The computer
uses a number of special memory units called registers to speed up the rate of
information transfer between various units of the computer. They are not
treated as part of main memory and are used to retain information on a
temporary basis. The number of registers varies from computer to computer. Most
computers use several types of registers each designed to perform a specific
function. These registers can receive information, retain it temporarily and
pass it on as directed by the control unit. The length of a register depends on
the number of bits it can store. Thus a register which can store 16 bits is
normally referred to as 16-bit register. Although the number of registers
varies from computer to computer, some registers are common to all computers
such as instruction register, program counter register, etc.
Types
of registers
1.
Memory address register.
This holds the
address of the active memory location. When an instruction is read from memory,
the address is loaded from the program into this register.
2.
Memory buffer register.
It holds the contents of the memory word read
from or written in memory. A word to be stored in memory location must first be
transferred to this register from where it is written in memory.
3.
Program counter register.
It holds the address of the next instruction
to be executed by the computer.
4.
Accumulator register.
It holds the initial data to be operated upon,
the intermediate results and also the final results of processing operations.
5.
Instruction register.
It holds the
current instruction that is being executed.
6.
Input - output register.
This holds all input information to be passed
to the memory and also the output information to be transferred to an output
device.
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