Read Only Memory

ROM stands for Read Only Memory. As the name implies the ROMs are designed in such a way that data can only be read out during normal operation. The ROM is written information during the manufacturing process. ROMs are non-volatile so they have non-destructive read out feature. Therefore monitor program which must be present immediately after power on is store in ROM. These memories are also used to hold frequently used programs such as subroutines and tables.

Figure(a) shows the internal block structure of a ROM. It has n address lines. The ROM chip can have 2ⁿ registers as there are 2ⁿ possible combinations of n binary digits. The register is identified by one of 2ⁿ output lines of the internal address decoder.

Applications of ROM

• ROM is used to store Basic Input-Output System (BIOS).
• ROMs are used in constructing look-up tables for computer systems. Look-up table are used to store logarithms of number, sines and cosines of angles, fixed physical constants and so on.
• ROM is used for stroing various sequences. This helps sequence generator to perform sequence generation.

Types of ROMs

ROMs are available in number of forms.

Mask-Programmable ROM

These devices are programmed at the factory as a part of the manufacturing process. According to user’s specification, the manufacturer makes special masks for fabrication of the ROM chip. The main drawback of this type of memory is that once fabricated the content cannot be altered.

PROMs

Programming of Programmable Read Only Memory (PROM) can be done by any user by blowing out fusable nicrome wires links inside the devices with the help of a PROM programmer. Although the programming can be done by an user it has the disadvantage that it can be done only once. No reprogramming is possible. So PROM is referred to as One-time programmable (OTP). This facility provides more convenience and flexibility to the user. PROMs are generally used in the early stages of product development. They are costlier than the Mask-PROMs but cheaper than other types of ROM.

EPROMs

The content of Erasable Programmable Read Only Memory (EPROM) can be erased by exposing the device to ultra violet light for period of 10 to 20 minutes through an optical window provided on the chip. Reprogramming is then possible with the help of an EPROM programmar.

EPROMs are generally used for system development and low-volume production. Normally the optical window is covered with opaque tape so that inadvertent erasure of contents can be avoided.

Disadvantages

• The chip has to be taken out of the system for erasing and programming
• Selective erasing is not possible

Advantage

EPROMs are more flexible than ROMs and PROMs due to its reprogrammablity feature.

EAROMs

Programming and reprogramming can be done on board in Electrically Alterable Read Only Memory (EAROM). It requires only few milliseconds for programming. These devices are more expensive.

Flash Memory

Flash Memory or Flash is Electrically Erasable Programmable ROM. Flash memory can be erased and rewritten faster than other types of ROMs. But flash requires the erasure of an entire page. It uses quantum cells to trap electrons.

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