B261

Systems Architecture

MIPS Instruction Set: Conditional Expressions

This is based on:

D.A. Patterson and J.L. Hennessy (1994) Computer Organisation and Design: The Hardware Software Interface, Morgan Kauffmann Publishers Inc., San Francisco.

Equality Test

In C++ we would write:

if(x==y) {

//executed if x==y

}

In MIPS this type of conditional expression is broken down into simpler elements.

beq R1,R2,Label#branch if equal

means that if the value in regester R1 == the value in register R2, then control jumps to the address specified by Label.

Similarly,

bne R1,R2,Label #branch if not equal

means

if(R1 != R2) goto Label

Consider the following example:

if(x==y) x++; else y++;

Assume that x is in $20 and y is in $21.

          bne $20,$21,ELSE   #if (x!=y) goto ELSE label
          addi $21,$21,1     #x = x+1 - addi (immediate)
          j CONTINUE         #jump to CONTINUE label
ELSE:     addi $21,$21,1     #y = y+1
CONTINUE: ???                #...whatever is required here... 

Note that the "labels" here are arbitrary symbolic names representing addresses in memory.

Here's another example. Suppose that X[100] is an array of ints, and it is required to add them all. In C++ we might write:

sum = 0;
for(i=0;i<100;++i) sum += x[i];

In MIPS, this would be expressed as:

Assume that x is at Xstart, sum is at register $20. i is at $10, and n = 100 is in $21

          move $10,$0           #i = 0
          move $20,$0           #sum = 0
          addi $21,$0,100       #n= 400, not 100, because of byte addressing
LOOP:     beq $10,$21,CONTINUE  #if(i==n) goto CONTINUE
          lw $22,Xstart($10)    #$22 = x[i]
          add $20,$20,$22       #sum += x[i]
          addi $10,$10,1        #i += 4, because of byte addressing
          j LOOP                #goto LOOP
CONTINUE:

Note the use of addi (add immediate) which is called immediate addressing. The actual number 1 is added to the register, rather than a reference through another register.
Note the byte addressing - so that to get at the words we have to increment through memory in steps of 4 (not 1). Hence the test is whether 400 has been reached, not 100.
Note also the use of move, which is not actually a MIPS instruction, but a symbolic name for programmer convenience. It copies the contents of the second register to the first. In the example, recall that $0 always contains zero. For example,

move $10,$0

addi $10,$0,0