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Supporting the 80286

The next milestone for PCx86 is complete 80286 emulation. My hope is to have it working by the end of the year.

PCx86 version 1.15.0 is the first step on the path to full 80286 support. It includes changes to the physical memory manager and separate real-mode and protected-mode address evaluators. The Debugger supports physical addresses (eg, %FE05B is the same as F000:E05B, assuming real-mode operation), along with breakpoint commands that stop execution on port input/output operations. And the ChipSet component now contains “infrastructure” (a fancy way of saying “partial support”) for multiple PICs, DMA controllers, the 8042 keyboard controller (including A20 support), and a bit more – but not much.

One of the challenges is creating a single “universal” version of PCx86 that can adapt itself to different machine types without impacting performance. There will not be a pc8088.js or a pc80286.js or whatever. There will only be pcx86.js.

Up until now, all PCx86 machine XML files assumed an 8088 CPU with a 20-bit bus and a model 5150 or 5160 motherboard. But now, a machine XML file can specify:

<computer name="IBM PC AT" buswidth="24"/>
<cpu model="80286"/>
<chipset model="5170"/>

Conventional emulators are usually NOT able to run original BIOS images, or simulate original PC hardware, or even run at the same speed as the original PC, making some software difficult or impossible to use. PCx86 takes a different approach, by attempting to simulate an entire PC as it originally existed. Which is why a PCx86 simulation of an IBM PC does NOT run at whatever speed your modern PC happens to run in V86-mode or whatever speed your browser’s JavaScript engine tops out at.

No, a PCx86 simulation of a 4.77Mhz IBM PC runs at 4.77Mhz. And a PCx86 simulation of a 6Mhz IBM PC AT will run at 6Mhz. If you want to run the simulation faster, you have that option, but that’s not the default. And I’m not saying that PCx86 is exact – exactness is an exercise I’m leaving for another day and/or to other developers who are even more obsessive than I am. I’m just saying that original PCs represent the targets that PCx86 is shooting for.

PCx86 1.15.0 can now load and run the IBM 5170 ROM BIOS up to the first 80286-specific opcode, so it’s off and running. Although “running” isn’t quite the right metaphor, because the process of bringing a new machine simulation to completion is a very long series of baby steps.

Also, in preparation for this new phase, I recently dug up a variety of old 80286 CPU Documentation and posted excerpts. I’m sure none of this information is “new” at this point, but it might have some historical interest.


Jeff Parsons
Aug 28, 2014