Primary Storage Devices

Primary Storage, also known as main storage or memory, is the main area in a computer in which data is stored for quick access by the computer’s processor. Information must be transferred to primary storage. On today’s smaller computers, especially personal computers and workstations, the term random access memory (RAM) – or just memory – is used instead of primary, main storage, core memory, or immediate access storage, internal storage, and the hard disk, diskette, CD, and DVD collectively describe secondary storage or auxiliary storage.

It allows very fast access to data. It is relatively move expensive. A CPU has three element or parts primary storage, arithmetic logic unit (ALU), and control unit. Primary storage is fast and expensive as compared to secondary storage.

Primary storage device are :-

• RAM (Random Access Memory)
• ROM (Read Only Memory)
• PROM (Programmable Read Only Memory)
• EPROM (Erasable Programmable Read Only Memory)

1. RAM (Random Access Memory)

RAM (Random Access Memory) is the hardware in a computing device where the operating system (OS), application programs and data in current use are kept so they can be quickly reached by the device’s processor. RAM is the main memory in a computer, and it is much faster to read from and write to than other kinds of storage, such as a hard disk drive (HDD), solid-state drive (SSD) or optical drive.

Random Access Memory is volatile. That means data is retained in RAM as long as the computer is on, but it is lost when the computer is turned off. When the computer is rebooted, the OS and other files are reloaded into RAM, usually from an HDD or SSD.

Function of RAM

Because of its volatility, RAM can’t store permanent data. RAM can be compared to a person’s short-term memory, and a hard disk drive to a person’s long-term memory. Short-term memory is focused on immediate work, but it can only keep a limited number of facts in view at any one time. When a person’s short-term memory fills up, it can be refreshed with facts stored in the brain’s long-term memory.

A computer also works this way. If RAM fills up, the computer’s processor must repeatedly go to the hard disk to overlay the old data in RAM with new data. This process slows the computer’s operation.

Types of RAM

RAM comes in two primary forms:

(i) Dynamic Random Access Memory (DRAM)

Dynamic Random Access Memory (DRAM) makes up the typical computing device’s RAM and, as was previously noted, it needs that power to be on to retain stored data.

Each DRAM cell has a charge or lack of charge held in an electrical capacitor. This data must be constantly refreshed with an electronic charge every few milliseconds to compensate for leaks from the capacitator. A transistor serves as a gate, determining whether a capacitor’s value can be read or written.

(ii) Static Random Access Memory (SRAM)

Static Random Access Memory (SRAM) also needs constant power to hold on to data, but it doesn’t need to be continually refreshed the way DRAM does.

In SRAM, instead of a capacitor holding the charge, the transistor acts as a switch, with one position serving as 1 and the other position as 0. Static RAM requires several transistors to retain one bit of data compared to dynamic RAM which needs only one transistor per bit. As a result, SRAM chips are much larger and more expensive than an equivalent amount of DRAM.

However, SRAM is significantly faster and uses less power than DRAM. The price and speed differences mean static RAM is mainly used in small amounts as cache memory inside a computer’s processor.

2. ROM (Read Only Memory)

ROM stands for Read Only Memory. The memory from which we can only read but cannot write on it. This type of memory is non-volatile. The information is stored permanently in such memories during manufacture. A ROM stores such instructions that are required to start a computer. This operation is referred to as bootstrap. ROM chips are not only used in the computer but also in other electronic items like washing machine and microwave oven.

Advantages of ROM

The advantages of ROM are as follows

• Non-volatile in nature
• Cannot be accidentally changed
• Cheaper than RAMs
• Easy to test
• More reliable than RAMs
• Static and do not require refreshing
• Contents are always known and can be verified

3. PROM (Programmable Read Only Memory)

PROM or programmable ROM (programmable read-only memory) is a computer memory chip that can be programmed once after it is created. Once the PROM is programmed, the information written is permanent and cannot be erased or deleted. PROM was first developed by Wen Tsing Chow in 1956. An example of a PROM is a computer BIOS in early computers. Today, PROM in computers has been replaced by EEPROM.

When the PROM is created, all bits read as “1” and during the programming, any bit needing to be changed to a “0” is etched or burned into the chip using a gang programmer. Below is an example of a gang programmer from Advin that programs multiple ROM chips at one time.

If a PROM has been programmed and has an error or needs to be updated, the chip is discarded and a new PROM is created, replacing the old chip. A variation of the PROM is an EPROM, which is a PROM that can be erased and reprogrammed without being replaced.

4. EPROM (Erasable Programmable Read Only Memory)

Erasable Programmable Read-Only Memory, EPROM is a non-volatile memory chip that was invented by Dov Frohman in 1971 while at Intel that can only be read. If exposed to ultraviolet light, an EPROM can be reprogrammed if needed, but otherwise does not accept or save any new data. Hardware manufactures use EPROM when it may be needed that the data contained on the EPROM needs to be changed. An EPROM chip is distinguishable by a small quartz crystal (not glass) circle window that exposes the chip so that can be reprogrammed.

EPROM (erasable programmable read-only memory) is programmable read-only memory (programmable ROM) that can be erased and re-used. Erasure is caused by shining an intense ultraviolet light through a window that is designed into the memory chip. (Although ordinary room lighting does not contain enough ultraviolet light to cause erasure, bright sunlight can cause erasure. For this reason, the window is usually covered with a label when not installed in the computer.)