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MIPS Procedures and Functions in CS61C Course - Prof. D. D. Garcia, Study notes of Computer Science

University of California - Berkeley Computer Science

Prof. D. D. Garcia

This document from the cs61c course at uc berkeley covers mips procedures, functions, decision making instructions, and memory allocation, with examples and exercises. It includes information about jal, jr, and register conventions.

Typology: Study notes

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Uploaded on 10/01/2009

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CS61C L8 MIPS Procedures (1) Garcia, Fall 2005 © UCB

Lecturer PSOE, new dad Dan Garcia

www.cs.berkeley.edu/~ddgarcia

inst.eecs.berkeley.edu/~cs61c

CS61C : Machine Structures

Lecture #8 – MIPS Procedures

2005-09-26 There is one handout

today at the front and

back of the room!

No. 11 Cal runs over the

Aggies 41-13 are are now 4-0 for 1st

time since 1996. Justin Forsett ran

for 235 yds, Joe Ayoob rushed for 3

TDs. Next Sat…Arizona @ home!

New Mexico St who? ⇒

calbears.collegesports.com/sports/m-footbl/recaps/092405aaa.html

Discover Study notes of Computer Science University of California - Berkeley

Partial preview of the text

Download MIPS Procedures and Functions in CS61C Course - Prof. D. D. Garcia and more Study notes Computer Science in PDF only on Docsity!

Lecturer PSOE, new dad Dan Garcia www.cs.berkeley.edu/~ddgarcia inst.eecs.berkeley.edu/~cs61c

CS61C : Machine Structures

Lecture #8 – MIPS Procedures

There is one handout today at the front and back of the room! No. 11 Cal runs over the Aggies 41-13 are are now 4-0 for 1 st time since 1996. Justin Forsett ran for 235 yds, Joe Ayoob rushed for 3 TDs. Next Sat…Arizona @ home!

New Mexico St who? ⇒

calbears.collegesports.com/sports/m-footbl/recaps/092405aaa.html

Review

A Decision allows us to decide what to execute at run-time rather than compile-time.
C Decisions are made using conditional statements within if , while , do while , for.
MIPS Decision making instructions are the conditional branches: beq and bne.
In order to help the conditional branches make decisions concerning inequalities, we introduce a single instruction: “Set on Less Than”called slt , slti , sltu , sltiu
Unsigned add/sub don ’ t cause overflow
New MIPS Instructions: beq, bne, j, sll, srl slt, slti, sltu, sltiu addu, addiu, subu

Function Call Bookkeeping

Registers play a major role in

keeping track of information for

function calls.

Register conventions:
- Return address $ra
- Arguments $a0, $a1, $a2, $a
- Return value $v0, $v
- Local variables $s0, $s1, … , $s
The stack is also used; more later.

Instruction Support for Functions (1/6)

... sum(a,b);... /* a,b:$s0,$s1 */

int sum(int x, int y) {

return x+y;

address

C

M

I

P

S

In MIPS, all instructions

are 4 bytes, and stored in

memory just like data. So

here we show the

addresses of where the

programs are stored.

Instruction Support for Functions (3/6)

... sum(a,b);... /* a,b:$s0,$s1 */

int sum(int x, int y) {

return x+y;

2000 sum: add $v0,$a0,$a

2004 jr $ra # new instruction

C

M

I

P

S

Question: Why use jr here? Why not simply use j?
Answer: sum might be called by many functions, so we can ’ t return to a fixed place. The calling proc to sum must be able to say “return here” somehow.

Instruction Support for Functions (4/6)

Single instruction to jump and save return address: jump and link ( jal )
Before: 1008 addi $ra,$zero,1016 #$ra= 1012 j sum #goto sum
After: 1008 jal sum # $ra=1012,goto sum
Why have a jal? Make the common case fast: function calls are very common. Also, you don ’ t have to know where the code is loaded into memory with jal.

Instruction Support for Functions (6/6)

Syntax for jr (jump register): jr register
Instead of providing a label to jump to,

the jr instruction provides a register

which contains an address to jump to.

Only useful if we know exact address to

jump to.

Very useful for function calls:
- jal stores return address in register ($ra )
- jr $ra jumps back to that address

Nested Procedures (1/2)

int sumSquare(int x, int y) {

return mult(x,x)+ y;

Something called sumSquare , now

sumSquare is calling mult.

