Primary Memory
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.
- It loads the word into the memory data register (MDR)
- It loads the address of the memory location into the memory address register (MAR)
- 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.
- CPU loads the address of the memory location into the memory address register (MAR)
- It issues a signal, read, to indicate that the word whose address is in MAR is to be read into MDR
- 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)
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