Memory - Upgrading and Repairing PCs - page 397

Extended Data Out RAM
In 1995, a newer type of DRAM called extended data out (EDO) RAM became available
for Pentium systems. EDO, a modified form of FPM memory, is sometimes referred to as
Hyper Page mode. EDO was invented and patented by Micron Technology, although Mi-
cron licensed production to many other memory manufacturers.
EDO memory consists of specially manufactured chips that allow a timing overlap
between successive accesses. The name extended data out refers specifically to the fact
that, unlike FPM, the data output drivers on the chip are not turned off when the memory
controllerremovesthecolumnaddresstobeginthenextcycle.Thisenablesthenextcycle
to overlap the previous one, saving approximately 10ns per cycle.
The effect of EDO is that cycle times are improved by enabling the memory controller
to begin a new column address instruction while it is reading data at the current address.
This is almost identical to what was achieved in older systems by interleaving banks of
memory, but unlike interleaving, with EDO you didn't need to install two identical banks
of memory in the system at a time.
EDORAMallowsforburstmodecyclingof5-2-2-2,comparedtothe5-3-3-3ofstandard
fast page mode memory. To do four memory transfers, then, EDO would require 11
total system cycles, compared to 14 total cycles for FPM. This is a 22% improvement
in overall cycling time. The resulting two-cycle (30ns) cycle time during burst transfers
equals a 33.3MHz effective clock rate, compared to 45ns/22MHz for FPM. On a system
with a 64-bit (8-byte) wide memory bus, this would result in a maximum throughput of
266MBps (33.3MHz × 8 bytes = 266MBps). Due to the processor cache, EDO typically
increased overall system benchmark speed by only 5% or less. Even though the overall
system improvement was small, the important thing about EDO was that it used the same
basic DRAM chip design as FPM, meaning that there was practically no additional cost
over FPM. In fact, in its heyday EDO cost less than FPM yet offered higher performance.
EDO RAM generally came in 72-pin SIMM form. Figure 6.4 (later in this chapter) shows
the physical characteristics of these SIMMs.
To actually use EDO memory, your motherboard chipset had to support it. Most mother-
board chipsets introduced on the market from 1995 (Intel 430FX) through 1997 (Intel
430TX) offered support for EDO, making EDO the most popular form of memory in PCs
from 1995 through 1998. Because EDO memory chips cost the same to manufacture as
standard chips, combined with Intel's support of EDO in motherboard chipsets, the PC
market jumped on the EDO bandwagon full force.
See “ Fifth-Generation (P5 Pentium Class) Chipsets , ” p. 179 and “ Sixth-Generation
(P6 Pentium Pro/II/III Class) Chipsets , ” p. 181 ( Chapter 4 ) .