Functions, types, and storage capacity of main memory

Main memory is also known as the working memory of a computer, internal memory, or primary storage. Its principal function is to hold data and programs temporarily when they are being processed. It seat between the processor and the secondary storage (hard disk). The computer main memory is fixed on a memory slot within the motherboard

It is volatile storage since it stores data only when it has power, but when the power goes off it losses every data that it had.

The main function of RAM is to temporarily hold data that is being processed in the computer processor. Types of primary memory include SRAM, DRAM, and ROM.

Functions of main memory (RAM)

  1. Holds data currently being processed.
  2. Stores intermediate results of processing.
  3. Stores the supervisor part of the operating system (kernel). It is loaded when the computer starts and remains there.
  4. Holds data being transmitted from one input device to another.
  5. Keep a record of the address of all data within the memory.

Primary memory is read and writes memory that has a fast access speed which allows processors to access data from it faster than from secondary storage.

Computer Main Memory

Primary memory storage capacity

Computer memory is calculated using bytes and the current computer has memory in Gigabyte size. Computers that have processor architecture of X86 had a maximum of 4GB of primary memory. With the X86-64 processors, they allow a memory address of upward of 4GB. Some computers can support 128GB. 

In 2017 Hewlett Packard (HP) unveiled a 160TB RAM computer which is the world’s largest single memory computer. The computer is part of new computing technology they are calling memory-driven computing to deal with big data.

Types of computer primary memory

Computer main memories are classified into 2 classes

1. Random Access Memory (RAM)

RAM is where all the data and programs being processed by the processor are held temporally. It is the working memory of the computer. There are 2 types of RAM according to the technology used to manufacture.

Static RAM (SRAM)

It uses transistors to store the data and does not require refreshing because once a flip-flop is set it doesn’t reset until it receives another electrical signal to say so. The transistors are arranged in a flip-flop manner either a 0 (flip) or 1 (flop).

Their main advantage is that they don’t require any refreshing and they are faster than DRAM but they are expensive to manufacture. Mostly they are used in cache memory.

Dynamic RAM (DRAM)

Instead of DRAM using transistors they use capacitors to store electrical charge. Since electrical charges have a short life span they require a system to keep them refreshed which means additional circuitry. The presence or absence of an electrical charge determines if what the memory holds is either a 1 or 0. The technology used is cheaper than the one used in SRAM but the refreshing factor lowers the performance. Mostly they are used in system RAM and Video RAM (VRAM).

Read Only Memory (ROM)

These are the type of memory that stores data and programs either permanently or semi-permanently by re-programmed. They are used by computer manufacturers to store data and information that they don’t want the user to tamper with. This includes the BIOS information, the POST program, and any other that users don’t need to change at all or frequently.

Types of Read Only Memory (ROM)

Programmable ROM (PROM)

These ROMs allow to be written on only once and they cannot be deleted later to have some other data in them. It can be programmed only once and that’s all.

Erasable Programmable ROM (EPROM)

These ROMs allow erasing of the content using Ultra Violet light and then re-programmed them again with new data. Since also sunlight has Ultra Violet light it is not advisable to expose them to the sun because it may alter the data.

Electrical Erasable Programmable ROM (EEPROM)

These are the main types of ROM used in many computers today because they allow users to upgrade them as long as they have the right upgrade i.e. BIOS upgrade. Most of these upgrades can be downloaded from the manufacturer’s company website. These ROM are also called flash memory because you can remove content when you need to. They can be reprogrammed electrically many times.


The cache is a very fast access memory that is used to store the most frequently requested data required by the processor. Since the processor is faster than RAM the speed of access slows down how fast the processor can access data.

To solve the issues of slow primary memory compared to the processor cache memory is used. Cache memory comes in between the RAM and the processor. There are two types of cache memory:

Internal or primary cache (Level 1)

This is the cache memory that is in-built into the processor chip. It is faster because it is inbuilt within the processor. The integration makes it more expensive hence capacity is limited.

External or secondary cache (Level 2)

This is built on the motherboard instead of the processor itself. This makes it less cheap but it is not fast compared to the primary cache. Some systems may have more than one secondary cache memory. 

Differences between RAM and ROM.

Random Access Memory (RAM)

Read Only Memory (ROM)

Store data temporarily

Store data semi-permanently/permanently



Read and write

Not all that is read and write

Example DRAM and SRAM


Used for Primary Memory

Used for BIOS and Firmware

Computer memory hierarchy 

Memory can be arranged in the hierarchy depending on several factors such as access speed, capacity, cost per byte, and volatility. The diagram below gives a summary of how memories are arranged.

computer Memory hierarchy diagram

  1. Level 0 are CPU registers that have a small capacity, are costly per bit of storage, and have very high data access speed.
  2. Level 1 (Cache): they are near to CPU after registers and have high access speed and low capacity compared to RAM.
  3. Level 2 is RAM or primary memory. They are slower than both register and cache but have high storage capacity.
  4. Level 3 Magneti disk (Hard disk). They are the main secondary storage of computers. Magnetic disks have a big storage capacity but they don’t have direct access to the CPU. They have low access speed.
  5. Level 4 magnetic tape and Optical disk. They have very low access speed but have high storage capacity. In today’s world of high magnetic disk and flash memory, these storage devices are not common.

 Which are the computer’s main memory manufacturers?

  1. Intel Corporation: it is an American corporation with deals with most computing electronic components,
  2. Kingston Technology Company: It is an American company with a global headquarter in Fountain Valley California,
  3. Samsung Electronics: It is a South Korean company based in Suwon. It deals with many computing devices including main memories manufacturing.
  4. Phison Electronics Corp.:It was established in 2000 in Hsinchu Taiwan.