Primary memory is also called main memory. It is a primary storage device of a computer system. It is a semiconductor device placed on the motherboard of the computer.

Memory Unit

A memory unit is a collection of storage registers and associated circuits that transfer information in and out of the registers. The storage registers in a memory unit is called memory registers. The binary cells of memory registers are small in size and have two-state property for binary representation. Memory registers are cheap but faster.

Memory Word

A memory unit stores group of binary information called words. Each word is stored in a memory register. A memory word contains n bits that moves in and out of storage unit. It may be an operand, an instruction, a group of alphanumeric characters or any binary coded information.

Read and Write Operations

A unique address is assigned to each word so that it can be moved in and out of the memory. This address is used to determine the memory location in which a given word is to be stored. This is called memory write operation. Similarly the address is used to determine the memory location from which a word is to be retrieved from the memory. This is called memory read operation. The CPU performs both read and write operations.

CPU performs the following steps to write a word into a memory location.

1. It loads the word into the memory data register (MDR)
2. It loads the address of the memory location into the memory address register (MAR)
3. It issues a signal, write, to indicate that the word in MDR is to be stored in the memory location whose address is in MAR

The following steps are required to read a word from a memory location.

1. CPU loads the address of the memory location into the memory address register (MAR)
2. It issues a signal, read, to indicate that the word whose address is in MAR is to be read into MDR
3. The memory loads the required word into MDR

Types of Primary Memory

The primary memory can be volatile or non-volatile. The content of the volatile memories will get erased once the power is turned off. The non-volatile memories retain the content even if the power is turned off.

There are two types.

Read Only Memory (ROM)

Random Access Memory (RAM)