COA-Computer Registers

Within the processor , there is a set of Registers which are very fast but hold only small amount of data, in order to speed up its operation by providing quick access to commonly used values.Because they are semiconductor devices whose contents can be accessed  at extremely high speed.And the most important thing about Registers is that Registers generally hold data temporarily, during execution of a program.And Registers are normally measured by the number of bits they can hold.

As an example: An 8-bit register means, it can store 8 bit of data, 32-bit register means it can store 32 bit of data.

Now, generally, Registers in the processor perform following two roles:

1. Enable the machine or assembly language programmer to minimise main memory reference by optimizing use of Registers.
2. Used by control unit to control the operation of the processor or by operating system programs to control the execution of programs.

Now, we will discuss about various type of registers that are commonly used in processors.These are:

1.Accumulator Register(AC):

    Accumulator Register is a general purpose Register.This Register is required for doing operation             on data.As this register holds the initial data to be operated on, the intermediate result and the final         result of processing operation.Means, the result of arithmetic operation are transfer to AC if no                 specific address of result operation is specified.

    And the number of bits that accumulator register contain is equal to the number of bits per                       word.That is, if word is of n bits, the AC is also of n bits.

2.Program Counter(PC):

    Program Counter(PC) holds the address of next instruction to be read from memory after the                   current instruction is executed.Means, Program Counter generally acts as a pointer which points to         the memory location where the next instruction is stored.

    And the no. of bits in PC, that is, the size of PC is equal to the width of the memory address.

3.Temporary Register(TR):

    Temporary Register(TR) is used for holding data during the processing.And since Temporary                   Register hold data, so, the no. of bits it contain is equal to no. of bits of word(data word).

4.Instruction Register(IR):

    Instruction Register(IR) is used for storing the instruction.Means the instruction register hold the               current instruction that is being executed.And since it contain instruction, so, the no. of bits(size) of         Instruction Register is equal to the no. of bits of instruction, and the size of instruction is n bit for               n-bit CPU.

5.Data Register(DR):

    The Data Register hold the operand from memory.Means when an operand is found, using either             direct or indirect addressing, it is placed in Data Register(DR).The processor then used in this                 value as data for its operation.

    Its size is equal to the size of word in memory.

6.Address Register(AR):

    The Address Register hold the address of memory word that is the Address Register contain the             address of memory location or Register from where data is stored or retrieved.Since it contain                 address, so the size of Address Register is equal to the width of memory address is directly related         to size of memory, 

    Means, if a memory is of size 2ⁿ *m , then, n bits are used to specify address.

7.Input Register(INPR):

    Input Register(INPR) hold(or receives) data from an input device.And the size of input register is             depend on the alphanumeric code that followed in computer, that is, if ASCII is followed, then the             size of INPR is 7 bits and if EBCDIC is followed, then the size of INPR is 8 bits.

8.Output Register(OUTR):

    Output Register(OUTR) hold data that need to be sent to an output device. Its size is also depend           on the alphanumeric code that followed in computer.

Now, thus, according to Basic computer, whose CPU is of 16 bit and memory is of 2¹²*16=4096 words, of 16 bits each, and according to definition of each type of register we discussed, the no. of bits allocated to each register is depicted in Figure below.Here we assume that EBCDIC code is followed in computer.