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Author Topic: RAM Boards  (Read 286 times)

blusnowkitty

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RAM Boards
« on: September 25, 2020, 01:29:11 pm »

Was looking at this PC card recently: https://texelec.com/product/lo-tech-1mb-ram/

Got me wondering - is there any reason the same couldn't be done for the Lisa? I realize that product's using SRAM and not DRAM but apparently you can still get 4Mbit DRAM new. Would be cool to make modern-day RAM boards to get people up to 2MB or beyond for not a whole lot.
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stepleton

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Re: RAM Boards
« Reply #1 on: September 25, 2020, 01:53:17 pm »

I've also wondered about the idea of homebrew ram boards. I've always thought it would look funny to see a tiny PCB poking out of one of the RAM slots.

This said, I think the MMU limits the Lisa to 2MiB max? Whatever the case, it's less than the 16MiB addressable by the 68000.
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rayarachelian

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Re: RAM Boards
« Reply #2 on: September 25, 2020, 07:22:54 pm »

Was looking at this PC card recently: https://texelec.com/product/lo-tech-1mb-ram/

Got me wondering - is there any reason the same couldn't be done for the Lisa? I realize that product's using SRAM and not DRAM but apparently you can still get 4Mbit DRAM new. Would be cool to make modern-day RAM boards to get people up to 2MB or beyond for not a whole lot.

Sounds wonderful. SRAM would make the Lisa a tiny bit faster since refresh won't be needed, maybe remove some wait states but yes, for LOS/Xenix/Unix, the limit is 2MB. James MacPhail has posted a way to modify the RAM board to get 4MB: https://lisalist2.com/index.php/topic,29.0.html but this will only work with MacWorks - not sure what happens if you boot LOS on such a machine, whether it will ignore the extra RAM or if it will fail. If it fails, perhaps some sort of switch with a cable out the back will do the trick to disable the extra 2MB.

Not sure what will happen to the DRAM refresh cycles and how they'd interact, I suspect (but likely am wrong) that all they'll do is read all the RAM every so many cycles, and I suspect this is tied in to the video refresh cycles, so not sure if you'd need to do something to block that cycle from reaching SRAM, or signal something back for completion, etc.
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Lisa2

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Re: RAM Boards
« Reply #3 on: September 28, 2020, 01:07:13 pm »

I assume the appeal of a modern static RAM board would be it's price, what price point would it need to be?  If it cost several hundred dollars would it still be appealing?

The Lisa memory slots use 21 address lines, this is where the max limit of 2Mb ( 2 to the power of 21) comes from.
The 4Mb hack re-purposes another signal in the slot to be a 22nd address line ( requiring a mod to the CPU card ).  To use this hack you must also use the ScreenMod ROM's (3A), so it is not possible to use any environment other than MacWorks with this mod installed ( like LOS or Xenix ).

Rick 

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compu_85

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Re: RAM Boards
« Reply #4 on: September 28, 2020, 02:23:10 pm »

What about having an off-slot jump lead back to the CPU card?
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rayarachelian

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Re: RAM Boards
« Reply #5 on: September 28, 2020, 02:50:33 pm »

What about having an off-slot jump lead back to the CPU card?

Might be easier to turn the CPU into a daughterboard, like the XLerator and then use modern SIMMs or socketed SRAMs, no?
You'd still need to ensure I/O, ROM, SIO, latches, etc. still show up where they belong. I/O space starts around 0x00FC0000 and ROM at 0x00FE0000, so it will leave most of the 16MB range for RAM (16515072 bytes)
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