So there ’ s a value in $ra that

sumSquare wants to jump back to, but

this will be overwritten by the call to

mult.

Need to save sumSquare return

address before call to mult.

C memory Allocation review

Address

Code Program

Static

Variables declared

once per program

Heap

Explicitly created space,

e.g., malloc(); C pointers

Stack

Space for saved

procedure information

$sp stack pointer

Using the Stack (1/2)

So we have a register $sp which

always points to the last used space

in the stack.

To use stack, we decrement this

pointer by the amount of space we

need and then fill it with info.

So, how do we compile this? int sumSquare(int x, int y) { return mult(x,x)+ y; }

Steps for Making a Procedure Call

1) Save necessary values onto stack.

2) Assign argument(s), if any.

3) jal call

4) Restore values from stack.

Rules for Procedures

Called with a jal instruction, returns

with a jr $ra

Accepts up to 4 arguments in $a0 ,

$a1 , $a2 and $a

Return value is always in $v0 (and if

necessary in $v1 )

Must follow register conventions

(even in functions that only you will

call)! So what are they?

We ’ ll see these in a few slides…

Administrivia

We have a midterm&review time & date
- Review: Sun 2005-10-16 @ 2pm in 10 Evans
- Midterm: Mon 2005-10-17, 5:30-8:30pm here!
- DSP or Conflicts? Email Jeremy
Dan ’ s before-class graphics videos: www.siggraph.org/publications/video-review/SVR.html
Project 1 due tonight @ 11:59pm
- An easy HW3 follows, due Friday

What C code properly fills in the blank in loop below?

Peer Instruction

do {i--;} while(__); Loop:addi $s0,$s0,-1 # i = i - 1 slti $t0,$s1,2 # $t0 = (j < 2) beq $t0,$0 ,Loop # goto Loop if $t0 == 0 slt $t0,$s1,$s0 # $t0 = (j < i) bne $t0,$0 ,Loop # goto Loop if $t0 != 0 1: j < 2 && j < i 2: j ≥ 2 && j < i 3: j < 2 && j ≥ i 4: j ≥ 2 && j ≥ i 5: j > 2 && j < i 6: j < 2 || j < i 7: j ≥ 2 || j < i 8: j < 2 || j ≥ i 9: j ≥ 2 || j ≥ i 0: j > 2 || j < i

MIPS Procedures and Functions in CS61C Course - Prof. D. D. Garcia, Study notes of Computer Science

Related documents

Partial preview of the text

Download MIPS Procedures and Functions in CS61C Course - Prof. D. D. Garcia and more Study notes Computer Science in PDF only on Docsity!

CS61C : Machine Structures

Lecture #8 – MIPS Procedures

New Mexico St who? ⇒

Review

Function Call Bookkeeping

keeping track of information for

function calls.

Instruction Support for Functions (1/6)

... sum(a,b);... /* a,b:$s0,$s1 */

int sum(int x, int y) {

return x+y;

address

C

M

I

P

S

In MIPS, all instructions

are 4 bytes, and stored in

memory just like data. So

here we show the

addresses of where the

programs are stored.

Instruction Support for Functions (3/6)

... sum(a,b);... /* a,b:$s0,$s1 */

int sum(int x, int y) {

return x+y;

2000 sum: add $v0,$a0,$a

2004 jr $ra # new instruction

C

M

I

P

S

Instruction Support for Functions (4/6)

Instruction Support for Functions (6/6)

the jr instruction provides a register

which contains an address to jump to.

jump to.

Nested Procedures (1/2)

int sumSquare(int x, int y) {

return mult(x,x)+ y;

sumSquare is calling mult.

sumSquare wants to jump back to, but

this will be overwritten by the call to

mult.

address before call to mult.

C memory Allocation review

Code Program

Static

Variables declared

once per program

Heap

Explicitly created space,

e.g., malloc(); C pointers

Stack

Space for saved

procedure information

Using the Stack (1/2)

always points to the last used space

in the stack.

pointer by the amount of space we

need and then fill it with info.

Steps for Making a Procedure Call

1) Save necessary values onto stack.

2) Assign argument(s), if any.

3) jal call

4) Restore values from stack.

Rules for Procedures

with a jr $ra

$a1 , $a2 and $a

necessary in $v1 )

(even in functions that only you will

call)! So what are they?

Administrivia

Peer Instruction