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ISA 8 Bit 8MB Extended Memory Card

NobodyIsHere

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Dec 21, 2006
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2,394
Hi

You realize the 8 bit ISA slot has only 20 address lines (A0-A19). That means a maximum of 1MB of directly accessible memory on an 8 bit ISA board -- less depending what is on the motherboard typically 256K to 1MB.

LIM might be able to have more memory but requires some sort of bank switching scheme and/or controller. A 16 bit ISA board could have as much as 16MB directly addressable memory but those are common boards.

Andrew Lynch
 

RubberTelly

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Sep 23, 2013
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Lismore, Australia
Oh crud...LIM EMS it is...I'll need to investigate PSOCs further....64K window mapped to main memory...386 does this inherently while the 286 needs a driver written...

Looks like I'm in for a fun time...Yay...
 

pearce_jj

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I put together a 1MB 8-bit RAM board based on AS6C4008 SRAM chips. Board features:

  • Full 1MB RAM on an 8-bit ISA card
  • Each physical 64KB page is separately switched
  • Jumper to disable first 16KB, enabling maximum RAM expansion on a 5150 with 16KB onboard.

Schematic here.
 

Trixter

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I put together a 1MB 8-bit RAM board based on AS6C4008 SRAM chips. Board features:

  • Full 1MB RAM on an 8-bit ISA card
  • Each physical 64KB page is separately switched
  • Jumper to disable first 16KB, enabling maximum RAM expansion on a 5150 with 16KB onboard.

Schematic here.

Did you ever make a page on your wiki for this board?

If the RAM is SRAM, I wonder if there is a way to dramatically shorten the DMA dram refresh routine for just the first 16k -- would result in a slight speedup. (just thinking out loud, not actually going to try it)
 

pearce_jj

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Did you ever make a page on your wiki for this board?

Not yet - only started this a couple of days ago :) If there is interest I'll take it forward.

If the RAM is SRAM, I wonder if there is a way to dramatically shorten the DMA dram refresh routine for just the first 16k

I wondered this too but is seems it's not possible - the DMA controller needs to visit 128 consecutive rows in 2ms, which the 66kHz refresh clock provides nearly exactly. You could though pull all the RAM logic from the system board, then re-gain DMA channel 0 too I guess.
 

RubberTelly

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Sep 23, 2013
Messages
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Location
Lismore, Australia
Anyway, I'm going to make a parts list;
5V to 3.3V Bi-Directional Logic Converter (Maybe 2) - TI

4Mb x 8 Memory SRAM - Cypress
or
4Mb x 16 Memory SRAM - Cypress

PSoC for Advanced Page Selection Algorithms (Maybe)

and a few other ICs
 
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