Hardware Reference
In-Depth Information
memory is stored on.)
•
ROM
—Read-only memory
•
DRAM
—Dynamic random access memory
•
SRAM
—Static RAM
The only type of memory you normally need to purchase and install in a system is DRAM. The other
types are built in to the motherboard (ROM), processor (SRAM), and other components such as the
video card, hard drives, and so on.
ROM
Read-only memory, or ROM, is a type of memory that can permanently or semi-permanently store
data. It is called read-only because it is either impossible or difficult to write to. ROM also is often
referred to as nonvolatile memory because any data stored in ROM remains there, even if the power
is turned off. As such, ROM is an ideal place to put the PC's startup instructions—that is, the
software that boots the system.
Note that ROM and RAM are not opposites, as some people seem to believe. Both are simply types
of memory. In fact, ROM technically could be classified as a subset of the system's RAM. In other
words, a portion of the system's random access memory address space is mapped into one or more
ROM chips. This is necessary to contain the software that enables the PC to boot; otherwise, the
processor would have no program in memory to execute when it was powered on.
The main ROM BIOS is contained in a ROM chip on the motherboard, but there are also adapter
cards with ROMs on them. ROMs on adapter cards contain auxiliary BIOS routines and drivers
needed by the particular card, especially for those cards that must be active early in the boot process,
such as video cards. Cards that don't need drivers active at boot time typically don't have a ROM
because those drivers can be loaded from the hard disk later in the boot process.
Most systems today use a type of ROM called
electrically erasable programmable ROM
(EEPROM)
, which is a form of flash memory. Flash is a truly nonvolatile memory that is rewritable,
enabling users to easily update the ROM or firmware in their motherboards or any other components
(video cards, SCSI cards, peripherals, and so on).
For more information on BIOS upgrades, see the
Chapter 5
section, “
Upgrading the BIOS
,
”
p.
274
.
DRAM
Dynamic RAM (DRAM) is the type of memory chip used for most of the main memory in a modern
PC. The main advantages of DRAM are that it is dense, meaning you can pack a lot of bits into a
small chip, and it is inexpensive, which makes purchasing large amounts of memory affordable.
The memory cells in a DRAM chip are tiny capacitors that retain a charge to indicate a bit. The
problem with DRAM is that it is dynamic—that is, its contents can be changed. With every keystroke
or every mouse swipe, the contents of RAM change. And the entire contents of RAM can be wiped
out by a system crash. Also, because of the design, it must be constantly refreshed; otherwise, the
electrical charges in the individual memory capacitors drain and the data is lost. Refresh occurs when
the system memory controller takes a tiny break and accesses all the rows of data in the memory
chips. The standard refresh time is 15ms (milliseconds), which means that every 15ms, all the rows
in the memory are automatically read to refresh the data.
