ROM, RAM and HDD design

Written by Dean Barnard

Copyright (c) 2007 Dean Barnard


ROM, Read only Memory

The data or information stored in these types of memories is not changeable. It therefore does not require electrical power to retain information. The ROM chip or IC contains rows and columns of diodes. The presence or absence of diode in a particular row defines weather it is a binary one or zero. The content of a ROM is permanent and care has to be taken while designing it. It is not possible to rewrite a ROM. If change is necessitated, the whole chip has to be discarded. But once properly designed, they are robust, consume less power and cheap to manufacture.

The different types of ROM are:

1.PROM, Programmable Read only Memory
The design of ROM is made easier by the PROM. PROM contains a fuse at the junction of each row and column of the grid. In the initial stage the complete grid represents a binary one. When an electric charge is passed through it, it ruptures the fuse and becomes a zero.

2.EPROM Erasable Programmable Read only Memory
This is an improvement on PROM. Ultraviolet light is used to erase the information. This is a tedious process and excess exposure may lead to destruction of the ROM.

3.EEPROM, Electrical Erasable Programmable Read only Memory
This kind of ROM is popular in memory sticks. The required portion of the memory can be erased or written on in a selective basis.

RAM, Random Access Memory

This type of memory is essential for the working of a computer. It is a integrated chip consisting of rows and columns of a pair of transistor and a capacitor forming a grid. If the capacitor is charged at 50 % or more, it reflects a binary one. Due to the nature of a capacitor (it discharges at a very fast rate), it is required to charge them periodically. This is called a refresh cycle. In addition to the charging circuitry, it consists of a control circuit which is used to read and write data. Since the data can be accessed and written dynamically, these are called dynamic random access memory. The combined working of all the devices makes it slow in working.

Another type of RAM is the static RAM, where the integrated chip consists of a series of Flip-flops instead of a combination of a transistor and a capacitor. In the absence of a capacitor, there is no need to cater for the refresh cycle and this makes the static RAM considerably faster than a dynamic RAM. A Flip-flop consists of a few transistors and therefore the cost of a static RAM is higher, though it is compact.

Hard Disk Drive (HDD)

A hard disk drive is used to store vast amount of data. The data access time and speed are not critical for this device. The speed of a HDD is specified by the RPM or the rotations per minute of the rotating disc. For an IDE based HDD, the RPM is 5400 or 7200. Faster hard discs with better access time are called SCSI drives which can attain much faster speeds. Since, the HDD works on the principle of rotating disc, its efficiency is restricted by the speed of rotation of mechanical components. Transporting a HDD while in operation may cause a crash and damage the disc.

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