MEMORY

Main Memory is located directly on the computer’s main circuit board, so that data can travel quickly to and from the CPU.  Data is stored at a specific location, called an address.  The contents of an address can be accessed to be read from, written to or processed.

TYPES OF MEMORY

1. Random Access Memory (RAM) – Inside a computer, you will find one or more memory chips called RAM.  These hold temporary operating instructions for the computer, its programs and the data. This is the place where the CPU receives instructions and data to do its job.

Advantage: the computer can access data held in RAM almost immediately.

Disadvantage: RAM is volatile, i.e. data held in RAM is lost when the computer is turned off or malfunctions.

Capacity: As computer programs and operating systems become more complex, the size of RAM must be increased. Older computers come with 128 Mb, 256 Mb or 512 Mb. More recently, the size of RAM can be as much as 4 Gb.

2. Read Only Memory (ROM) – ROM chips hold data and instructions necessary for starting up the computer.  The information is fixed at the time of manufacture and is sometimes described as hard wired.

Advantage: ROM is non-volatile, i.e. the instructions are available every time the computer is switched on. These instructions can only be read and cannot be changed.

3. Programmable ROM (PROM) – This is one step up from ROM.  Once PROM has been programmed with data its contents cannot be changed. They usually do not have any data locked into it upon manufacture. However, a company using those PROMs to make a computer can put its own information. If the code or data stored in PROM must be changed, the device must be discarded. PROMs are ‘one-time programmable’ devices.

4. Erasable and Programmable ROM (EPROM) – EPROM is programmed in exactly the same manner as PROM.  However, EPROMs can be erased and re-programmed repeatedly. An EPROM is erased by exposing the device to strong Ultraviolet (UV) light and this resets the chip to its initial, un-programmed state. EPROMs are more expensive than PROMs.