an error was detected, but that is not all bad. If a byte starts out with six 1 bits but ends up
with only five or gains one and has seven, there is definitely a condition in memory of
which you should be aware.
Parity memory systems are able to detect only a 1-bit error and cannot fix the error.
When a parity error is detected, normally an error message is displayed to the monitor
and the system halts.ECC Memory
Error-correcting code (ECC)goes beyond simple parity systems to detect errors of up to four
bits and correct all 1-bit errors in memory. Four-bit errors in memory are extremely rare
and when detected are an indication of a serious memory problem. However, 1-bit errors
are quite common and ECC memory is able to correct them without reporting errors and
keep the system running. Errors detected of two, three, or four bits are reported as parity
errors and the system halts.DRAM Technologies
As microprocessors and chipsets evolve, so do memory technologies. Since DRAM is still
the primary type of memory used in the PC, it has had to adapt to keep pace. The result is
that new DRAM technologies are created that improve on the previous DRAM technol-
ogy in a sort of memory one-upmanship. Each new DRAM technology is based at least in
part on a preceding DRAM technology, usually improving its organization, speed, and
access method.
Some of the more common DRAM technologies are: Fast Page Mode (FPM) FPM DRAM, which is also known as non-EDO
DRAM, is compatible with virtually all motherboards with bus speeds
under 66MHz.
Extended Data Out (EDO) EDO, the most common technology of DRAM,
is slightly faster than FPM DRAM and is very common in Pentium and later
PCs with bus speeds under 75MHz.
Synchronous DRAM (SDRAM) SDRAM (pronounced “ess-dee-ram”) is
synchronized to the system clock to read and write memory in burst mode.
This type of DRAM is more common on systems with higher bus speeds.
Burst Extended Data Out (BEDO) DRAM BEDO (pronounced “beado”)
is EDO memory with pipelining technology added.Pipeliningallows BEDO
DRAM to transfer data and accept the next request from the CPU at the same
time. BEDO DRAM is common on PCs with clock speeds of up to 66MHz.
Enhanced DRAM (EDRAM) EDRAM (pronounced “ee-dee-ram”) is a
combination of SRAM and DRAM used for Level 2 cache (see Chapter 8).
Double Data Rate (DDR) SDRAM A special form of SDRAM that is
designed for systems with bus speeds over 200MHz.Chapter 7: Computer Memory^145