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The IBM PC AT: Alive and Booting

My first IBM PC AT (Model 5170) Test Configuration finally boots to a PC DOS prompt. The configuration uses the original IBM Model 5170 ROM BIOS, dated January 10, 1984.

Getting through the BIOS “POST” (Power-On Self Test) diagnostics was like running an obstacle course, with various tests derailing the simulation at every turn.

Sometimes the problems were as simple as missing hardware. For example, I knew that the PC AT contained two DMA controllers, for a total of 8 DMA channels, but what I didn’t know (or had forgotten) is that it also contained 16 DMA page registers, some of which the BIOS uses as scratch registers. Since DMA page registers are accessed with I/O instructions that work identically in both real-mode and protected-mode, they obviously offer some advantages over RAM, especially when the BIOS hasn’t yet tested all the RAM, or determined how much RAM is installed, or set up descriptors that allow the RAM to be accessed from protected-mode.

Aside from the additional DMA Controller, other major new motherboard components on the PC AT included a second 8259 Interrupt Controller, an 8042 Keyboard (“Kitchen Sink”) Controller, and an MC146818 Real-Time Clock/CMOS chip. The Keyboard Controller and Real-Time Clock/CMOS components required the most tinkering to pass through the ROM BIOS gauntlet.

For example, at one point, the BIOS (“TEST.21”) reset the keyboard (“KBD_RESET”), which unmasked the keyboard IRQ and waited for an interrupt, using a loop where CX was initialized to zero and then decremented until either CX wrapped around to zero again or an interrupt occurred. The “TEST.21” code then assumed that if “KBD_RESET” returned zero in CX, no interrupt had occurred.

Unfortunately, my Keyboard Controller was a bit too fast: it generated an interrupt as soon as the keyboard IRQ was unmasked; as a result, CX was never decremented, leaving it at zero.

Most of the effort getting to this point involved adding support for 80286 protected-mode. That work is still far from complete, but getting through multiple real-mode/protected-mode round trips in the BIOS was an important milestone. Some of the work was outside the CPU component, such as A20 support and processor reset via the 8042 controller. Work inside the CPU component included:

Another “feature” I spent considerable time on was ensuring that 80286 protected-mode support did not adversely real-mode performance, so that the PC and PC XT simulations still run (almost) as fast as before. PCjs dynamically reconfigures itself according to the requirements of the processor and platform it’s emulating.

There’s still no support for LOADALL or triple-fault resets, nor for call gates or task gates, nor for conforming code segments or expand-down data segments. 80286-specific cycle counts haven’t been incorporated yet, either. The list of remaining 80286 features is long.

And there’s plenty of hardware support left to do: I haven’t looked at the AT hard drive controller yet (which I believe is significantly different from the XT hard drive controller), and the keyboard barely works; the 8042 Keyboard Controller and AT keyboard had a number of features that older PC/XT keyboards did not (like LEDs and programmable repeat rate).

And there are plenty of issues to investigate. For example, PC DOS is picking up the correct RTC time, but not the date (PCjs initializes the RTC to the browser’s current date/time, unless a hard-coded date/time is specified in the machine XML). And diskette I/O seems a bit slow; I’m concerned that the BIOS is spinning its wheels somewhere unnecessarily. And even though the test machine is configured with 640Kb of RAM, the BIOS is reporting only 64Kb.

Hmmmm.

Jeff Parsons
Sep 13, 2014