MIPS Control Flow

Abstract

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• MIPS executes instructions in a sequential manner. It follows the instructions one by one in order. If we want to have loops, we need the ability to change the order of executing instructions

Labels are not instructions

Labels usually point to the target of branching or jumping. They aren’t stored in the generated machine codes!

Important

Any form of loop can be written in Assembly language with the help of Conditional Branch and Unconditional Jump.

For for, we convert it to a while loop first. For while loop, we can rewrite it with if statements and goto.

For if statements, we can rewrite it with conditional branch. For goto, we can rewrite it with unconditional jump.

Conditional Branch

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• bne $t0, $t1, label branches to label when $t0 != $t1. This is similar to if ($t0 != $t1) goto label
• beq $t0, $t1, label branches to label when $t0 == $t1. This is similar to if ($t0 == $t1) goto label

Translating from C to MIPS

To optimise the number of MIPS instructions, consider inverting the condition. For example, use bne (branch if not equal) for if ($t0 == $t1) goto label and beq (branch if equal) for if ($t0 != $t1) goto label.

New address

When branch is taken, Program Counter PC = (PC + 4) + (Immediate Value * 4).

Unconditional Jump

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j label
j label

• j label branches to label. This is similar to goto label

MIPS Inequality

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• slt $t0, $s1, $s2 is same as $t0 = $s1 < $s2 ? 1 : 0

Important

blt $s1, $s2, L is a Pseudo Instruction which gets translated to slt $t0, $s1, $s2, followed by bne $t0, $zero, L by the Assembler.