Question

Following table indicates the latencies of operations between the instructions producing the result and instruction using the result.

Instruction production the result	Instruction using the result	Latency
ALU Operation	ALU Operation	2
ALU Operation	Store	2
Load	ALU Operation	1
Load	Store	0

Consider the following code segment.

Load R1, Loc1 ; Load R1 from memory location Loc1

Load R2, Loc2 ; Load R2 from memory location Loc2

Add R1, R2, R1 ; Add R1 and R2 and save result in R1

Dec R2 ; Decrement R2

Dec R1 ; Decrement R1

Mpy R1, R2, R3 ; Multiply R1 and R2 and save result in R3

Store R3, Loc3 ; Store R3 in memory location Loc3

What is the number of cycles needed to execute the above code segment assuming each instruction takes one cycle to execute?

• A. 13
• B. 7
• C. 14
• D. 10

Answer 1

The correct option is C 14

Instruction	Latency	No. of cycles
Load R1, Loc1	0	0
Load R2, Loc2	0	0
Add R1, R2, R1	1 + 1 + 2 = 4	4 $\times$ 1 cycle = 4
Dec R2	1 + 2 = 3	3 $\times$ 1 cycle = 3
Dec R1	1 + 2 = 3	3 $\times$ 1 cycle = 3
Mpy R1, R2, R3	1 + 1 + 2 = 4	4 $\times$ 1 cycle = 4
Store R3, Loc3	0	0
		Total = 14