Hardware register

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In digital electronics, especially computing, 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 (CPU) are called processor registers. 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, 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. video memory for graphics cards, input/output (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 outside the CPU -- involves accessing its memory-mapped I/O 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 bits 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 bits they can hold, for example, an "8-bit register" or a "32-bit register". Registers can be implemented in a wide variety of ways, including register files, standard SRAM, individual flip-flops, 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.

Commercial design tools such as Socrates Bitwise by Duolog Technologies^[1], simplify and automate memory-mapped register specification and code generation for hardware, firmware, hardware verification, testing and documentation.^[2], ^[3], ^[4]

SPIRIT IP-XACT and DITA SIDSC XML define standard XML formats for memory-mapped registers.^[5], ^[6], ^[7]

Because write-only registers make debugging almost impossible^[8], lead to the read-modify-write problem, and also make it unnecessarily difficult for the Advanced Configuration and Power Interface to determine the device's state when entering sleep mode in order to restore that state when exiting sleep mode^[9], many programmers tell hardware designers to make sure that all writable registers are also readable. However, there are some cases when reading certain types of hardware registers is useless. For example, a strobe register bit that generates a one cycle pulse into specialized hardware will always read logic 0.

See also

• Register Transfer Language
• Input/Output Base Address

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