1.4 TYPES OF MAIN MEMORY

The data and instructions which are entered into the computer system through input devices (keyboard, mouse etc.) have to be stored inside the computer before the actual processing starts. Similarly, after processing, the results produced by the computer must be kept somewhere inside the computer system before being passed onto the output devices (monitor/screen or printer). Thus, the storage unit (primary/main memory) of a computer system is designed to take care of all these needs.

The memory unit is the electronic device that holds data and instructions for processing.

It consists of binary cells (i.e., 0 or 1 signals) to store the information. Thus,

 • the memory unit that directly communicate with the CPU is called the primary storage or primary memory or main storage or main memory.

• it basically stores the necessary programs of system software, which are required to execute the user’s program.

• when we load software from floppy disk, pen drive, hard disk or CD-ROM, it is stored in the main memory.

• there are two types of computer (main) memory inside the computer (as shown in Figure 9):

1. RAM (Random Access Memory)

2. ROM (Read Only Memory)

1.4.1 Random Access Memory (RAM)

• Random Access Memory (RAM) is really the main store and is the place where the program and software we load gets stored. When CPU runs a program, it fetches the program instructions from the RAM and carries them out. Similarly, if the CPU needs to store the final results of calculations, it stores those in RAM. Thus, you (CPU) can both READ data from RAM and WRITE data into the RAM.

• If your computer has large RAM, you can run larger programs.

• RAM is known as volatile memory, that is, the stored data are lost, if the power goes off. The following Figure 10 shows a common RAM chip.


Mother board is a microcomputer circuit board that contains slots for connecting peripherals like RAM modules, CPU and adapter cards. Motherboards also have electronic circuitry for handling such tasks as I/O signals from those peripheral devices.

A motherboard is the backbone of a computer system: The power of a PC highly depends on the peripherals that its motherboard supports.

In older days, the first home PC’s uses a 64 KB of RAM memory. Today’s modern computers need a minimum of 64 MB (recommended 128 MB or 256 MB) to run Windows or Linux Operating system with latest number of software. A RAM memory chips come in many different sizes ranging from 1 MB to 2 GB.

1.4.2 Read Only Memory (ROM)

• A Read-Only memory (RAM) is one in which information is permanently stored. Computer almost always contains a small amount of Read-Only Memory (RAM).

• ROM is used for storing special set of instruction, which the computer needs when it start up (boot up).

• Unlike RAM, the information from ROM can only be READ and it is not possible to WRITE fresh information to it. That is the CPU can only fetch or READ instructions from ROM. This is the reason why it is called ROM.

• When we switch the computer off, the contents of the ROM does not get erased. Therefore, it is non-volatile memory. Thus, a Read Only Memory (ROM) is one in which information is permanently stored. The following figure shows a relationship between the CPU and main memory (RAM and ROM).

1.4.3 Programmable Read Only Memory (PROM)

• A variation of ROM chip is programmable read only memory (PROM). A PROM is a memory chip on which data can be written only once.
• ROM chips are supplied by computer manufacturer and it is not possible for a user to modify the programs stored inside the ROM chip. However, in case of PROM, it is possible for a user to customize a system by storing own program in a PROM chip.
• Once a program has been written on to a PROM chip, the recorded information cannot be changed i.e., the PROM becomes a ROM and it is only possible to read the stored information.
• PROM is also a non-volatile memory i.e., the stored information remains even if power is switched off. 
• The basic difference between PROM and a ROM is that a PROM is manufactured as blank memory, whereas a ROM is programmed during the manufacturing process. To write data on a PROM chip, you need a special device called a PROM programmer or a PROM burner. The process of programming a PROM is sometimes called burning the PROM.

1.4.4 Erasable Programmable Read Only Memory (EPROM)

• Once the information is stored in a ROM chip or a PROM chip, it cannot be changed. However, there is another type of memory chip called erasable programmable read-only memory (EPROM) that overcomes this problem.
• As the name implies, it is possible to erase information stored in an EPROM chip and the chip can be reprogrammed to store new information by using a special device called a PROM programmer or a PROM burner.
• The information stored in an EPROM chip is erased by exposing the chip for some time to ultraviolet light.
• When an EPROM chip is in use, information can only be READ and the information remains on the chip until it is erased.
• An EPROM differs from a PROM in that a PROM can be written to only once and can not be erased.
• EPROMs are widely used in personal computer they enable the manufacturer to change the contents of the PROM before the computer is actually delivered. This means that bugs can be removed and new versions installed shortly before delivery.

 

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