CPU

CPU Performance

The performance of a computer is determined by three key factors

1. Instruction count: The number of instructions executed by the program.
2. Clock Cycle Time
3. CPI = Clock cycles / instruction

$\text{CPI}=\frac{\text{CPU clock cycles}}{\text{Instruction Count (IC)}}$ IPC = 1 / CPI

$\text{CPU clock cycles}={\sum }_{i=1}^n{\text{CPI}}_i\times I{C}_i$

• What does $C_i$ stand for??

$\text{CPU execution time}=\text{Instruction count (IC)}\times \text{CPI}\times \text{Clock cycle time}$

• “Clock cycle time” is also called “clock period”

$\text{CPU execution time}=\frac{\text{Instruction count (IC)}\times \text{CPI}}{\text{Clock Rate}}$

• “Clock rate” is also called “clock frequency”

• You can derive the other values from this formula by moving things around slightly

CPU time does NOT include time waiting for I/O or in OS waiting for task to be performed by CPU.

Power (Physics): Dynamic power of circuit is proportional to $\frac{1}{2}C{V}^{2}f$

• $C$ is Capacitance
• $V$ is the power supply (Vdd)
• $f$ is the Clock frequency

Related

• See Performance for comparing the performance of 2 CPUs