12-12-2012, 05:51 PM
Types of RAM and ROM
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Introduction
Decades of intensive research into integrated circuits has produced classes of electronic
devices with amazing performance. Consider a music player the size of a deck of cards
that can store all the works of Beethoven—with enough room left over to store a set of
encyclopedias. At the heart of this success story are two technologies: processors and
storage. This article examines two key storage technologies in modern electronic devices:
random access memory (RAM) and read only memory (ROM). Because they provide
high-speed storage, they enable the performance we expect in modern computers.
Memory chips are everywhere: in your car, television, and phone. They come in many
different flavors, although they all perform essentially the same function—to store data
and provide for its retrieval. The economic driver for the development of these chips has
historically been the computer industry. However, as the price has plummeted, the
consumer electronics industry has embraced them wholeheartedly. Two of the most
successful gadgets in the last decade (the digital camera and the portable music player)
are built around memory chips.
Both types of memory are normally packaged as integrated circuits, which are small
electronic circuits that consist mostly of semiconductors. These are referred to as
microchips, ICs, memory chips, or chips. Beyond that point, the differences between the
two systems begin to become important. Random access memory is examined in the next
section, and then it is followed with a discussion of read only memory.
Random Access Memory (RAM)
The name random access memory is an artifact of hardware evolution. Random access
means that the stored data can be accessed in any order, which is in contrast to the more
restricted access provided by other memory systems, such as tape and disk drive. The
access time to any piece of data stored on in RAM is essentially the same.
RAM is normally used in computer systems for main memory or primary storage. This is
where running programs and the data they use are stored. Moving data from primary
storage to the processor requires only a few cycles, although retrieving data from a hard
drive can take considerable longer. For this reason, modern operating systems run
primarily in RAM, and as they load and run additional applications, they move these
programs and their data into RAM for faster processing.
RAM can be categorized as volatile or non-volatile. Volatile means that all data is lost
when the chip is powered down. Historically, non-volatile RAM was a tiny part of the
market, although consumer electronics have changed that situation. When referring to
computer systems, most RAM remains volatile. Non-volatile RAM and its uses are
discussed later in this essay.
Dynamic RAM (DRAM)
Most computers incorporate two types of volatile RAM: static and dynamic. Although
both types require constant electrical current to function, they have some important
differences. Dynamic RAM is less expensive, and therefore it is the most widely used.
When a computer is said to have 512 megabytes or one gigabyte of RAM, the
specification refers to dynamic RAM (DRAM). DRAM stores each bit of information in
a separate capacitor on the integrated circuit. The DRAM chip requires only one
transistor and one capacitor for each bit of storage. This makes it both cheap and space
efficient.
One disadvantage with using capacitors for storage is that they gradually dissipate their
charge, so the charge must be refreshed regularly (current specifications are for the
refresh to occur every 64 milliseconds or less). This refresh requirement is what makes
this technology dynamic. DRAM also suffers periodic access limitations, because it
cannot be read during the refresh cycle.
One specialized type of DRAM is more common is embedded DRAM, or EDRAM.
EDRAM is DRAM integrated onto the same chip as the processor and used as cache
memory. This is a common solution in gaming consoles, and it will likely become a
staple in embedded systems.
Static RAM (SRAM)
Static RAM (SRAM) has the advantage of being faster than DRAM, although the
disadvantage is that it is more expensive. SRAM is static in the sense that it doesn’t
require constant electrical refreshes; however, it still requires constant current to maintain
the voltage differentials. SRAM generally requires less power than DRAM, although its
power requirements vary depending on clock speed. At higher clock speeds, it can use as
much power as DRAM; however, at more moderate speeds, it requires only a fraction of
what DRAM uses. When idle, SRAM power requirements are low.
Each bit in a SRAM chip requires a cell of six transistors, although DRAM needs only
one transistor and one capacitor. This means that SRAM cannot achieve the storage
densities of the DRAM family. As with DRAM, SRAM chips are mostly large arrays of
these cells of transistors.
The two primary applications of SRAM are embedded use and in computers. Embedded
use refers to SRAM use in automotive and consumer electronics, industrial equipment,
and almost all appliances or toys with an electronic user interface. Devices, such as cell
phones and music synthesizers, can incorporate several megabytes of SRAM.
SRAM in computer systems is usually delegated to roles where a small amount of highspeed
memory is required, such as processor caches and I/O buffers. Printers and liquid
crystal displays (LCDs) often use SRAM to buffer images. SRAM is also widely used in
networking devices, such as routers, switches, and cable modems, to buffer transmitted
information.
SRAM should not be confused with synchronous DRAM (SDRAM) or pseudostatic
RAM (PSRAM).
Read Only Memory (ROM)
Non-volatile memory retains data even when not powered. The two common types of
non-volatile memory are read only memory and flash memory. There are several types of
read only memory (ROM), although most are obsolete. These ROMs are called read only
because they cannot be modified by the casual user (and some types cannot be modified
at all). ROMs have traditionally been used in computer systems to store configuration
data, such as bootstrap or BIOS code, which requires fast access.
The first ROMs were mask-programmed ROMs, which had 1s and 0s actually burned
into the integrated circuit. This technique was simple but inflexible, and it was often used
to contain the startup code (bootstrap) for early microcomputers. Mask ROM is now
obsolete.