CS201 Lab: Control Flow Structures
In this lab, we are going to explore how to make loops and how to implement if-then-else statement in SPIM.
Control Structure Instructions
In this section, the control structure instructions and examples are given for your reference.
Jump Instruction
The simplest control instruction jump j addr means jump to address addr which makes the processor to fetch the next instruction from the word at address addr. Actually, the implementation is just copying the value of the address to the program counter. There are another two jump instructions jal addr and jr Rsrc They are used for procedure/function calls. We will talk more about them later.
Conditional Branches
The jump instruction is also known as an unconditional jump. That means the processor always goes to the jump target address without any conditions. The conditional branch is another kind of interesting control structure. The MIPS conditional branch instructions have names that begin with b (b is for branches and j is for jump).Here are some examples of MIPS jump, branch and compare instructions:
beq $t0, $t1, addr means branch to addr if $t0 == $t1 beqz $t0 addr means branch to addr if $t0 == 0 bne $t0, $t1, addr means branch to addr if $t0 != $t1 blt $t0, $t1, addr means branch to addr if $t0 < $t1 ble $t0, 5, loop means branch to loop if $t0 <= 5 bgt $t0, $t1, addr means branch to addr if $t0 > $t1 bge $t0, 5, loop means branch to loop if $t0 >= 5
Example Programs Using Loops and Conditional Branches
Example 1:
The following program prints out the length of a given string.
##
## The program --- length.s
##
## - will print out the length of character
## string "str".
##
##
## - $t0 - holds each byte from string in turn
## - $t1 - contains count of characters
## - $t2 - points to the string
##
#################################################
# #
# text segment #
# #
#################################################
.text
.globl __start
__start: # execution starts here
la $t2,str # t2 points to the string
li $t1,0 # t1 holds the count
nextCh: lb $t0,($t2) # get a byte from the string
beqz $t0,strEnd # zero means end of string
addi $t1, $t1, 1 # increment count
addi $t2, $t2, 1 # move pointer one character
j nextCh # go round the loop again
strEnd:
la $a0,ans # System call
li $v0,4 # to print out
syscall # the string message
move $a0,$t1 # copy the count to a0
li $v0,1 # System call 1
syscall # to print the length worked out
la $a0,endl # syscall to print out
li $v0,4 # a newline
syscall
li $v0,10
syscall # Bye!
#################################################
# #
# data segment #
# #
#################################################
.data
str: .asciiz "hello world"
ans: .asciiz "Length is "
endl: .asciiz "\n"
##
## end of file length.s
Example 2:
The following program replaces some characters in a given string.
##
## The program --- loop1.s
##
## - will replace all occurrences of 'a' with
## 'A' in the string "chararray" and
## print the resulting string.
##
## - chararray: "abbbaabbbabababab\n"
## - output: "AbbbAAbbbAbAbAbAb"
##
##
#################################################
# #
# text segment #
# #
#################################################
.text
.globl __start
__start: # execution starts here
la $t2,chararray # t2 points to the string
li $t1,'A' # t1 holds the char 'A'
nextCh: lb $t0,($t2) # get a byte from the string
beqz $t0,strEnd # zero means end of string
bne $t0,'a',nota # if not 'a' go to nota
sb $t1,($t2) # store upper case 'A'
nota: addi $t2, $t2, 1 # move pointer one character
j nextCh # go round the loop again
strEnd:
la $a0,chararray # System call
li $v0,4 # to print out
syscall # the modified string
li $v0,10
syscall # Bye!
#################################################
# #
# data segment #
# #
#################################################
.data
chararray: .asciiz "abbbaabbbabababab\n"
##
## end of file loop1.s
Lab Assignment
Copyright: Department of Computer Science, University of Regina.