Memory System Design II - Fundamentals of Computer Organization and Architecture - page 157

TABLE 7.3 Patterns Stored at Four ROM Locations

Address
lines

Word line
activated

Output
pattern

00 W 0 1001

01 W 1 0110

10 W 2 1010

11 W 3 0101

bit lines is used to output a 1 at the output of those pulled down bit lines. Table 7.3
shows the patterns stored at each of the four ROM locations.
Mask-programmed ROMs are primarily used to store machine microcode, desk-
top bootstrap loaders, and video game cartridges. Because they can be programmed
only once by the manufacturer, mask-programmed ROMs are inflexible. If the user
would like to program his her ROM on site, then a different type of ROM, called the

Programmable ROM (PROM) should be used. In this case, fuses, instead of transis-
tors, are placed at the intersection of word and bit lines. The user can program the
PROM by selectively blowing up the appropriate fuses. This can be done by allow-
ing a high current to flow in those particular fuses, thus causing them to blow up.
This process is known as “burning the ROM.”
Although it allows for some added flexibility, PROM is still restricted by the fact
that it can only be programmed once (by the user). A third type of ROM, called
Erasable PROM (EPROM), is reprogrammable; that is, it allows stored data to be
erased and new data to be stored. In order to provide such flexibility, EPROMs
are constructed using a special type of transistors. These transistors are able to
assume one of two statuses, normal or disabled. A disabled transistor acts like a
switch that is turned off all the time. A normal transistor can be programmed to
become open all the time by inducing a certain amount of charge to be trapped
under its gate. A disabled transistor can become normal again by removing
the induced charge. This requires exposing those transistors to ultraviolet light.
Exposing the EPROM chip to such ultraviolet light will lead to the erasure of the
entire chip contents. This is considered a major drawback of EPROMs. Both
PROMs and EPROMs are used in prototyping, of moderate size systems.
Flash EPROMs (FEPROMs) have emerged as strong contenders to EPROMs.
This is because FEPROMs are more compact, faster, and removable compared to
EPROMs. The erasure time of a FEPROM is far faster than that of an EPROM.
A different type of ROM, which overcomes the drawback of the EPROM, is the
Electrically EPROM or EEPROM. In this case, the erasure of the EPROM can be
done electrically and, moreover, selectively; that is, only the contents of selective
cells can be erased, leaving the other cells' contents untouched. Both FEPROMs
and EEPROMs are used in applications requiring occasional updating of infor-
mation, such as Programmable TVs, VCR, and automotives.
Table 7.4 summarizes the main characteristics of the different types of ROM
discussed above.

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