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A project log for Improbable AVR -> 8088 substitution for PC/XT

Probability this can work: 98%, working well: 50% A LOT of work, and utterly ridiculous.

Eric HertzEric Hertz 03/31/2017 at 06:592 Comments

Moved here from my project-ideas page... *long* after having-begun.

Processor Replacement... (2-4-16) (and some new thoughts 2-5-16)

What about placing a microcontroller on an old motherboard's CPU socket...?

Kinda digging this idea, but haven't really thought it out too much, yet.

AVR would be difficult-ish, since code can only run from FLASH. (Though, have seen some impressive work on that front, running 'code' externally from SRAM, SD-Card, etc. via function-calls...?)

PIC32... well, there's already linux's for that, right? So plug a MIPS core onto an old 486 board, get some ISA slots, PCI, whatnot... Maybe even SDRAM... And plausibly be able to use the already-available linux drivers for those cards...

Not sure how much effort this would take... might need some nitty-gritty details on the bridge-chip(s), OTOH, e.g. 486-era bridges were pretty durn simple and pretty standardized... right?

So, obviously, the BIOS won't be executed (though, I suppose, execution of the BIOS could be emulated), but since it'd be a microcontroller, it could have its own BIOS in firmware...

Not sure why exactly this seems like a cool idea to me...

Some thoughts...

Per my recent experiments with 486 chips, in #Random Ridiculosities and Experiments, it would seem the 486-era was the transition from 5V logic to 3.3V and below... 486DX4's, for instance, have 3.3V core logic, with 5V tolerant I/O, whereas the 486SX is 5V-only... The DX4 was, it would seem, designed to be placed in an SX's socket as an upgrade. (Or, plausibly more important, designed to be used with industry-standard design-practices at the time, which just happen to be somewhat compatible with most hobbiests' abilities). Thus, a 486 mobo being a reasonable starting-point for such an endeavor. I haven't looked into Pentiums and beyond, but I'm guessing as of the later processors, there's probably some likelihood that their interfaces may be 3.3V or even lower. This might lend itself well to e.g. a PIC32 replacement, BUT (again, I haven't looked into them), it's also quite plausible that later processors use a less-standard I/O scheme, being that they may *rely* on the fact of bridge-chips. E.G. newer processors may use 1.8V, or LVDS, or who knows what... It's plausible they don't even use an Address/Data I/O scheme at all, in favor of some sort of newer "transport" scheme made specifically to work with bridge-chips. I really don't know. All I know is that I was pleasantly-surprised to find that 486's (which I just happened to grab at-random) still supported an I/O interface that makes sense to me...

Oooh, a *QUICK* overview of : http://download.intel.com/design/pentium/datashts/24199710.pdf Suggests that Pentium processors may be quite similar to 486's interface-wise. A BRIEF overview (and an utter lack of knowledge) suggests that the major difference is a 64-bit data-bus, as opposed to 32-bit. OK, that's Doable... The P66 uses 5V signals, the 75-200MHz chips use 3.3V... Might be doable. 64bit, well... I guess a few+ 74574's could latch two 32-bit data-cycles into 64bit. Doable, anyhow.

And, who knows what those bridge chips are capable of... The 486 definitely had 8-bit and 16-bit interface-*modes*, the Pentium likely has a 32-bit *mode* (despite having a 64bit data-width)... Is there a PIC32 with a Parallel Master Port that supports 32-bits...? hmm... And, even if not, is it possible to *send* the lower-bit-width mode (rather'n receive)? By which I mean... (it's been a while, I could be *completely* mistaken), I think the bit-width is determined by the device... Would it be possible to specify somehow that *all* devices are less than or equal to 32bits (or whatever bit-width the selected uC can support)...? Then, maybe, it'd be possible to rely on the "bridge" to break even a 64-bit device into 8, 16, or 32 bit transactions for our processor... and avoid the necessity for latches altogether......?

Ridonculous? Probably.

Here's a cool one: Someone plugged an ARM into a Commodore 64's CPU socket:

http://hackaday.com/2016/07/02/the-dual-core-arm-powered-commodore-64/

And this dude Booted Linux on an AVR emulating an ARM:

http://dmitry.gr/index.php?r=05.Projects&proj=07. Linux on 8bit

Discussions

jaromir.sukuba wrote 03/31/2017 at 07:15 point

"PIC32... well, there's already linux's for that, right?"

BSD Unix - http://retrobsd.org/wiki/doku.php

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Eric Hertz wrote 03/31/2017 at 07:40 point

Shoulda known!

#MiniBSD laptop computer ;)

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