C to MIPS - Instruction references undefined QTSpim - c

Question

I was trying to do an assignment translating from C to MIPS but I get a instruction reference error in jal main. Here is what I have to translate:
void swap (int a, int b)
{
int temp=a;
a=b;
b=temp;
}
int distance (int a, int b)
{
if (b > a)
swap (a,b);
return (a-b)
}
And here is what I wrote:
.data #declare the variables
var1: .word 4, 7, 12, 5
var2: .word 15, 3, 6, 14
result: .space 4
.text
main:
la $t0, var1 #load address 'var1' into $t0
la $t1, var2 #load address 'var2' into $t1
la $t2, result #load address 'result' into $t2
li $t3, 0 # load imm (i=0)
for_loop:
bgt $t3, 4, for_done #when i>4 do not meet condition, exit
lw $t4, 0($t4) #result[i] = tmp
jal distance
addi $t3, $t3, 1 #i++
j for_loop
for_done:
la $t2, distance
ori $v0, $0, 4
syscall
distance:
blt $t0, $t1, exit
jal swap
sub $t5, $t0, $t1
jr $t5
swap:
lw $t6, 0($t0)
lw $t7, 0($t1)
sw $t6, 0($t1)
sw $t7, 0($t0)
exit:
I actually don't know what am I doing, just the basics of Assembly. I hope some of you could help me. :)

Answer 1

I'm sorry but your asm code had [at least] 15 bugs.
I've created two versions: one with the bugs annotated and a second with the bugs fixed and a working program
Note that due to the vagueness of the C code, I had to guess the true intent of the program and take [considerable] poetic license.
Here's the unchanged version with the bugs annotated [please pardon the gratuitous style cleanup]:
.data # declare the variables
var1: .word 4, 7, 12, 5
var2: .word 15, 3, 6, 14
# NOTE/BUG: this only reserves 4 bytes instead of the 16 we need to hold all
# four values
result: .space 4
.text
main:
la $t0,var1 # load address 'var1' into $t0
la $t1,var2 # load address 'var2' into $t1
la $t2,result # load address 'result' into $t2
li $t3,0 # load imm (i=0)
for_loop:
# NOTE/BUG: this goes one too far (i.e. we want i>=4)
bgt $t3,4,for_done # when i>4 do not meet condition, exit
# NOTE/BUG: $t4 is never intialized to anything, so this instruction will fault
# (e.g. equivalent to dererencing a null pointer in C)
lw $t4,0($t4) # result[i] = tmp
jal distance
# NOTE/BUG: the index variable 'i' is incremented, but distance does _not_ use
# (i.e.) distance will always use var1[0] and var2[0] on each iteration
addi $t3,$t3,1 # i++
j for_loop
# NOTE/BUG: what do we want to do here? -- print the result vector presumably
# NOTE/BUG: syscall 4 is to print a string -- it would require setting up $a0
# and _not_ $t2 but, even then, using 'distance' is wrong as distance is the
# function name and _not_ a string so we'd get garbage
# NOTE/BUG: we probably wouldn't even get that far because QtSpim would
# probably fault because distance is in the .text segment and not the .data
# segment
for_done:
la $t2,distance
ori $v0,$0,4
syscall
distance:
# NOTE/BUG: this is comparing _addresses_ instead of _values_ (i.e.) this
# compares (&var1[i] > &var2[i]) instead of var1[i] > var2[i])
# NOTE/BUG: this test is _reversed_, because this guarantees negative numbers
blt $t0,$t1,exit
# NOTE/BUG: jal is calling swap as a function, but swap is merely a label here
jal swap
# NOTE/BUG: based on the mips ABI, return values go into $v0
sub $t5,$t0,$t1
# NOTE/BUG: when 'jal distance' is called, the return address goes into $ra
# and to return to the place in main that called us, we want to do 'jr $ra'
# NOTE/BUG: this 'jr' should be at exit:
jr $t5
# NOTE/BUG: this actually swaps var1[i] and var2[i] -- would this be correct to
# modify the original arrays???
swap:
lw $t6,0($t0)
lw $t7,0($t1)
sw $t6,0($t1)
sw $t7,0($t0)
# NOTE/BUG: this is where the 'jr' should go
exit:
Here's the cleaned up and working version. I decided that it should store the distance in the result vector and then show all three vectors:
.data
# NOTE: lw/sw must be four byte aligned so keep these first
var1: .word 4, 7, 12, 5
var2: .word 15, 3, 6, 14
result: .space 16
msg_var1: .asciiz "var1:"
msg_var2: .asciiz "var2:"
msg_result: .asciiz "dist:"
msg_space: .asciiz " "
msg_nl: .asciiz "\n"
.text
main:
la $s0,var1 # load address of 'var1'
la $s1,var2 # load address of 'var2'
la $s2,result # load address of 'result'
li $s3,4 # number of elements in a given vector
li $s4,0 # load imm (i=0)
for_loop:
bge $s4,$s3,for_done # i <= count? if no, fly
jal distance
addi $s4,$s4,1 # i++
j for_loop
for_done:
la $a0,msg_var1
la $a1,var1
jal show
la $a0,msg_var2
la $a1,var2
jal show
la $a0,msg_result
la $a1,result
jal show
# exit program
li $v0,10
syscall
# distance -- calculate distance between two numbers in two vectors
#
# RETURNS:
# stores into 'result' vector
#
# global registers:
# s0 -- pointer to var1
# s1 -- pointer to var2
# s2 -- pointer to result
# s4 -- array index
#
# registers:
# t0 -- address and value of var1[i]
# t1 -- address and value of var2[i]
# t2 -- temp value
# t7 -- byte offset corresponding to index 'i'
distance:
sll $t7,$s4,2 # convert index to byte offset
addu $t0,$s0,$t7 # get &var1[i]
lw $t0,0($t0) # fetch var1[i]
addu $t1,$s1,$t7 # get &var2[i]
lw $t1,0($t1) # fetch var2[i]
bge $t0,$t1,distance_done # swap a/b to get abs val? if no, fly
# swap a/b
move $t2,$t0 # temp = a
move $t0,$t1 # a = b
move $t1,$t2 # b = temp
distance_done:
sub $v0,$t0,$t1 # get distance (i.e.) abs(a-b)
addu $t2,$s2,$t7 # get &result[i]
sw $v0,0($t2) # result[i] = distance
jr $ra # return
# show -- show vector
#
# arguments:
# a0 -- vector name
# a1 -- pointer to vector
#
# registers:
# t3 -- array remaining count
#
# clobbers:
# v0
show:
li $v0,4 # syscall to print string
syscall
move $t3,$s3 # get number of elements in vector
show_loop:
blez $t3,show_done # more to do? if no, fly
li $v0,4
la $a0,msg_space # output a space
syscall
# output vector[i]
li $v0,1 # syscall to output value
lw $a0,0($a1) # get vector value
syscall
addiu $a1,$a1,4 # advance pointer to next array element
addi $t3,$t3,-1 # bump down count
j show_loop
show_done:
# output newline
la $v0,4
la $a0,msg_nl
syscall
jr $ra # return