A common question many people ask i

A common question many people ask is “why are there so many different types of
computer memory?” The answer is that new technologies continue to be introduced
in an attempt to match the improvements in CPU design—the speed of
memory has to, somewhat, keep pace with the CPU, or the memory becomes a
bottleneck. Although we have seen many improvements in CPUs over the past
few years, improving main memory to keep pace with the CPU is actually not as
critical because of the use of cache memory. Cache memory is a small, highspeed
(and thus high-cost) type of memory that serves as a buffer for frequently
accessed data. The additional expense of using very fast technologies for memory
cannot always be justified because slower memories can often be “hidden” by
high-performance cache systems. However, before we discuss cache memory, we
will explain the various memory technologies.
Even though a large number of memory technologies exist, there are only two
basic types of memory: RAM (random access memory) and ROM (read-only
memory). RAM is somewhat of a misnomer; a more appropriate name is readwrite
memory. RAM is the memory to which computer specifications refer; if
you buy a computer with 128 megabytes of memory, it has 128MB of RAM.
RAM is also the “main memory” we have continually referred to throughout this
book. Often called primary memory, RAM is used to store programs and data that
the computer needs when executing programs; but RAM is volatile, and loses this
information once the power is turned off. There are two general types of chips
used to build the bulk of RAM memory in today’s computers: SRAM and DRAM
(static and dynamic random access memory).
Dynamic RAM is constructed of tiny capacitors that leak electricity. DRAM
requires a recharge every few milliseconds to maintain its data. Static RAM technology,
in contrast, holds its contents as long as power is available. SRAM consists
of circuits similar to the D flip-flops we studied in Chapter 3. SRAM is
faster and much more expensive than DRAM; however, designers use DRAM
because it is much denser (can store many bits per chip), uses less power, and
generates less heat than SRAM. For these reasons, both technologies are often
used in combination: DRAM for main memory and SRAM for cache. The basic
operation of all DRAM memories is the same, but there are many flavors, including
Multibank DRAM (MDRAM), Fast-Page Mode (FPM) DRAM, Extended
Data Out (EDO) DRAM, Burst EDO DRAM (BEDO DRAM), Synchronous
Dynamic Random Access Memory (SDRAM), Synchronous-Link (SL) DRAM,
Double Data Rate (DDR) SDRAM, and Direct Rambus (DR) DRAM. The different
types of SRAM include asynchronous SRAM, synchronous SRAM, and
pipeline burst SRAM. For more information about these types of memory, refer
to the references listed at the end of the chapter.
In addition to RAM, most computers contain a small amount of ROM (readonly
memory) that stores critical information necessary to operate the system,
such as the program necessary to boot the computer. ROM is not volatile and
always retains its data. This type of memory is also used in embedded systems or
any systems where the programming does not need to change. Many appliances,
toys, and most automobiles use ROM chips to maintain information when the
power is shut off. ROMs are also used extensively in calculators and peripheral
devices such as laser printers, which store their fonts in ROMs. There are five
basic different types of ROM: ROM, PROM, EPROM, EEPROM, and flash
memory. PROM (programmable read-only memory) is a variation on ROM.
PROMs can be programmed by the user with the appropriate equipment. Whereas
ROMs are hardwired, PROMs have fuses that can be blown to program the chip.
Once programmed, the data and instructions in PROM cannot be changed.
EPROM (erasable PROM) is programmable with the added advantage of being
reprogrammable (erasing an EPROM requires a special tool that emits ultraviolet
light). To reprogram an EPROM, the entire chip must first be erased. EEPROM
(electrically erasable PROM) removes many of the disadvantages of EPROM: no
special tools are required for erasure (this is performed by applying an electric
field) and you can erase only portions of the chip, one byte at a time. Flash memory
is essentially EEPROM with the added benefit that data can be written or
erased in blocks, removing the one-byte-at-a-time limitation. This makes flash
memory faster than EEPROM.