Cycles per instruction

Cycles per instruction

In computer architecture, cycles per instruction (aka clock cycles per instruction, clocks per instruction, or CPI) is a term used to describe one aspect of a processor's performance: the number of clock cycles that happen when an instruction is being executed. It is the multiplicative inverse of instructions per cycle.

Contents

Explanation

Let us assume a classic RISC pipeline, with the following 5 stages:

  1. Instruction fetch cycle (IF)
  2. Instruction decode/Register fetch cycle (ID)
  3. Execution/Effective address cycle (EX)
  4. Memory access (MEM)
  5. Write-back cycle (WB)

Each stage requires one clock cycle and an instruction passes through the stages sequentially. Without pipelining, a new instruction is fetched in stage 1 only after the previous instruction finishes at stage 5. Therefore without pipelining the number of cycles it takes to execute an instruction is 5. This is the definition of CPI.

With pipelining we can improve the CPI by exploiting instruction level parallelism. For example, what if an instruction is fetched every cycle? We could theoretically have 5 instructions in the 5 pipeline stages at once (one instruction per stage). In this case, a different instruction would complete stage 5 in every clock cycle, and therefore on average we have one clock cycle per instruction (CPI = 1).

With a single-issue processor, the best CPI attainable is 1. However with multiple-issue processors, we may achieve even better CPI values. For example a processor that issues two instructions per clock cycle (see Superscalar) can achieve a CPI of 0.5 when two instructions are completing every clock cycle.

Calculations

Multi-cycle example

For the multi-cycle MIPS, there are 5 types of instructions:

  • Load (5 cycles)
  • Store (4 cycles)
  • R-type (4 cycles)
  • Branch (3 cycles)
  • Jump (3 cycles)

If a program has:

  • 50% R-type instructions
  • 15% load instructions
  • 25% store instructions
  • 8% branch instructions
  • 2% jump instructions

then, the CPI is:

CPI = (4 × 50 + 5 × 15 + 4 × 25 + 3 × 8 + 3 × 2) / 100 = 4.05.

Example

A 40-MHz processor was used to execute a benchmark program with the following instruction mix and clock cycle count:

Instruction type Instruction count Clock cycle count
Integer arithmetic 45000 1
Data transfer 32000 2
Floating point 15000 2
Control transfer 8000 2

Determine the effective CPI, MIPS rate, and execution time for this program.

Total instruction count = 100000.
CPI = (45000*1 + 32000*2 + 15000*2 + 8000*2)/100000 = 155000/100000 = 1.55.
MIPS = clock frequency/(CPI*1000000) = (40*1000000)/(1.55*1000000) = 25.8.

Therefore:

Execution time (T) = CPI*Instruction count*clock time = CPI*Instruction count/frequency = 1.55*100000/40000000 = 1.55/400 = 3.87 ms.

See also


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