Hardware Reference
In-Depth Information
checksum, the motherboard tests each adapter ROM during the POST and flags any that appear to
have been corrupted.
The motherboard ROM automatically runs the programs in any adapter ROMs it finds during the scan.
You see this in most systems when you turn them on, and during the POST you see the video card
BIOS initialize and announce its presence.
ROM Shadowing
ROM chips by nature are slow, with access times of 100ns (nanoseconds, or billionths of a second)
or more, compared to dynamic RAM (DRAM) access times of well under 10ns on most systems.
Because of this, in virtually all systems the ROMs are
shadowed
, which means they are copied into
RAM at startup to allow faster access during normal operation. The shadowing procedure copies the
ROM into RAM and then assigns that RAM the same address as the ROM originally used, disabling
the actual ROM in the process. This makes the system seem as though it has ROM running at the same
speed as RAM.
ROM Chip Types
The four main types of ROM chips that have been used in PCs are as follows:
•
ROM
—Read-only memory
•
PROM
—Programmable ROM
•
EPROM
—Erasable PROM
•
EEPROM
—Electrically erasable PROM, also sometimes called a flash ROM
No matter which type of ROM your system uses, the data stored in a ROM chip is nonvolatile and
remains indefinitely unless intentionally erased or overwritten (in those cases where that is possible).
ROM (True or Mask ROM)
Originally, most ROMs were manufactured with the binary data (0s and 1s) already “cast in” or
integrated into the die. The die represents the actual silicon chip. These are called
mask ROMs
because the data is formed into the mask from which the ROM die is photo-lithographically produced.
This type of manufacturing method is economical if you are making hundreds of thousands of ROMs
with the same information. If you must change a single bit, however, you must remake the mask, which
is an expensive proposition. Because of costs and inflexibility, nobody uses mask ROMs anymore.
PROM
PROMs
are a type of ROM that is blank when new and must be programmed with whatever data you
want. The PROM was invented in the late 1970s by Texas Instruments and has been available in sizes
from 1KB (8Kb) to 2MB (16Mb) or more. PROMs can be identified by their part numbers, which
usually are 27nnnn—where the 27 indicates the TI type PROM and the nnnn indicates the size of the
chip in kilobits (not bytes). For example, most PCs that used PROMs came with 27512 or 271000
chips, which indicate 512Kb (64KB) or 1Mb (128KB), respectively.
Note
Since 1981, all cars sold in the United States have used onboard computers with some form of
ROM containing the control software. For example, the 1989 Pontiac Turbo Trans Am I had
came with an onboard computer containing a 2732 PROM, which was a 32Kb (4KB) chip in
