Aha, I though they are pin2pin compatible.
I couldn't find IBM datasheet I found only Cyrix486SLC datasheet. I'll check the difference to 386SX...

But opposite, when SLC is replaced by SX it should work?

snorg wrote on 2016-02-28, 16:07:

I haven't decided if this is a project worth pursuing just yet, even if I could get this to work I'm still limited to the 640k RAM in my system, and since I've only got an 8 bit expansion bus I'm not sure the worth of investing a ton of money into memory expansion boards.

But are you really limited to 640k?

This is something I’ve thought about since the mid 90’s
many 286’s for one reason or another are limited to some obscure amount of Ram (Tandys mainly but laptops and even some more mainstream brands had a 640k or 1m limit)
Other 286’s just simply wouldn’t allow you to put in the bios more than a specific amount of memory, resulting in you being able to “see” the memory with an app but the max memory was wrong so himem wouldn’t map it.
I even had an odd NCR 286 that had 1mb of memory, bios counted it but the upper 384k could only be umb as the board had no method to relocate it.

In the day I knew people who removed all the dram from their systems to replace with sram (thinking XT class) for a speed boost

What is strange is, how did they do that in back in the day?

Next sram these days is available in large sizes like 4mb and should be able to be plopped right onto the address lines of your cpu with minimal circuitry

So what is stopping us from building a daughterboard like your overdrive that also includes extended sram memory soldered on?

Line A0?

What is stopping us from taking an 8bit ISA slot and adding the missing control lines off the cpu ISA extension style for an external 16bit ISA slot? Buffers, missing dma? (The components to make 16bit ISA are so crude and obsolete one would think in this era they could be wired in)

What’s worse systems like the 1000rlx can accept but not use high density ram, I desoldered the 512k put in 2mb of hi density and still get 1mb of Ram, install 4mb still get 1mb of Ram.

I’m told the memory controller has limits on rows, columns and refresh and that the bios memory size is gimped but one has to wonder why either of these things would be complicated to address.

Also worth noting my 1000rlx left stock easily overclocks to 15mhz, you might find your Tandys motherboard can overclock to 10,12 or even 15mhz without modifications just need a variety of clock crystals

Ah well