machine cycle


Also found in: Dictionary, Thesaurus, Medical, Legal, Financial.

machine cycle

[mə′shēn ‚sī·kəl]
(computer science)
The shortest period of time at the end of which a series of events in the operation of a computer is repeated.
The series of events itself.

machine cycle

(processor)
The four steps which the CPU carries out for each machine language instruction: fetch, decode, execute, and store. These steps are performed by the control unit, and may be fixed in the logic of the CPU or may be programmed as microcode which is itself usually fixed (in ROM) but may be (partially) modifiable (stored in RAM).

The fetch cycle places the current program counter contents (the address of the next instruction to execute) on the address bus and reads in the word at that location into the instruction register (IR). In RISC CPUs instructions are usually a single word but in other architectures an instruction may be several words long, necessitating several fetches.

The decode cycle uses the contents of the IR to determine which gates should be opened between the CPU's various functional units and busses and what operation the ALU(s) should perform (e.g. add, bitwise and). Each gate allows data to flow from one unit to another (e.g. from register 0 to ALU input 1) or enables data from one output onto a certain bus. In the simplest case ("horizontal encoding") each bit of the instruction register controls a single gate or several bits may control the ALU operation. This is rarely used because it requires long instruction words (such an architecture is sometimes called a very long instruction word architecture). Commonly, groups of bits from the IR are fed through decoders to control higher level aspects of the CPU's operation, e.g. source and destination registers, addressing mode and ALU operation. This is known as vertical encoding. One way RISC processors gain their advantage in speed is by having simple instruction decoding which can be performed quickly.

The execute cycle occurs when the decoding logic has settled and entails the passing of values between the various function units and busses and the operation of the ALU. A simple instruction will require only a single execute cycle whereas a complex instruction (e.g. subroutine call or one using memory indirect addressing) may require three or four. Instructions in a RISC typically (but not invariably) take only a single cycle.

The store cycle is when the result of the instruction is written to its destination, either a register or a memory location. This is really part of the execute cycle because some instructions may write to multiple destinations as part of their execution.

machine cycle

The shortest interval in which an elementary operation can take place within the processor. It is made up of some number of clock cycles.
References in periodicals archive ?
Cycle times should be determined for each process, and the effective machine cycle time should be determined for automated processes.
Rapid traverse rates of 1,181ipm and 787ipm mean accelerated machine cycle times and reduced non-cut time that increases overall throughput.
A cat that went through a washing machine cycle and a dog that swallowed two golf balls were two of the more bizarre claims by pet insurance policyholders last year.
A CAT that went through a washing machine cycle and a dog that swallowed two golf balls are two of the more bizarre pet insurance claims revealed by insurers.
A cat which went through a washing machine cycle and a dog that swallowed two golf balls are two of the more bizarre pet insurance claims made to Lloyds TSB insurance last year.
The actual completion of the primary function of that machine--in this case, the printing of solder paste--is generally additive to the accepted elements that define machine cycle time.
This, I was consuming with one hand, while reading a newspaper with the other, at the same time as waiting for the washing machine cycle to finish so I could unload the contents, and mentally running through the phone calls I needed to make.
Features a new runner block that can help machine builders shorten machine cycle times, increase production and reduce lubrication and maintenance costs Contact: Susan Baker, 1-704-583-4338
All tools are readily available to the robot during each machine cycle to maximize efficiency as it switches between drilling holes, cutting panels and trimming the perimeter of the composite parts.
Post-machining quality control can be performed while maintaining machine cycle times, with parts clamping, orientation and quality assurance accomplished with the machines running.
In addition the P89C66x line achieves, according to the company, a 2x performance improvement over standard devices by supporting six clocks per machine cycle.
He brings to the table an impressive 20-year track record of robust technology development and commercialization -- spanning many fields and applications -- that have consistently resulted in best-in-class machine cycle time, footprint and cost.