return a value.

compile

PC

PC+4

jal: jumps to simple and saves PC+4 in the return address register ($ra). In this case, $ra = 0x00400204 after jal executes

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MIPS Instructions for Accessing Procedures

• Return instruction:jr $ra #return // pc <- $ra

Instruction format (R format):

5

0 31 0x08

High-level code

int main() { simple(); a = b + c;}

void simple() { return;}

MIPS assembly code

0x00400200 main: jal simple

0x00400204 add $s0, $s1, $s2

...

0x00401020 simple: jr $ra

compile

$ra contains 0x00400204

jr $ra: jumps to address in $ra, in this case 0x00400204.

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Procedure Calls

Procedure calling conventions:• Caller:

passes arguments to callee. jumps to the callee

• Callee: performs the procedure returns the result to caller returns to the point of call Must not overwrite registers or memory needed by the

caller

MIPS conventions:• Call procedure: jump and link (jal) • Return from procedure: jump register (jr)• Argument values: $a0 - $a3• Return values: $v0, $v1

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Input Arguments and Return Values

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High-level code

int main() { int y; ... y = diffofsums(2, 3, 4, 5); // 4 arguments ...}

int diffofsums(int f, int g, int h, int i){ int result; result = (f + g) - (h + i); return result; // return value}

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Input Arguments and Return Values

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MIPS assembly code

# $s0 = y

main: ... addi $a0, $0, 2 # argument 0 = 2 addi $a1, $0, 3 # argument 1 = 3 addi $a2, $0, 4 # argument 2 = 4 addi $a3, $0, 5 # argument 3 = 5 jal diffofsums # call procedure add $s0, $v0, $0 # y = returned value ...

# $s0 = resultdiffofsums: add $t0, $a0, $a1 # $t0 = f + g add $t1, $a2, $a3 # $t1 = h + i sub $s0, $t0, $t1 # result = (f + g) - (h + i) add $v0, $s0, $0 # put return value in $v0 jr $ra # return to caller

High-level code

int main() { int y; ... y = diffofsums(2, 3, 4, 5); // 4 arguments ...}

int diffofsums(int f, int g, int h, int i){ int result; result = (f + g) - (h + i); return result; // return value}

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Input Arguments and Return Values

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MIPS assembly code

# $s0 = resultdiffofsums: add $t0, $a0, $a1 # $t0 = f + g add $t1, $a2, $a3 # $t1 = h + i sub $s0, $t0, $t1 # result = (f + g) - (h + i) add $v0, $s0, $0 # put return value in $v0 jr $ra # return to caller

• We need a place (called stack) to temporarily store arguments and registers because … diffofsums overwrote 3 registers: $t0, $t1, and $s0 , which may

be being used in the main routine What if the callee (diffofsums) needs to use more registers than

allocated to argument ($a0 - $a3) and return values ($v0, $v1)

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The Stack

• Memory used to temporarily save variables

• Like a stack of dishes, stack is a data structure for spilling registers organized as a last-in-first-out (LIFO) queue

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The Stack - Spilling Registers

• Stack is a data structure for spilling registers organized as a last-in-first-out (LIFO) queue

• One of the general registers, $sp ($29), is used to point to the top of the stack The stack “grows” from high

address to low address in MIPS Push: add data onto the stack

• $sp = $sp – 4 • data on stack at new $sp

Pop: remove data from the stack• Data from stack at $sp• $sp = $sp + 4

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low addr

high addr

$sptop of stack

Main Memory

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Stack Pointer (SP)

• Stack pointer ($sp) points to the top of the stack

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Data

7FFFFFFC 12345678

7FFFFFF8

7FFFFFF4

7FFFFFF0

Address

$sp 7FFFFFFC

7FFFFFF8

7FFFFFF4

7FFFFFF0

Address Data

12345678

$sp

AABBCCDD

11223344

Main Memory

Main Memory

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How Procedures use the Stack

• Called procedures (callee) must not have any unintended side effects to the caller But, diffofsums overwrites 3 registers ($t0,

$t1, $s0) as duplicated below:

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# MIPS assembly

# $s0 = resultdiffofsums: add $t0, $a0, $a1 # $t0 = f + g add $t1, $a2, $a3 # $t1 = h + i sub $s0, $t0, $t1 # result = (f + g) - (h + i) add $v0, $s0, $0 # put return value in $v0 jr $ra # return to caller

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Storing Register Values on the Stack

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# $s0 = resultdiffofsums: addi $sp, $sp, -12 # make space on stack # to store 3 registers sw $s0, 8($sp) # save $s0 on stack sw $t0, 4($sp) # save $t0 on stack sw $t1, 0($sp) # save $t1 on stack add $t0, $a0, $a1 # $t0 = f + g add $t1, $a2, $a3 # $t1 = h + i sub $s0, $t0, $t1 # result = (f + g) - (h +

i) add $v0, $s0, $0 # put return value in $v0 lw $t1, 0($sp) # restore $t1 from stack lw $t0, 4($sp) # restore $t0 from stack lw $s0, 8($sp) # restore $s0 from stack addi $sp, $sp, 12 # deallocate stack space jr $ra # return to caller

