What is Processor Register? Example, Functions & More

What is Processor Register?

A processor register (CPU register) refers to a small local storage space on the CPU. It holds those specific data sets that are being processed by the processor currently. It may also contain the address of the memory location rather than the actual data itself.

Technically, these registers are usually found on the top-most part of the memory hierarchy. This offers a high speed access to the stored data.

KEY TAKEAWAYS

• The processor register is a small storage space that normally contains the data of the ongoing process.
• Typically, the processor registers can have arbitrary names or may even be numbered depending on its design and language policy.
• The two broad categories of CPU registers are the general-purpose registers and the special-purpose registers, which can be further divided into several subgroups.
• The registers on a processor can come in different sizes such as 8, 16, 32, or 64 bits.
• These registers can store a variety of information such as data and Instruction, bit sequences, characters and memory location addresses.

Understanding Processor Register

Typically, every processor comes with a local storage space of its own, which is called a register.

Usually, it is the register of the processor that identifies any kind of data before it is worked on by the CPU.

In simple terms, a processor register is the place to hold any kind of information such as:

• Data
• Instruction
• Memory location address
• Individual characters
• Bit sequence

More commonly, the processor registers are classified into two major categories namely:

• General purpose registers and
• Special purpose registers.

Here, the general purpose registers store data temporarily in them, especially those that are needed by the processes that are being processed currently by the processor.

On the other hand, the special purpose registers typically store instruction counters.

These registers normally hold the address of the subsequent instruction that is to be processed in a sequence.

Ideally, the processor has several index registers. These are also known by different names such as:

• Registers of modification
• Address registers

In the index registers, usually the effective address of an entity is stored. This address can have varied contents such as:

• Base
• Index
• Relative addresses

Another type of processor register is the shift register. Bits enter it from one end and leave from the other.

There are six different types of shift registers found in a CPU such as:

• A serial in – serial out shift register that streams in and streams out data serially
• A serial in – parallel out shift register that converts data in serial manner but sends them out in parallel
• A parallel in – serial out shift register that takes in data in parallel and sends it out serially
• A parallel in – parallel out shift register that takes in and sends out data in parallel
• A bidirectional shift register that can perform data shift in left, right or in both directions
• Counters that send the output to the device as input in such a way that it creates a specific sequence or pattern

Depending on the type of instruction handled by these registers, they can be further categorized into several subgroups such as:

• Conditional
• Data
• Address
• Vector
• Model-specific
• Control
• Status

Since the size of the memory in the registers is small and fixed, if the instruction is large, the process will have to access and use the cache memory and the main memory apart from its own register. This will slow down the performance of the CPU.

Therefore, the number of registers available in the processor and the size of the instructions significantly impact the time taken to execute a code.

Hence, it is necessary to have sufficient registers in the processor to ensure smoother and faster execution of instructions.

Processor Register Examples

There are different examples or types of general and special purpose registers that normally perform most of the particular tasks that the CPU is not able to execute directly, such as index registers, shift registers, vector registers, constant registers and more.

Apart from that, based on the functions performed as well as the values stored by each, the other different types of processor registers are:

• User-accessible registers that include data registers to hold data and address registers to hold memory address specifically
• Status registers that hold the true values that help the processor to determine whether or not an instruction should be carried out
• Floating Point Registers, also known as FPRs, which store floating-point numbers in different architectures
• Constant registers that contain the read-only values such as 0, 1, or pi
• Vector registers that contain the specific data required for vector processing that are typically performed by Single Instruction, Multiple Data (SIMD) instructions
• Internal registers that are used internally during the current processing operations and cannot be accessed by instructions such as the Memory Buffer Register (MBR), which is also called the Memory Data Register (MDR), and the Memory Address Register (MAR)
• Architectural registers that are defined by a specific architecture and are discernible to software but may not match with the underlying physical hardware if it is performing register renaming operation.

A few other types of architectures that also come with data registers, address registers, FP registers and other General Purpose Registers are:

• AT&T Hobbit
• Cray-1
• 4004
• 8008
• 8080
• iAPX432
• 16-bit x86
• IA-32
• x86-64
• Fairchild F8
• Geode GX
• SunPlus SPG
• VM Labs Nuon
• Nios II
• Motorola 6800
• Motorola 68k
• SH 16-bit
• Emotion Engine
• CUDA
• CDC 6000 series
• MMIX
• NS320xx
• Xelerated X10
• Parallax Propeller
• Itanium
• SPARC
• IBM POWER
• Power ISA
• Blackfin
• IBM Cell SPE
• PDP-10
• PDP-11
• VAX    16
• Alpha 31
• 6502
• W65C816S
• MeP
• PIC microcontroller
• AVR microcontroller
• ARM 32-bit
• ARM 64-bit
• MIPS
• RISC-V
• Epiphany

Also, in all CPUs that are x86 compatible, the stack pointer is considered to be integer registers as well.

