Instructions per second (IPS) is a measure of a computer's processor speed. Many reported IPS values have represented "peak" execution rates on artificial instruction sequences with few branches, whereas realistic workloads typically lead to significantly lower IPS values. The performance of the memory hierarchy also greatly affects processor performance, an issue barely considered in MIPS calculations . Because of these problems, researchers created standardized tests such as SPECint to attempt to measure the real effective performance in commonly used applications, and raw IPS has fallen into disuse.
The term is commonly used in association with a numeric value such as thousand instructions per second (kIPS), million instructions per second (MIPS), or Million Operations per Second (MOPS).
Thousand instructions per second
Before standard benchmarks were available, average speed rating of computers was based on calculations for a mix of instructions with the results given in kilo Instructions Per Second (kIPS). The most famous was the Gibson Mix, produced by Jack Clark Gibson of IBM for scientific applications. Other ratings were also produced for commercial applications. Computer Speeds From Instruction Mixes pre-1960 to 1971 has results for around 175 computers, providing scientific and commercial ratings. For IBM, the earliest Gibson Mix calculations shown are the 1954 IBM 650 at 0.06 kIPS and 1956 IBM 705 at 0.5 kIPS. The results are mainly for IBM and others known as the BUNCH - Burroughs, Univac, NCR, CDC and Honeywell.
A thousand instructions per second (kIPS) is rarely used, as most current microprocessors can execute a least a billion instructions per second. The thousand means 1000, not 1024.
kIPS is also a common joke name for 16 bit microprocessor designs developed in undergraduate computer engineering courses that use the text Computer Organization and Design by Patterson and Hennessy (ISBN 1-55860-428-6), which explains computer architecture concepts in terms of the MIPS architecture. Such architectures tend to be scaled down versions of the MIPS R2000 architecture.
Million instructions per second
Comparison of processors speeds requires thorough analysis, as the speed of a given CPU is dependent upon many factors, such as the type of instructions being executed, the execution order and the presence of branch instructions (problematic in CPU pipelines). CPU instruction rates are different from clock frequencies, usually reported in Hz, as each instruction may require several clock cycles to complete or may be capable of executing multiple independent instructions at once. Additionally, the number of cycles required for instructions to complete is dependent upon the instruction being executed. MIPS are difficult to compare between CPU architectures. This and other limitations of the unit lead many computer engineers to define MIPS as Meaningless Indicator of Processor Speed.
In the late 1970s, minicomputer performance was compared using VAX MIPS, where computers were measured on a task and their performance rated against the VAX 11/780 that was marketed as a 1 MIPS machine. (The measure was also known as the VAX Unit of Performance or VUP. Though orthographically incorrect, the s in VUPs is sometimes written in upper case.) This was chosen because the 11/780 was roughly equivalent in performance to an IBM System/370 model 158-3, which was commonly accepted in the computing industry as running at 1 MIPS.
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