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Decision Making

Decision Making. if and loops. Need to do one of two things if : do next instruction or skip to another (else) loop: continue in loop or exit loop. Decision instructions in MIPS. Only two such instructions: beq $t0, $t1, label If value in $t0 equals value in $t1, go to instruction at label

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Decision Making

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  1. Decision Making

  2. if and loops • Need to do one of two things • if : do next instruction or skip to another (else) • loop: continue in loop or exit loop

  3. Decision instructions in MIPS • Only two such instructions: • beq $t0, $t1, label • If value in $t0 equals value in $t1, go to instruction at label • otherwise, execute next instruction in order • bne $t0, $t1, label • If value in $t0 does not equal value in $t1, go to instruction at label • otherwise, execute next instruction in order

  4. if statement in assembly if(a == b) // a in $t0, b in $t1 {c = 1} // c in $s0 // next instruction bne $t0, $t1, endif addi $s0, $zero, 1 endif: # next instruction

  5. if - else if(a == b) // a in $t0, b in $t1 {c = 1} // c in $s0 else {c = 0} // next instruction

  6. if – else in assembly bne $t0, $t1, else addi $s0, $zero, 1 j endif else: addi $s0, $zero, 0 endif: # next instruction

  7. What about other comparisons? • a < b • a <= b • etc…

  8. Set less than instruction • slt $t0, $s1, $s2 • If value in $s1 is less than value in $s2, then set the value in $t0 to 1, otherwise set $t0 to 0 • Use slt combined with beq or bne, comparing to the $zero register, for an if comparison.

  9. If again if(a < b) // a in $t0, b in $t1 {c = 1} // c in $s0 // next instruction slt $t2, $t0, $t1 beq $t2, $zero, endif addi $s0, $zero, 1 endif: # next instruction

  10. while loop while (condition) { loop body } Each time at the top of the loop, check the condition. If true, continue the loop. At the end of the loop, go back to check the condition again.

  11. while loop example - c while(count < 10) { // do something count ++; } // next instruction

  12. while loop example - assembly # Assume count is in register $t0, # and register $s0 contains 10 loop: slt $t1, $t0, $s0 b?? $t1, $zero, loopend # do something addi $t0, $t0, 1 j loop loopend: # next instruction

  13. while loop # Assume count is in register $t0, # and register $s0 contains 10 loop: slt $t1, $t0, $s0 beq $t1, $zero, loopend # do something addi $t0, $t0, 1 j loop loopend: # next instruction

  14. for loop example - c for(cnt = 5, cnt >= 0, cnt--) { // do something } // next instruction

  15. for loop example - assembly addi $t0, $zero, 5 loop: slt $t1, $t0, $zero bne $t1, $zero, loopend # do something addi $t0, $t0, -1 j loop loopend: # next instruction

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