Memory Classification in Computer Architecture

Memory Classification – The memory is technically any form of electronic storage. It is used to identify fast and temporary forms of storage. Although the computer memory is simple in concept but exhibits the widest range of types, technologies, organisation, performance and cost of any feature of a computer system.

No single technology is optimal in satisfying the memory requirements for a computer system. Therefore, a computer system has a hierarchy of memory subsystems. Out of these subsystems of memory. Some are internal to the computer, and some are external.

The internal memory of a computer is directly accessible by the processor, whereas external memory is accessible by the processor through an input-output module. The object of every memory system is to provide adequate storage capacity with an acceptable level of performance and cost. 

Memory Classification

The memory of any computer system consists of two sections: 

1. Internal memory section for storage of active data and instructions. 
2. External memory section for long-term storage.

These two sections are further divided into various types of memories. Figure belows shows the various types of semiconductor memories discussed in the next articles.

The above figure shows various types of memories. These can be classified in a number of ways. There are three basic ways for classification:

1. Volatile and Non-Volatile Memory
2. Primary and Secondary Memory
3. Static and Dynamic Memory 

Volatile and Non-Volatile Memory

The memory that loses its contents when the power is turned off is called volatile memory. Examples of volatile memories are all types of random access memories (RAM) except the CMOS RAM. Whereas the non-volatile memory is a memory that can retain the stored information even when not powered. Examples of non-volatile memory include ROM, flash memory and magnetic memory.

The volatile memory is used as primary memory, but the non-volatile memory is typically used for the task of secondary memory. Most forms of non-volatile memory have limitations that make them unsuitable for use as primary storage. Typically, non-volatile memory either costs more or performs worse than volatile random access memory.

Primary and Secondary Memory

The computer system memory can be classified as primary and secondary memory. The primary, also known as the main memory, is a part of the main computer system. The processor directly stores and retrieves information from it. This memory is accessed by the CPU in a random fashion. That means any location of this memory can be accessed by the processor to either read information from it or to store information in it 

The primary memory itself is implemented by two types of memory technologies: (1) Random Access Memory (RAM) and (2) Read-only Memory (ROM). A more appropriate name for RAM is RWM (Read Write Memory). The CPU can write and read information from any primary memory location implemented using RAM. The other part of primary memory is implemented using ROM.

Primary memory is much faster, and it is more cost-effective. But the secondary memory is much slower and also less costly. It stores the data permanently unless it is erased. The secondary memory is usually available in the form of a floppy disk, a hard disk, a CD, a DVD, a pen drive, memory chips, etc. The data on a floppy disk is organised in terms of tracks and sectors. A hard disk is made up of a large number of platters and can have a large capacity. A hard disk is usually much faster than a floppy disk. CD can store upto 750 MB of data. Information on CD-ROM is organised in terms of spiral tracks. A DVD is a Digital Versatile Disk and can store 4.6GB of information.

Static and Dynamic Memory

The random access memories are classified as static and dynamic random access memories. Static Random Access Memory (SRAM) is a type of semiconductor memory where the word static indicates that it does not need to be periodically refreshed. SRAM is a volatile memory.

The Dynamic Random Access Memory (DRAM) is a type of memory that stores each bit of data in a separate capacitor within an integrated circuit. Since real capacitors leak charge, the information eventually fades unless the capacitor charge is refreshed periodically. Because of this refresh requirement, it is a dynamic memory as opposed to SRAM. The advantage of DRAM is its structural simplicity; only one transistor and one capacitor are required per bit compared to four transistors in SRAM. This allows DRAM to reach very high density. It is also a volatile memory and loses its data when the power supply is removed.

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