Once ROM operations are completed, the computer
is ready for normal operational use. For all this to take
place, ROM uses circuits in the computer that we have
already discussed. In some cases, they are circuits
specific for ROM operations. They include:
Registers and flip-flops
READ-ONLY MEMORY TYPES
Types of ROMs include the basic ROM that once
manufactured cannot be written on again. Other types,
called programmable read-only memories (PROMS),
can be written on again and again.
ROMs are prepared at the factory. They are not
meant to be changed by the user or the technician. They
are only to be changed when a newer version is
authorized and supplied to replace the old one.
Programmable Read-Only Memory (PROM)
A PROM is a programmable ROM. Once
programmed it acts like a ROM. It can be
field-programmed by an. authorized technician. Each
cell is identified by selecting the row and the column
just like locating an address in read/write memory.
There are two types of PROMs erasable and
nonerasable. Erasable PROMS can be erased and
Nonerasable PROMS cannot be
changed once they are programmed.
There are a couple of ways to create or erase the
ones in the array; electrically or with ultraviolet (UV)
light. Some PROMS are electrically programmed but
erased with UV light. Others are erased electrically and
programmed with the UV light.
ELECTRICAL. An electric charge can be used
to either blow fusible links permanently in a cell or used
on a special transistor with two gates. With the special
transistors, the gate between the memory cell and the
column wire is disabled by the electrical charge.
UV LIGHT. UV light is used to erase data in a
cell by exposing the IC die to the UV light for a few
minutes (usually less than 30 minutes). UV light can
also be used to restore ones to a cell by dissipating the
electrical charge that disabled the gate.
ELECTRICALLY ALTERABLE OR
( E A P R O M O R
EEPROM). The EAPROM or EEPROM can be
programmed (modified) or erased while it is still in the
circuit and used like a nonvolatile read/write memory.
EAPROMs/EEPROMs use an electric charge to erase
the ones. Some types of EAPROMs/EEPROMs are
more versatile; individual cells can be reprogrammed
by reversing the voltage used to create a zero. There
are some timing constraints that cause the part to need
more time for erasure or programming than is needed
to read data from the part. Some EAPROM/EEPROMs
have a word or byte erase mode.
ULTRAVIOLET-ERASABLE PROM (UV
EPROM OR EPROM). UV EPROMs/EPROMs
trap a charge (1) in the cells to represent the data. To
release the charge, the cells are exposed to the UV light
for 30 minutes or less. UV EPROMs/EPROMs are
usually programmed out of circuit. Figure 6-38 is an
example of a 2K × 8 UV EPROM; block diagram and
This chapter introduced you to memory types. The
following information summarizes important points
you should have learned.
MEMORY The main memory of a computer is
used for storing programs, data, calculations, and
MEMORY MODULES Memory modules are
made up of multiple pcbs (support circuitry) and
memory components (stacks [core or film] and
semiconductor pcbs) to form one memory module or
unit. Memory modules are interchangeable with other
modules of the same type and size in the same computer
set. Each module provides a fixed number of memory
words with a fixed number of bit positions for each
MEMORY ARCHITECTURE Memories are
typically organized in square form so that they have an
equal number of rows (x) and columns (y). Each
intersection of a row and a column comprises a memory
word address. Each memory address will contain a
MEMORY OPERATIONS -Memory opera-
tions operate on a request, selection, and initiate basis.
A memory request or selection and a memory word