Tomek TRV wrote on 2023-03-09, 22:40:
So now I can say that Highscreen really did something with this BIOS because after flashing new one computer see more SIMM modules but only 4MB each and only FPM. My EDO 8, 16, 32MB modules are not recognized. I have to find some FPM bigger than 8MB but I thought that in 486 times it was not matter and all motherboards worked with FPM and EDO.
I'm going to put it how I understand it in layman's terms. The details are more complicated and several prolific posters here have explained them, and that's where much of my info comes from.
The whole point of EDO is that the memory chips don't "let go" of the bus in as many portions of the memory read cycle as FPM chips do--after all, it's "extended data out." This increases speed when the CPU is reading multiple addresses from EDO in rapid succession, if the chipset takes advantage of this by doing more aggressive timing.
Many Pentium chipsets take advantage of this to have a faster burst read cycle. Only a few very "end of the line" 486 chipsets can, specifically the SiS 496 and UM8881F, and even then it seems dependent on the particular board and the exact date code of the chipset. And it may not be any faster in benchmarks.
Even a 486 board doesn't take advantage of EDO RAM, it could still be made EDO "tolerant" to make the EDO RAM emulate the behavior of FPM RAM by temporarily disconnecting the SIMMs from the bus when the EDO feature would otherwise cause a problem. The chips even have an output enable pin but it isn't connected to the edge connector on the 72-pin SIMM. The board can get around that by introducing buffer chips between the SIMMs and the bus. But all that accomplishes is turning the EDO RAM back into FPM RAM. It doesn't take advantage of its faster speed capability. In particular, the datasheet for your VT82C496 doesn't say anything about EDO support.
It's possible that a cacheless board might be more tolerant of EDO RAM even without those buffer chips(?) -- but ultimately, I think the motivation for making those end-of-the-line 486es EDO capable was not performance but because the price of EDO SIMMs was dropping below FPM SIMMs of the same capacity because of economy of scale.