Data

FC

F8

F4

F0

Address

$sp

(a)

Data

FC

F8

F4

F0

Address

$sp

(b)

$s0

Data

$sp

(c)

$t0

FC

F8

F4

F0

Address

? ??

sta

ck fr

am

e$t1

“Push” (back up) the registers to be used in the callee to the stack

“Pop” (restore) the registers from the stack prior to returning to the caller

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Preserved and NonPreserved Registers

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Preserved(Callee-Saved)

Nonpreserved(Caller-Saved)

$s0 - $s7 $t0 - $t9

$ra $a0 - $a3

$sp $v0 - $v1

stack above $sp

stack below $sp

• In the previous example, if the calling procedure does not use the temporary registers ($t0, $t1), the effort to save and restore them is wasted

• To avoid this waste, MIPS divides registers into preserved and nonpreserved categories• The preserved registers include $s0 ~ $s7 (saved)• The nonpreserved registers include $t0 ~ $t9 (temporary)• So, a procedure must save and restore any of the preserved registers it

wishes to use, but it can change the nonpreserved registers freely• The callee must save and restore any preserved registers it wishes

to use• The callee may change any of the nonpreserved registers

• But, if the caller is holding active data in a nonpreserved register, the caller needs to save and restore it

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Storing Saved Registers on the Stack

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# $s0 = resultdiffofsums: addi $sp, $sp, -4 # make space on stack to

# store one register sw $s0, 0($sp) # save $s0 on stack # no need to save $t0 or $t1 add $t0, $a0, $a1 # $t0 = f + g add $t1, $a2, $a3 # $t1 = h + i sub $s0, $t0, $t1 # result = (f + g) - (h + i) add $v0, $s0, $0 # put return value in $v0 lw $s0, 0($sp) # restore $s0 from stack addi $sp, $sp, 4 # deallocate stack space jr $ra # return to caller

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Nested Procedure Calls

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proc1: addi $sp, $sp, -4 # make space on stack sw $ra, 0($sp) # save $ra on stack jal proc2 ... lw $ra, 0($sp) # restore $s0 from stack addi $sp, $sp, 4 # deallocate stack space jr $ra # return to caller

• Procedures that do not call others are called leaf procedures• Life would be simple if all procedures were leaf procedures, but

they aren’t• For example,

• the main program calls procedure A with an argument of 3 (by placing the value 3 into register $a0 and then using jal A)

• Procedure A calls procedure B via jal B with an argument 7 (also placed in $a0)

• There is a conflict over the use of register $a0 and $ra• Use stack to preserve registers

• The caller pushes any argument registers ($a0~$a4) or temporary registers ($t0~$t9) that are needed after the call

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Recursive Procedure Call

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MIPS assembly code

0x90 factorial: addi $sp, $sp, -8 # make room0x94 sw $a0, 4($sp) # store $a00x98 sw $ra, 0($sp) # store $ra0x9C addi $t0, $0, 2 0xA0 slt $t0, $a0, $t0 # a <= 1 ?0xA4 beq $t0, $0, else # no: go to else 0xA8 addi $v0, $0, 1 # yes: return 10xAC addi $sp, $sp, 8 # restore $sp0xB0 jr $ra # return0xB4 else: addi $a0, $a0, -1 # n = n - 10xB8 jal factorial # recursive call0xBC lw $ra, 0($sp) # restore $ra0xC0 lw $a0, 4($sp) # restore $a00xC4 addi $sp, $sp, 8 # restore $sp0xC8 mul $v0, $a0, $v0 # n * factorial(n-1)0xCC jr $ra # return

High-level code

int factorial(int n) { if (n <= 1) return 1; else return (n * factorial(n-1));}

• Recursive procedures invoke “clones” of themselves

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Stack during Recursive Call

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$sp FC

F8

F4

F0

$ra

EC

E8

E4

E0

DC

FC

F8

F4

F0

EC

E8

E4

E0

DC

FC

F8

F4

F0

EC

E8

E4

E0

DC

$sp

$sp

$sp

$sp

$a0 = 1$v0 = 1 x 1

$a0 = 2$v0 = 2 x 1

$a0 = 3$v0 = 3 x 2

$v0 = 6

$sp

$sp

$sp

$sp

DataAddress DataAddress DataAddress

$a0 (0x3)

$ra (0xBC)

$a0 (0x2)

$ra (0xBC)

$a0 (0x1)

$ra

$a0 (0x3)

$ra (0xBC)

$a0 (0x2)

$ra (0xBC)

$a0 (0x1)