Main Store
A computer’s main store usually consists of integrated circuit chips. There are several types:
RAM-RANDOM ACCESS MEMORY
1 Random access memory is store which data can be written to and read from at high speed.
2 The processor accesses a location by setting the correct address for it. Decoding circuitry makes sure that the right location is accessed.
3 Usually RAM is volatile and loses data when switched off.
Uses of RAM
1 As the working store of the computer.
2 As buffers holding data being transferred to and from peripherals.
3 To store the contents of the screen in microcomputers.
Fig 1 A section of semiconductor random access memory (RAM)
How RAM works
Each bit of the store is represented in a chip as a separate storage cell. The cells are arranged in a lattice (i.e. a squared pattern) (see Fig 1). Usually the hits which make up a particular storage location are each on a different lattice. Often they are on different chips. This is so that the bits for a location can all be accessed at the same time.
The lattice has X-select lines going one way and Y-select lines going the other. A cell is accessed by sending a signal along the X-select and the Y -select lines which pass through it.
ROM-READ ONLY MEMORY
1 Read only memory is store that can be read but nothing can be written on to it.
2 The ROM in the main store is divided into locations which are addressed in the same way as for RAM.
3 it is nonvolatile-the programs and data in it are stored there permanently.
Uses of ROM
ROM is used to store programs essential to the normal running of the computer. Usually cheap microcomputers have more programs on ROM than do large computers. This is because large computers usually have very fast backing store so that programs can be loaded easily.
Programs which might be stored on ROM include:
1 The computer’s operating system or a program to load the operating system automatically .
2 Utility programs .
3 An interpreter or a compiler for a high-level language .
4 A word processor program .
PROM-PROGRAMMABLE READ ONLY MEMORY
Programmable read only memory is a form of ROM which is manufactured without anything stored on it. Programs or data can be written into and it then becomes permanent as read only memory.
EPROM-ERASABLE PROGRAMMABLE READ ONLY MEMORY
The contents of erasable programmable read only memories can be erased at any time using a special EPROM eraser. (One method involves shining ultraviolet light through a small window on the top of the chip-see Fig 3.)
New programs or data can then be written on to the EPROM using a device called an
EPROM programmer.
Uses of EPROMS
1 To store programs which might need to be changed.
2 To copy the contents of existing ROMs. (This should not be done without the copyright owner’s permission.)
THE ROLE OF RAM AND ROM
A computer contains both ROM and RAM:
1 In powerful computers with fast backing storage devices there is not much ROM. The ROM just contains a program to load the operating system automatically when the computer is switched on. The operating system and all other programs are loaded from backing storage Âusually disc. This makes the computer completely adaptable as to which software is used in it.
2 In cheap microcomputers a lot of the software is on ROM-operating system, utilities, language interpreters, word processor packages, etc. This means that the computer has all of these facilities stored in it permanently. They can be accessed without fuss and without expensive backing storage.
3 Dedicated microprocessor . If a microprocessor is dedicated to a control task there is no need to load the program into it from backing storage. The program can be stored on ROM and the complete computer consists of the processor, some RAM, some ROM and interfaces to the devices being controlled.