Processor Register Functions

The processor registers play a crucial role by storing the input given by the users for the CPUs to use and process. These registers also store the processing results, memory addresses, the instructions to be executed, the instructions that are being executed, an execution time counter, and more, apart from performing the basic operations.

The basic operations performed by the processor registers are:

• It fetches the instructions given by the users as well as the instructions stored in the main system memory in an organized manner.
• It decodes the instructions that are fetched and need to be executed. This enables the CPU to know what the instructions are in particular.
• When the instructions are executed by the processor and the results are obtained, it stores them and displays them on the screen when asked.

All the results of computations are also stored in the registers in the interim.

Ideally, the functions of the processor registers can be best understood by their types. A few of them can be explained as follows:

• The Data Register or DR of 16 bits stores the memory operands
• The Address Register or AR of 12 bits stores addresses for the memory
• The Instruction Register or IR of 16 bits stores the instruction codes and instructions read from the memory
• The Program Counter of 12 bits stores the instructions
• The Temporary Register or TR of 16 bits stores temporary data during operation
• The Input Register or INPR of 8 bits stores the input character
• The Output Register or OUTR of 8 bits stores the output character
• The Stack Control Register or SCR stores sets of memory blocks and follows FILO or First In Last Out principle for storing and retrieval of the same
• The Memory Buffer Register or MBR stores the data and instructions read from the memory before transferring the instructions to the IR and data to I/O register or the Accumulator
• Flag Register or FR of one or two bytes validates the diverse occurrences of a state in the CPU
• Instruction Pointer Register or IPR stores memory addresses that are to be executed in the next level in a proper sequence

Processor Register Size

There can be different sizes of the processor registers, which are typically measured in bits. These are 8 bits, 16 bits, 32 bits, or 64 bits. In some cases, it can be even more.

The size of the processor registers in bits indicates the amount of data it can hold. The two most commonly used terms that typically indicate the size of the register on a CPU are:

• 32-bit processors and
• 64-bit processors.

This means that in a 32-bit processor, each of the registers in it will measure 32 bits in width, which means that they will be able to manipulate 32 bits of data in one time. As for the 64-bit processors, things are just double that.

However, there may be a few specific types of instruction sets where the registers can operate in a diverse range of modes.

This helps them to break down their storage memory into even smaller parts. For example, a 32-bit memory can be broken down into four equal parts of 8 bits each.

Though this does not increase the overall size of the storage of the registers, it surely helps in different aspects, including but not limited to the following:

• Being able to load multiple data types such as a vector or one-dimensional set of data and
• Being able to load and operate on them at the same time.

These additional bits are however implemented typically by adding extra registers. These additional registers help in mapping the memory of these smaller registers into the larger register.

Is the Processor Register a Memory or a Storage?

Ideally, you can say that the processor register is more of a storage space than a memory due to the primary difference between the two.

This register stores the data that the CPU is using currently to process an instruction. On the other hand, a memory stores the data that will be needed for processing any instruction in the near future.

Apart from that, a CPU register is typically much smaller in size and in data holding capacity in comparison to a regular memory, being just 32-bits or 64-bits as opposed to the memory capacity that often can go up to several gigabytes or terabytes.

Therefore, you can say that the register is the smallest storage space of a computer that resides within the processor to hold the specific elements that are typically built into the CPU itself.

They typically store the operands, instructions, memory locations and units of it that can be accessed easily, quickly and directly by the CPU.

It allows the processor to use it to transfer and store data for immediate use by it in processing data.

Register vs Counter

• A register refers to a group or combinations of D-type, edge-triggered Flip Flops, but a counter is an example of a register and a connection of Flip Flops.
• A register stores binary information that can be manipulated when needed, but a counter is used for counting binary and decimal number systems or electrical pulses for practical applications.
• A register typically does not follow any particular sequence of states if it is not designed for performing a specific application, but the counter is bound to follow a sequence of states.
• A register will have the same clock, but a counter would not.

What are Processor Registers Made of?

More often than not, the registers on the processors are made from Static Random Access Memory (SRAM) or Dynamic Random Access memory (DRAM) cells.

One of the most significant differences between these two memory cells is that the SRAM cells are much faster at providing access in comparison to the DRAM cells.

What is the Need for CPU Registers?

The registers are needed by the processor to quickly access and use the data stored in them.

This expedites the speed of operation of the CPU and makes it much smoother and more meaningful.

These registers are also required to ensure that the performance of the compilers is optimized to increase the efficiency of code generation.

Conclusion

In the processor of the computer there are several registers, each of which performs specific functions with the common goal to help the CPU in performing its functions at the best possible level.

Through this article, you now have a fair amount of knowledge about the CPU registers, their types, functions and more.