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what does a cpu cycle mean?

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A cycle generally refers to a clock cycle (a single tick of the internal clock.) Cycles per second are also called Hertz, or Hz, thus a 2 GHz CPU (two gigahertz) goes through 2,000,000,000 cycles every second.

Some instructions on a CPU take multiple cycles to execute, and optimization means in most cases multiple instructions are executed in a single cycle... Thus the number of instructions executed in a given cycle is variable, but the number of CPU cycles any given instruction takes is usually fixed.

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Additional: modern CPUs will vary their clock rate according to demand for processing. Eg. if the CPU is mostly idle it will slow down. This is done to save power. Newer multi-core processors can do this per-core (ie. one core might be running faster than the others). –  Richard Jan 29 '11 at 8:11

In the world of X86-type CPUs (and many others), the running of the processor is controlled by a timing clock to synchronise what's happening with other parts of the system (RAM, ROM etc.) and this determines the overall speed of the CPU, but the things that the processor can do also go through a number of specific 'states' - called timing states, or 'T states'. Most CPUs run through around 3-6 T states in one cycle - for example, the classic Intel model is:

  • Memory read (MEMR)
  • Memory write (MEMW)
  • I/O read (IOR)
  • I/O write (IOW)

Each T state takes a certain number of clock periods to complete and can vary - for example, if your computer is fitted with (relatively) slow RAM, the memory read/write T states will be extended with additional 'wait states' to allow time for the CPU to issue a memory read instruction, for the instruction to be received by the memory controller, the required memory location addressed and the required data from that memory to appear on the memory bus for reading my the CPU.

Eg: MEMR - W - W - W - MEMW - IOR - IOW (3 wait states)

Running through all those operations is known as a machine cycle

Obviously, at some point in the cycle, the CPU is also busy doing some processing and adding this amount of time to the machine cycle give you a period of time known as the instruction cycle

Clever things can happen to skip unwanted states (like DMA and streaming instructions), but in simple terms, the time taken to complete one operation of the CPU (running through the required T and processing states) is known as a 'cycle'.

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does it mean that machine cycle is different from clock cycle? a single machine cycle consists of many clock cycles? –  Saurabh Aug 30 '13 at 4:50

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