Question

Explain what has changed in CPU architecture to deal with clock rate

Answer 1

Solution:

The clock rate (or clock speed) is stated in megahertz (MHz) or gigahertz (GHz), and refers to the speed at which the processor can execute instructions.  The fast a CPU can process instructions, the faster the CPU. CPU’s clock rate determines how fast a CPU can work. The speed of a CPU is the rate at which a CPU can complete a processing cycle. You know about the clock cycle.

Suppose, your CPU’s clock rate is 1 GHz. If your friend has a CPU which clock rate is 2 GHz, then your friend’s CPU’s clock speed is twice of your CPU’s. But it cannot be said that the 2 GHz CPU is two times faster than the 1 GHz CPU. It depends on many things including computer architecture. Suppose, the 2 GHz CPU can complete an instruction in 4 Cycles and the 1GHz CPU needs 6 Cycles to complete the same instruction then the 2 GHz CPU will perform the same operation more than two times faster than the 1 GHz CPU.
If the situation is opposite, that means, the 2 GHz CPU needs more cycles than the 1 GHz CPU to perform the same operation, then the 2 GHz CPU will work less than 2x fast than the 1 GHz CPU.

Some processors can execute only one instruction during a clock cycle. Most of the today’s CPUs can execute more than one instructions per clock cycle. So, the CPUs that can complete more instructions per cycle than others will work faster. In the same way, the computer with a 64-bit bus architecture will work faster than the computer work in a 32-bit bus architecture. So, you cannot find any simplistic or universal relation among clock speed, bus architecture.

CPI : CPI stands for Clocks per Instructions. That means, how many clocks are needed to complete an instruction.

CPU Time = I * T * CPI

Here,

I = Total number instructions in a program

CPI = Clocks needed to complete an instruction

T = Clock cycle time

CPU Time = I * CPI/R

Here,

R = Clock rate which can be measured by the inverse of Clock cycle time that means, R = 1/T