51 lines
2.8 KiB
Markdown
51 lines
2.8 KiB
Markdown
---
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layout: post
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title: Supporting the 80286
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date: 2014-08-28 11:00:00
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category: 80286
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permalink: /blog/2014/08/28/
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---
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The next milestone for PCjs is complete 80286 emulation. My hope is to have it working by the end of the year.
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PCjs version 1.15.0 is the first step on the path to full 80286 support. It includes changes to the physical
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memory manager and separate real-mode and protected-mode address evaluators. The Debugger supports physical
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addresses (eg, %FE05B is the same as F000:E05B, assuming real-mode operation), along with breakpoint commands that
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stop execution on port input/output operations. And the ChipSet component now contains "infrastructure" (a
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fancy way of saying "partial support") for multiple PICs, DMA controllers, the 8042 keyboard controller (including
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A20 support), and a bit more -- but not much.
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One of the challenges is creating a single "universal" version of PCjs that can adapt itself to different machine
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types without impacting performance. There will not be a **pc8088.js** or a **pc80286.js** or whatever. There will
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only be **pc.js**.
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Up until now, all PCjs machine XML files assumed an 8088 CPU with a 20-bit bus and a model 5150 or 5160 motherboard.
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But now, a machine XML file can specify:
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<computer name="IBM PC AT" buswidth="24"/>
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<cpu model="80286"/>
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<chipset model="5170"/>
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...
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Conventional emulators are usually NOT able to run original BIOS images, or simulate original PC hardware,
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or even run at the same speed as the original PC, making some software difficult or impossible to use. PCjs takes a
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different approach, by attempting to simulate an entire PC as it originally existed. Which is why a PCjs simulation
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of an IBM PC does NOT run at whatever speed your modern PC happens to run in V86-mode or whatever speed your
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browser's JavaScript engine tops out at.
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No, a PCjs simulation of a 4.77Mhz IBM PC runs at 4.77Mhz. And a PCjs simulation of a 6Mhz IBM PC AT will run at
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6Mhz. If you want to run the simulation faster, you have that option, but that's not the default. And I'm not saying
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that PCjs is *exact* -- exactness is an exercise I'm leaving for another day and/or to other developers who are even
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more obsessive than I am. I'm just saying that original PCs represent the targets that PCjs is shooting for.
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PCjs 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.
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Although "running" isn't quite the right metaphor, because the process of bringing a new machine simulation to
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completion is a *very* long series of baby steps.
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Also, in preparation for this new phase, I recently dug up a variety of old [80286 CPU Documentation](/pubs/pc/reference/intel/80286/)
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and posted excerpts. I'm sure none of this information is "new" at this point, but it might have some historical interest.
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Enjoy.
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*[@jeffpar](http://twitter.com/jeffpar)*
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*August 28, 2014*
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