In
computer architecture, instructions per cycle (IPC), commonly called instructions per clock, is one aspect of a
processor's performance: the average number of
instructions executed for each
clock cycle
In electronics and especially synchronous digital circuits, a clock signal (historically also known as ''logic beat'') is an electronic logic signal (voltage or current) which oscillates between a high and a low state at a constant frequency and ...
. It is the
multiplicative inverse
In mathematics, a multiplicative inverse or reciprocal for a number ''x'', denoted by 1/''x'' or ''x''−1, is a number which when Multiplication, multiplied by ''x'' yields the multiplicative identity, 1. The multiplicative inverse of a ra ...
of
cycles per instruction
In computer architecture, cycles per instruction (aka clock cycles per instruction, clocks per instruction, or CPI) is one aspect of a processor's performance: the average number of clock cycles per instruction for a program or program fragmen ...
.
Explanation
While early generations of CPUs carried out all the steps to execute an instruction sequentially, modern CPUs can do many things in parallel. As it is impossible to just keep doubling the speed of the clock,
instruction pipelining and
superscalar processor design have evolved so CPUs can use a variety of execution units in parallel - looking ahead through the incoming instructions in order to optimise them. This leads to the ''instructions per cycle completed'' being much higher than 1 and is responsible for much of the speed improvements in subsequent CPU generations.
Calculation of IPC
The calculation of IPC is done through running a set piece of code, calculating the number of machine-level instructions required to complete it, then using high-performance timers to calculate the number of clock cycles required to complete it on the actual hardware. The final result comes from dividing the number of instructions by the number of CPU clock cycles.
The number of
instructions per second
Instructions per second (IPS) is a measure of a computer's Central processing unit, processor speed. For complex instruction set computers (CISCs), different Machine code, instructions take different amounts of time, so the value measured depen ...
and
floating point operations per second for a processor can be derived by multiplying the number of instructions per cycle with the
clock rate
Clock rate or clock speed in computing typically refers to the frequency at which the clock generator of a processor can generate pulses used to synchronize the operations of its components. It is used as an indicator of the processor's s ...
(cycles per second given in
Hertz
The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), often described as being equivalent to one event (or Cycle per second, cycle) per second. The hertz is an SI derived unit whose formal expression in ter ...
) of the processor in question. The number of instructions per second is an approximate indicator of the likely performance of the processor.
The number of instructions executed per clock is not a constant for a given processor; it depends on how the particular
software
Software consists of computer programs that instruct the Execution (computing), execution of a computer. Software also includes design documents and specifications.
The history of software is closely tied to the development of digital comput ...
being run interacts with the processor, and indeed the entire machine, particularly the
memory hierarchy
In computer architecture, the memory hierarchy separates computer storage into a hierarchy based on response time. Since response time, complexity, and capacity are related, the levels may also be distinguished by their performance and contr ...
. However, certain processor features tend to lead to designs that have higher-than-average IPC values; the presence of multiple
arithmetic logic unit
In computing, an arithmetic logic unit (ALU) is a Combinational logic, combinational digital circuit that performs arithmetic and bitwise operations on integer binary numbers. This is in contrast to a floating-point unit (FPU), which operates on ...
s (an ALU is a processor subsystem that can perform elementary arithmetic and logical operations), and short pipelines. When comparing different
instruction set
In computer science, an instruction set architecture (ISA) is an abstract model that generally defines how software controls the CPU in a computer or a family of computers. A device or program that executes instructions described by that ISA, s ...
s, a simpler instruction set may lead to a higher IPC figure than an implementation of a more complex instruction set using the same chip technology; however, the more complex instruction set may be able to achieve more useful work with fewer instructions. As such comparing IPC figures between different instruction sets (for example x86 vs ARM) is usually meaningless.
Computer speed
The useful work that can be done with any computer depends on many factors besides the processor speed. These factors include the
instruction set architecture
In computer science, an instruction set architecture (ISA) is an abstract model that generally defines how software controls the CPU in a computer or a family of computers. A device or program that executes instructions described by that ISA, ...
, the processor's
microarchitecture
In electronics, computer science and computer engineering, microarchitecture, also called computer organization and sometimes abbreviated as μarch or uarch, is the way a given instruction set architecture (ISA) is implemented in a particular ...
, and the computer system organization (such as the design of the
disk storage
Disc or disk may refer to:
* Disk (mathematics)
In geometry, a disk (Spelling of disc, also spelled disc) is the region in a plane (geometry), plane bounded by a circle. A disk is said to be ''closed'' if it contains the circle that constitut ...
system and the capabilities and performance of other attached devices), the efficiency of the
operating system
An operating system (OS) is system software that manages computer hardware and software resources, and provides common daemon (computing), services for computer programs.
Time-sharing operating systems scheduler (computing), schedule tasks for ...
, and the high-level design of
application software
Application software is any computer program that is intended for end-user use not operating, administering or programming the computer. An application (app, application program, software application) is any program that can be categorized as ...
.
For computer users and purchasers, application
benchmarks, rather than instructions per cycle, are typically a much more useful indication of system performance. However, IPC does provide an example of why
clock speed
Clock rate or clock speed in computing typically refers to the frequency at which the clock generator of a processor can generate pulses used to synchronize the operations of its components. It is used as an indicator of the processor's ...
is not the only factor relevant to computer performance.
See also
*
Instructions per second
Instructions per second (IPS) is a measure of a computer's Central processing unit, processor speed. For complex instruction set computers (CISCs), different Machine code, instructions take different amounts of time, so the value measured depen ...
*
Cycles per instruction
In computer architecture, cycles per instruction (aka clock cycles per instruction, clocks per instruction, or CPI) is one aspect of a processor's performance: the average number of clock cycles per instruction for a program or program fragmen ...
*
FLOPS
Floating point operations per second (FLOPS, flops or flop/s) is a measure of computer performance in computing, useful in fields of scientific computations that require floating-point calculations.
For such cases, it is a more accurate measu ...
*
Megahertz myth
*
Benchmark (computing)
References
{{CPU technologies
Instruction processing
Clock signal
Rates
Computer performance