Hardware register

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Hardware register

In digital electronics, especially computing

Computing

Computing is usually defined as the activity of using and developing computer technology, computer hardware and computer software. It is the computer-specific part of information technology....
, a hardware register stores bits of information, in a way that all the bits can be written to or read out simultaneously. The hardware registers inside a central processing unit

Central processing unit

A central processing unit is an electronic circuit that can execute computer programs. This broad definition can easily be applied to many early computers that existed long before the term "CPU" ever came into widespread usage....
(CPU) are called processor register

Processor register

In computer architecture, a processor register is a small amount of Computer storage available on the CPU whose contents can be accessed more quickly than storage available elsewhere....
s. Signals from a state machine to the register control when registers transmit to or accept information from other registers. Sometimes the state machine routes information from one register through a functional transform, such as an adder unit

Adder (electronics)

In electronics, an adder or summer is a digital circuit that performs addition of numbers.In modern computers adders reside in the arithmetic logic unit where other operations are performed....
, and then to another register that stores the results.

Typical uses of hardware registers include configuration and start-up of certain features, especially during initialization, buffer storage e.g.

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Encyclopedia

In digital electronics, especially computing

Computing

Central processing unit

Processor register

Adder (electronics)

Input/output

In computing, input/output, or I/O, refers to the communication between an information processing system , and the outside world ? possibly a human, or another information processing system....
(I/O) of different kinds, and status reporting such as whether a certain event has occurred in the hardware unit.

Reading a hardware register in "peripheral units" -- computer hardware

Computer hardware

A personal computer is made up of computer hardware, multiple physical components onto which can be loaded into a multitude of software that perform the functions of the computer....
outside the CPU -- involves accessing its memory-mapped I/O

Memory-mapped I/O

Memory-mapped I/O and port I/O are two complementary methods of performing input/output between the Central processing unit and peripheral devices in a computer....
address or port-mapped I/O address with a "load" or "store" instruction, issued by the processor. Hardware registers are addressed in words, but sometimes only use a few bit

Bit

A bit is a binary numeral system numerical digit, taking a value of either 0 or 1. Binary digits are a basic unit of information Computer data storage and transmission in digital computing and digital information theory....
s of the word read in to, or written out to the register.

Strobe registers have the same interface as normal hardware registers, but instead of storing data, they trigger an action each time they are written to (or, in rare cases, read from). They are a means of signaling.

Registers are normally measured by the number of bit

Bit

8-bit

Eight-bit CPUs normally use an 8-bit data bus and a 16-bit address bus which means that their address space is limited to 64 KBs. This is not a "natural law", however, so there are exceptions....
register" or a "32-bit

32-bit

The range of integer values that can be stored in 32 bits is 0 through 4,294,967,295 or -2,147,483,648 through 2,147,483,647 using two's complement encoding....
register". Registers can be implemented in a wide variety of ways, including register file

A register file is an array of processor registers in a central processing unit. Modern integrated circuit-based register files are usually implemented by way of fast static RAMs with multiple ports....
s, standard SRAM

Static random access memory

Static random access memory is a type of semiconductor memory where the word static indicates that, unlike dynamic random access memory, it does not need to be periodically memory refresh, as SRAM uses bistable latch to store each bit....
, individual flip-flop

Flip-flop (electronics)

In digital circuits, a flip-flop is a term referring to an electronic circuit that has two stable states and thereby is capable of serving as one bit of computer storage....
s, or high speed core memory.

In addition to the "programmer-visible" registers that can be read and written with software, many chips have internal registers that are used for state machines and pipelining; for example, registered memory

Registered memory

Registered memory module have a Hardware register between the Dynamic_random_access_memory modules and the system's memory controller. They place less electrical load on the memory controller and allow single systems to remain stable with more memory modules than they would have otherwise....
.

Because write-only registers make debugging almost impossible, lead to the read-modify-write

Read-modify-write

In computer science, read-modify-write is a class of Linearizability such as test-and-set, fetch-and-add, and compare-and-swap which both read a memory location and write a new value into it simultaneously, either with a completely new value or some function of the previous value....
problem, and also make it unnecessarily difficult for the Advanced Configuration and Power Interface

Advanced Configuration and Power Interface

The Advanced Configuration and Power Interface specification is an open standard for unified operating system-centric device configuration and power management....
to determine the device's state when entering sleep mode

Sleep mode

Sleep mode refers to a low power mode for electronic devices such as computers, televisions, and remote controlled devices. These modes save significant electrical consumption compared to leaving a device fully on and idle but allow the user to avoid having to reset programming codes or wait for a machine to reboot....
in order to restore that state when exiting sleep mode, many programmers tell hardware designers to make sure that all writable registers are also readable.

Hardware register

See also