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5.1 / SEMICONDUCTOR MAIN MEMORY 163

A ROM is created like any other integrated circuit chip, with the data actually
wired into the chip as part of the fabrication process.This presents two problems:
• The data insertion step includes a relatively large fixed cost, whether one or
thousands of copies of a particular ROM are fabricated.
• There is no room for error. If one bit is wrong, the whole batch of ROMs must
be thrown out.
When only a small number of ROMs with a particular memory content is
needed, a less expensive alternative is the programmable ROM (PROM). Like the
ROM, the PROM is nonvolatile and may be written into only once. For the PROM,
the writing process is performed electrically and may be performed by a supplier or
customer at a time later than the original chip fabrication. Special equipment is re-
quired for the writing or “programming” process. PROMs provide flexibility and
convenience.The ROM remains attractive for high-volume production runs.
Another variation on read-only memory is the read-mostly memory, which is
useful for applications in which read operations are far more frequent than write
operations but for which nonvolatile storage is required. There are three common
forms of read-mostly memory: EPROM, EEPROM, and flash memory.
The optically erasable programmable read-only memory (EPROM) is read
and written electrically, as with PROM. However, before a write operation, all the
storage cells must be erased to the same initial state by exposure of the packaged
chip to ultraviolet radiation. Erasure is performed by shining an intense ultraviolet
light through a window that is designed into the memory chip. This erasure process
can be performed repeatedly; each erasure can take as much as 20 minutes to per-
form. Thus, the EPROM can be altered multiple times and, like the ROM and
PROM, holds its data virtually indefinitely. For comparable amounts of storage, the
EPROM is more expensive than PROM, but it has the advantage of the multiple up-
date capability.
A more attractive form of read-mostly memory is electrically erasable pro-
grammable read-only memory (EEPROM). This is a read-mostly memory that can
be written into at any time without erasing prior contents; only the byte or bytes ad-
dressed are updated. The write operation takes considerably longer than the read
operation, on the order of several hundred microseconds per byte. The EEPROM
combines the advantage of nonvolatility with the flexibility of being updatable in
place, using ordinary bus control, address, and data lines. EEPROM is more expen-
sive than EPROM and also is less dense, supporting fewer bits per chip.
Another form of semiconductor memory is flash memory (so named because
of the speed with which it can be reprogrammed). First introduced in the mid-1980s,
flash memory is intermediate between EPROM and EEPROM in both cost and
functionality. Like EEPROM, flash memory uses an electrical erasing technology.
An entire flash memory can be erased in one or a few seconds, which is much faster
than EPROM.In addition,it is possible to erase just blocks of memory rather than an
entire chip. Flash memory gets its name because the microchip is organized so that a
section of memory cells are erased in a single action or “flash.” However, flash mem-
ory does not provide byte-level erasure. Like EPROM, flash memory uses only one
transistor per bit, and so achieves the high density (compared with EEPROM) of
EPROM.