5. The following MIPS subroutine computes an interesting function. Address Label

Question

5. The following MIPS subroutine computes an interesting function. Address Label Instruction 1000start: addi $2 1004 1008 1012 3, $0, 1 al mvste en myster exit andi 1020 1024 1028 5, $2, 1 5, $0, s 4, $4, $3 sr 1036 1040 bne S2150, Toop 31 Consider the execution of the fragment of code beginning at start and ending at end. Assume all registers and data memory locations are cleared prior to the execution of the code sequence. List the results of these registers after the code fragment completes a. i. $2 ii. $3 iv. $5 b. Determine the number of instructions executed for this code fragment (includ ing the call of mystery). Count all instructions executed beginning at start and ending at end, inclusive. (Show your work) c. Briefly describe the function computed by the subroutine mystery.

Explanation / Answer

Part a

1. $2 = $0 + 85;

2. $3 = $0 +1;

3. $4 = $0 + $0;

4. $5 register value is clear as it is never reached.

Instructions executed are

Part b

addi $2, $0, 85

addi $3, $0, 1

jal mystery

add $4, $0, $0

j exit

Total 5 instructions are executed

Part c

The function computed by the subroutine mystery is

add $4, $0, $0

which means to add the values in the registers $0 and $0 and then assign the resulted value to register $4, i.e., $4 = $0 + $0;

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