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[ The PC Guide | Systems and Components Reference Guide | System Memory | DRAM Technologies ]

Extended Data Out (EDO) DRAM

The most common type of asynchronous DRAM used is called extended data out or EDO memory; sometimes it is also called hyper page mode DRAM. It is slightly faster than FPM memory due to another evolutionary tweak in how the memory access works. In simplified terms, EDO memory has had its timing circuits modified so one access to the memory can begin before the last one has finished. It is therefore slightly faster than FPM memory, giving a performance boost of around 3-5% over FPM in most systems. EDO memory has been hyped up a great deal, but in real world performance it offers a minimal speed increase over FPM memory.

EDO memory costs the same amount to manufacture as FPM, and due to its prominence in the market now is actually usually cheaper than FPM despite being newer and faster. It was originally more expensive but the reduced demand for slower FPM memory now makes FPM harder to find in most cases. Until recently, EDO was the standard for fifth- and sixth-generation systems. It still is found in most later model Pentium-class PCs, but SDRAM has now replaced it as the technology of choice for sixth-generation systems. EDO memory is still not usually suitable for high-speed (75 MHz and higher) memory buses, since it is really not that different than FPM overall. EDO typically allows burst system timings as fast as 5-2-2-2 at 66 MHz, when using an optimized chipset. It will run on faster buses but the memory timing may need to be reduced.

EDO memory requires support from the system chipset. Invented in 1994, most newer Pentium systems, as well as some of the latest PCI-based 486 motherboards will support EDO. Older systems will not work properly with EDO; some are "EDO tolerant", meaning that they will work with EDO but will run it at as if it were FPM memory (slower). Others will not work at all with EDO memory.

Warning: Some systems allow the use of EDO in one bank of memory and FPM in another, but others will not work with this sort of setup. Some will work with it but will run all of the memory at the slower speed used for FPM. The motherboard manual or manufacturer should be able to supply the particular information you need about your board.

Next: Burst Extended Data Out (BEDO) DRAM


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