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Reference for how ram works on Atari 8 bit

Discussion in 'SIG: Atari 8-Bit Hardware' started by Wildstar87, Apr 21, 2021.

  1. by Wildstar87

    Wildstar87 Galley Slave

    Blog Posts:
    Hi guys, been lurking here for a little bit. Based on what I have read here already, I thought maybe you could help me.

    I'm looking to make some ram ugrades for a 600XL and an 800XL. The 600XL is stock, and the 800XL has bad ram. I know the conventional means of getting DRAMs and doing it that way. What I'm looking for is the create SRAM versions, in addition to upgrading an additional 512k.

    I'm using these two mods as my basis:

    I have also been suggested to look at the 576NUC, since that is doing pretty much exactly what I want, memory wise.

    I know about all the ram upgrades available for purchase, and I have a few of them, but seeing as I'm an electronics technician by trade, I really want to build these myself. I've also built my own 1088XEL, so I'm definitely familiar with building this type of stuff.

    When looking at all the schematics, they all do things really similarly up to a point, then things start to differ, and I don't understand why. So I'm obviously missing some specifics of how ram is accessed and addressed in the Atari 8 bits.

    The 576NUC uses an XEGS MMU, plus a custom EMMU GAL, based on the Hias 512K design, but not exactly. I'm not clear on the differences between the XEGS and XL MMU differences, as the information I have been able to find seems to indicate the same pinout, but I've been told they are not the same.

    Also for the base 64Kb, I'm trying to use a 64K SRAM, and not waste 64K. If I use the equivalent 64K SRAM from Alliance Memory that the 512Kb, it only has 13 Address lines, where there are 15 or more on the other aforementioned mods. Looking at the standard ram, the ram itself only uses 8 lines, to be expected for 8k x 8, with the additional lines being used for the refresh circuit.

    I feel like I'm circling around the understanding, but can't quite put everything together, so would like to see if there is a reference to understand how all this works. I'm not looking for someone to create it for me, but am in this for learning how to do it.

    I thank you for your time, especially reading through all this.
  2. by nysavant

    nysavant Deckhand

    Blog Posts:
    I'm afraid I don't have the technical know how to help you but I'm sure one of our more advanced (in experience not years) group will be along soon to assist.

    In the meantime, welcome :)

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