Hardware Reference
In-Depth Information
EDORAMwasusedinsystemswithCPUbusspeedsofupto66MHz,whichfitperfectly
with the PC market up through 1998. However, starting in 1998, with the advent of
100MHz and faster system bus speeds, the market for EDO rapidly declined, and faster
SDRAM architecture became the standard.
One variation of EDO that never caught on was called
burst EDO
(BEDO). BEDO added
burst capabilities for even speedier data transfers than standard EDO. Unfortunately, the
technology was owned by Micron and not a free industry standard, so only one chipset
(Intel 440FX Natoma) ever supported it. BEDO was quickly overshadowed by industry-
standard SDRAM, which came into favoramong PCsystem chipset andsystem designers
over proprietary designs. As such, BEDO never really saw the light of production, and to
my knowledge no systems ever used it.
SDRAM
SDRAM is short for synchronous DRAM
, a JEDEC standard for a type of DRAM that
runsinsynchronization with the memory bus.SDRAM delivers information inveryhigh-
speed bursts using a high-speed clocked interface. SDRAM removes most of the latency
involved in asynchronous DRAM because the signals are already in synchronization with
the motherboard clock.
As with any newly introduced type of memory on the market, motherboard chipset sup-
port is required before it can be usable in systems. Starting in 1996 with the 430VX and
430TX, most of Intel's chipsets began to support industry-standard SDRAM, and in 1998
the introduction of the 440BX chipset caused SDRAM to eclipse EDO as the most popu-
lar type on the market.
SDRAM performance is dramatically improved over that of FPM or EDO RAM.
However, because SDRAM is still a type of DRAM, the initial latency is the same, but
burst mode cycle times are much faster than with FPM or EDO. SDRAM timing for a
burst access would be 5-1-1-1, meaning that four memory reads would complete in only
eight system bus cycles, compared to 11 cycles for EDO and 14 cycles for FPM. This
makes SDRAM almost 20% faster than EDO.
Besides being capable of working in fewer cycles, SDRAM is capable of supporting up
to 133MHz (7.5ns) system bus cycling. Most PC systems sold from 1998 through 2002
included SDRAM memory.
SDRAM is sold in DIMM form and is normally rated by clock speed (MHz) rather than
cycling time (ns), which was confusing during the initial change from FPM and EDO
DRAM.
Figure 6.5
(later in this chapter) shows the physical characteristics of DIMMs.
