EDO

11/2/96

Extended Data Out DRAM (EDO Dynamic RAM) is conventional Fast Page Mode DRAM that has been modified to hold the last read data on the data lines while the next address to be read is being output on the address lines. This allows some amount of overlap between consecutive DRAM read cycles since the next cycle can be started slightly before the previous one has completed.

EDO does not speed up the setup or data retrieval phases of data read, but it does reduce or eliminate the data return phase when a DRAM read is followed by another read. Other than the data read/write overlap, EDO DRAM signaling is conventional. EDO DRAM uses conventional pinouts and can be plugged into computers not designed for EDO DRAM -- in which case it will act like non-EDO (Fast Page Mode) DRAM. On the other hand, non-EDO DRAM will not work in a computer designed to use only EDO DRAM. Chipsets can be designed to determine whether EDO is present and use the feature if it is present.

EDO improves timing only for reads followed immediately by other reads, not by reads following writes; writes following reads; or reads followed by idle time. EDO helps only on the last read access of interleaved reads. While a large percentage (70-80%) of CPU "memory" operations are read operations, most reads are handled by internal or external memory cache. Very few consecutive read operations actually make it to DRAM. Measured improvement from use of EDO DRAM for general computing are on the order of 3% to 5%. However for specialized applications such as video output, savings can be considerably higher

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Note, I'm only repeating what I've read elsewhere -- which is that EDO helps video applications a lot. Surely properly designed video output is interleaved delivering one word per clock. How can EDO help?

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