Question

The following instruction is not included in the MIPS instruction set:

rpt $t2, loop # if(R[rs]>0) R[rs]=R[rs]−1, PC=PC+4+BranchAddr

2.25.1 [5] <§2.7> If this instruction were to be implemented in the MIPS

instruction set, what is the most appropriate instruction format?

2.25.2 [5] <§2.7> What is the shortest sequence of MIPS instructions that

performs the same operation?

Short answer

2.25.1

The most appropriate instruction format will be “I type” for implementing the given MIPS instruction set.

2.25.2

The MIPS instructions sequence that performs as same as the given MIPS instructions sequence:

LOOP : slt $t3, $0, $t2

beq $t3, $0, Exit

sub $t2, $t2, 1

EXIT:

Step-by-step solution

Define the concept of the type of MIPS instruction and the purpose of using MIPS instructions.

2.25.1

There are three common types of MIPS instruction in MIPS assembly language – the “I type”, the “J type”, and the “R type”. In the name of “I type instructions”, “I” stands for immediate. One of the “I type” MIPS instruction is “addi $t3 $t4 1” where “$t4” is the source register, “$t3” is the destination register, and “1” is the immediate value.

2.25.2

The full form of “slt” MIPS instruction is set on less than. The MIPS instruction “slt $t3, $0, $t2” is used in the sequence where “$t3” is the register destination, “$0” is the source register, “$t2” is the destination register. If $0<$t2 then $t3 = 1 else $t3 = 0. The full form of “beq” MIPS instruction is branch-on-equal. The purpose of using this MIPS assembly instruction “beq $t3, $0, EXIT” is going to the statement “EXIT” if the value of “$t3” is equal to the “$0”. The “sub” MIPS instruction is used for subtracting. The MIPS instruction “sub $t2, $t2, 1” is used in the sequence where $t2=$t2-1.

Determine the instruction format and the sequence of MIPS instructions.

2.25.1

In the given code:

rpt $t2

loop # if(R[rs]>0) R[rs]=R[rs]−1

PC=PC+4+BranchAddr

The “I” type of instruction can be able to load or store the operations, the branch operations, and also the immediate ALU operations. This type of instruction is used when there is a need of operating on an immediate value and a register value. Hence, The most appropriate instruction format will be “I type” for implementing the given MIPS instruction set.

2.25.2

The MIPS instructions sequence that performs as same as the given MIPS instructions sequence:

LOOP:slt $t3, $0, $t2 //If $0 is less than $t2 then $t3=1 else $t3=0

beq $t3, $0, Exit//if $t3 is equal to the $0 then go to Exit

sub $t2, $t2, 1// $t2=$t2-1

EXIT: