Electronic – eZ430-Chronos Development on *nix

You basically have two options here:

• A current-generation 486-based CPU
  • Upside:
    • Everything will likely be integrated into the main CPU - No southbridge, IO controllers, etc...
    • you can actually buy the ICs off the shelf.
  • Downside:
    • Devices may be BGA
    • few exposed busses
    • may want to run at a few hundred Mhz.
  • By "Currently Produced", I mean something like a AMD Geode, or similar device from VIA, etc... There is a pretty big market for small, low power, low speed x86 CPUs in embedded applications.
• A old-stock or old-design 386/486 CPU
  • Upside:
    • Probably more educational
  • Downsides:
    • Requires a LOT of devices (e.g. Southbridge, a UART, etc...), all of which are additional ICs.
    • ^{_{Note: the external devices will vary depending on which 386/486 you decide on. The early ones had few integrated peripherals. Later on, many of the peripherals got integrated into the CPU itself.}}

There are some midpoint-devices, like the 386EX, which is a 386 intended for embedded applications. It's old enough that it's available in TQFP-144 (released in 1994), yet it includes most of the necessary peripherals on-die.
_Datasheet

Some resources, off the top of my head:

CoreBoot:

A FOSS BIOS alternative.

Interesting forum thread about building a IBM XT compatible computer.

Other stuff:

Dieter's Homepage
Some nutjob who build a discrete Transistor CPU!
He also has a bunch of other homemade CPU projects

Really, If I were you, I would go with an ARM device. You can get big ARM CPUs that have MMUs, and will run linux fine.

Alternatively, an 8088 or 8086 may be significantly more approachable. There is lots of information about people homebrewing 8088 computers out there.

I've had lots of success with FreeRTOS, combine this with an ARM Cortex dev board such as one from Olimex (available from Farnell) - see This Page for a list of supported devices. A Cortex M3 will run at 75MHz and deliver over 80MIPS. ARM code is dead effecient and some ARM Cortex devices include fixed point math functions.

If you want even more grunt, try a Beagleboard or RasberryPI.

As well as FreeRTOS, the latter will run Linux with Linux compiled with config_preempt_rt config option.