Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
I am looking to build a high memory board for an i486 motherboard. I've seen controller-less boards on Ebay that inspire me to do it that way. Any old diagrams for Intel above boards or Kicad schematics would help.
EISA appears to be a simple non-multiplexed bus & I believe XMS is just continuous RAM area beyond the 1MB point?
As far as I can see from a quick look, all it needs is basic buffering and decode to start at memory address zero, but omitting (not enabling the data bus buffers for read, at least) the area of RAM on the motherboard so there is no conflict.
As long as it's seen as a continuous block with the onboard RAM by the BIOS memory test, I think it should just work?
(You could offset the entire address range to start it above the onboard RAM, but omitting something from 1 - 16MB out of 4GB seems a lot simpler and pretty irrelevant!)
You may have to take care of Dynamic RAM refresh yourself, though.
I remember problems with older systems, ISA / PCI at least, as they only had a limited number of bits in the internal refresh address counter for automatic DRAM refresh, and newer DRAM ICs that needed more bits in the refresh address did not work.
I've not done anything with DRAM design-wise for decades though, so it may well be that the RAM has an internal refresh counter and only needs regular refresh triggers?
There are two types of memory... EMS and XMS.. EMS is Extended ( AS JRW said passed the 1MB ) But XMS is Expanded ( I my view a bad idea, but all they had back then ) is paged in.. You can swap 64k in and out at any time.. There was a limit to the pages you could swap ( again down to the limitations of Ye ol'de address bus way back when.. )
What was good.. A decent gamer could draw screens and page them in as the "ye olde" Graphics cards needed around this to display a full picture. I doubt you will get XMS anymore, BUT!! windows still support it??