any ideas? Why I am getting: Runtime exception at 0x00400020: fetch address not aligned on word boundary 0x00000007 Problem line is: lw $s1,0($a1) #copy arg2 = size of array
.data .align 4 #added this, didnt work size: .word 7 .align 4 #added this, didnt work search: .word 30 .align 4 #added this,didnt work array: .word 10,20,30,40,50,60,70 .align 4 .text main: la $a0,array #$a0 = address of array lw $a1,size #a1 = size of array lw $a2,search #$a2 = search key COUNT: lw $s0,0($a0) #copy arg1 = address array addi $s1,$zero,7 lw $s1,0($a1) #copy arg2 = size of array lw $s2,0($a2) #copy arg3 = search key (n) addi $s2,$zero,30 COUNTLOOP: add $v0,$zero,$zero #v0 = res add $t0,$zero,$zero #$t0 = init i to 0 slt $t1,$t0,$s1 #check if i > size of array beq $t1,$zero,DONECOUNT #i is n so end sll $t2,$s0,2 #$t2 = get off set for a[i] lw $t3,0($t2) #$t3 = get value of a[i] bne $t3,$s2,CLOOPBTM #check if a[i] == seach key addi $v0,$v0,1 #if above then increment res CLOOPBTM: addi $t0,$t0,1 j COUNTLOOP DONECOUNT: 1 Answer
The problem with the code is, that you're not using the address where the size is stored but the size itself:
Here you load the address into A0 and the size (7) into A1:
la $a0,array lw $a1,size #a1 = size of array Here you load the first word stored at your array (that will load a 10). This is not what you've intended.
lw $s0,0($a0) #copy arg1 = address array addi $s1,$zero,7 Here you load the first word stored at location 0x000007. (your size). This is probably also not intended and will cause an exception because the address is not aligned:
lw $s1,0($a1) #copy arg2 = size of array and so on.
It seems to me, that you have a misunderstanding what the LW instruction does. It reads a memory location into a register. What you want in the prolog of your loop is to make copies of a register.
To do so you can use the move pseudo instruction if your assembler supports it. Otherwise use the OR instruction to copy registers like this:
COUNT: or $s0, $a0, $a0 #copy arg1 = address array addi $s1, $zero,7 or $s1, $a1, $a1 #copy arg2 = size of array or $s2, $a2, $a2 #copy arg3 = search key (n) addi $s2, $zero,30 COUNTLOOP: ... for a complete example of a linear search loop try this (untested and expects that the assembler cares about the delay slots)
main: la $a0,array # $a0 = address of array lw $a1,size # $a1 = size of array lw $a2,search # $a2 = search key beq $a1, $zero, NOTFOUND # handle the size==0 case.. or $v0, $zero, $zero # init counter to zero LOOP: lw $s0, 0($a0) # load element beq $s0, $a2, FOUND # branch if key found: addiu $a0, $a0, 4 # increment array pointer addiu $v0, $v0, 1 # increment loop counter bne $v0, $a1, LOOP # repeat until we've processed the array. NOTFOUND: # -------------------------------------- # if you reach this, key does not exist: # -------------------------------------- li $v0, -1 # load a -1 to signal key not found. jr $lr # return to caller FOUND: # ----------------------------------------- # v0 now contains the position of the key. # ----------------------------------------- jr $lr 
