A memory unit is a collection of storage registers and the necessary circuits needed to transfer information in and out of the registers as and when required. A memory unit can store information in T and ‘O’s i.e., in bits. A group of bits together which can be stored in a memory unit is known as WORD. Each word is stored in one memory register. A group of eight bits is known as a byte.
Word length is defined as the number of bits present in the computer memory word. The word length indicates the number of bits the CPU can process at a time. The memory size indicates the total storage capacity of the CPU. The word length is indicated in bits or bytes whereas the memory is indicated in kilo or mega bytes. The word in a memory unit can represent a number, an instruction code, alphanumeric characters or other information. The selection of a specific word inside the memory is done by giving the address of the word on the selection lines.
Bit Lengths of Registers of the CPU:
The digital function commonly used for holding binary information in a digital computer is called a register. A register is a group of binary storage cells. Hence the register size is denoted depending upon the number of bits it can handle i.e., the word length is the indication for the bit length of each register. For an 8-bit processor the accumulator can handle 8-bits at a time therefore it is a 8-bit register. Generally the general purpose registers will also have the same bit length as that of the accumulator. Program counter is a 16-bit register i.e., double the word length of the processor. Similarly stack pointer is also a 16-bit register. Depending upon the processors bit handling capacity, other register sizes are estimated.