MIPS Function Continued

Character and String Operations

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Characters are encoded as 0’s and 1’s using ASCII most commonly American Standard Code for Information

Interchange Each character is represented using 8 bits (or

a byte) MIPS provides instructions to move bytes

Load byte (lb) loads a byte to the rightmost 8 bits of a register

Store byte (sb) write the rightmost 8 bits of a register to memory

SPIM syscalls

li $v0,1 # print an integer in $a0li $a0,100syscall

li $v0,5 # read an integer into $v0syscall

li $v0,4 # print an ASCIIZ string at $a0la $a0,msg_hellosyscall

li $v0,10 #exitsyscall

String Copy Procedure

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.datamsg_hello:

.asciiz "Hello! This is CDA3100!\n"

msg_empty: .space 400

.text

.globl mainmain:

li $v0,4la $a0,msg_hellosyscall

li $v0,4la $a0,msg_emptysyscall

la $a0,msg_empty #dstla $a1,msg_hello #srcjal strcpy

li $v0,4la $a0,msg_emptysyscall

li $v0,10 #exitsyscall

strcpy:ori $t0, $a0, 0ori $t1, $a1, 0

strcpyloop:lb $t3, 0($t1)sb $t3, 0($t0)beq $t3, $0, strcpydoneaddi $t0, $t0, 1addi $t1, $t1, 1j strcpyloop

strcpydone:jr $ra

Stack

Key things to keep in mind: Stack is a software concept – last in first out,

that’s it. In MIPS, you implement the stack by yourself by

keeping $sp always pointing to the top element on the stack

Stack can be used in functions to save register values, and is the standard approach to save register values. But You can also use stack for other purposes This is not the only way to save register values.

.datamsg: .asciiz "hello world"endl: .asciiz "\n"

.text .globl mainmain:

addi $sp,$sp,-1

sb $0,($sp)

la $t1, msg

L0:lb $t0,($t1)beq $t0,$0, L1sub $sp,$sp,1sb $t0,($sp)add $t1,1j L0

L1:la $t1,msg

L2:lb $t0,($sp)add $sp,$sp,1sb $t0,($t1)beq $t0, $0, L3

add $t1,1j L2

L3:la $a0,msgli $v0,4syscall

la $a0,endlli $v0,4syscall

li $v0,10 #exitsyscall

Implementing a Recursive Function Suppose we want to implement this in MIPS:

It is a recursive function – a function that calls itself.

It will keep on calling itself, with different parameters, until a terminating condition is met.

The Recursive Function

• What happens if we call fact(4)?– First time call fact, compare 4 with 1, no less

than 1, call fact again – fact(3).– Second time call fact, compare 3 with 1, no

less than 1, call fact again – fact(2).– Third time call fact, compare 2 with 1, no less

than 1, call fact again – fact(1).– Fourth time call fact, compare 1 with 1, no less

than 1, call fact again – fact(0).– Fifth time call fact, compare 1 with 1, less than

1, return 1.– Return to the time when fact(0) was called

(when calling fact(1)). Multiply 1 with 1, return 1.

– Return to the time when fact(1) was called (when calling fact(2)). Multiply 2 with 1, return 2.

– Return to the time when fact(2) was called (when calling fact(3)). Multiply 3 with 2, return 6.

– Return to the time when fact(3) was called (when calling fact(4)). Multiply 4 with 6, return 24.

The Recursive Function In MIPS, we say calling a function as going to

the function. So we go to the function over and over again, until the terminating condition is met.

Here, the function is called “fact,” so we will have a line of code inside the fact function:jal fact

The Recursive Function The parameter should be passed in $a0. In the

C function, every time we call fact, we call with n-1. So, in the MIPS function, before we do “jal fact”, we should have “addi $a0, $a0,-1.”addi $a0, $a0, -1

jal fact

The Recursive Function After calling fact, we multiply the return result

with n, soaddi $a0, $a0, -1

jal fact

mul $v0, $v0, $a0

The Recursive Function After multiplying, we return, soaddi $a0, $a0, -1

jal fact

mul $v0, $v0, $a0

jr $ra

The Recursive Function So, one if else branch is done. The other

branch is slti $t0, $a0, 1

beq $t0, $0, L1

ori $v0, $0, 1

jr $ra

where L1 is where we should go to if n >= 1.

The Recursive Function

fact:slti $t0, $a0, 1beq $t0, $zero, L1ori $v0, $0, 1jr $ra

L1:addi $a0, $a0, -1jal factmul $v0, $v0, $a0jr $ra

Any problems?

The Recursive Function

• The problem is that the function will call itself, as we have expected, but it will not return correctly!

• You need to save $ra, because you made another function call inside the function. You should always do so.

• Is this enough?

fact:addi $sp, $sp, -4sw $ra, 0($sp)

slti $t0, $a0, 1beq $t0, $zero, L1ori $v0, $0, 1

lw $ra, 0($sp)addi $sp, $sp, 4

jr $raL1:

addi $a0, $a0, -1jal factmul $v0, $v0, $a0

lw $ra, 0($sp)addi $sp, $sp, 4

jr $ra

The Recursive Function So now you can return to the main function,

but the return result is 0, why? Because you did not return to the correct

time. Time, here, means what was the parameter

you called fact with. So, should also save $a0!

The Recursive Function

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The Stack During Recursion

04/20/23week04-3.ppt19

.data

.text

.globl mainmain:

li $a0, 4jal fact

done:li $v0,10syscall

fact:addi $sp, $sp, -8sw $ra, 4($sp)sw $a0, 0($sp)

slti $t0, $a0, 1beq $t0, $zero, L1ori $v0, $0, 1addi $sp, $sp, 8jr $ra

L1:addi $a0, $a0, -1jal fact

lw $ra, 4($sp)lw $a0, 0($sp)

mul $v0, $v0, $a0addi $sp, $sp, 8jr $ra

Two other MIPS pointers

• $fp: When you call a C function, the function may declare an array of size 100 like int A[100]. It is on the stack. You would want to access it, but the stack pointer may keep changing, so you need a fixed reference. $fp is the “frame pointer,” which should always point to the first word that is used by this function.

• $gp: the “global pointer.” A reference to access the static data.