It is insane because IBM wanted a wireless keyboard. If you're not familiar with the PCjr keyboard, it has two IR transmitters (they are electrically the same, just having two gives you a better transmission path). The PCjr system has an IR receiver.

Now, a normal keyboard cable is a standard two-wire serial protocol with clock and data. Being wireless, you have no clock, so you have to synchronize clocks, or have clock recovery built into the protocol. The PCjr does the former, using the 8253 timer chip.

The keyboard has no way of knowing if the system has received a keystroke, so the system must receive every keystroke - they can't be stored in a fifo, they can't be resent if the CPU missed it.

This rules out the possibility of using system interrupts - so when you type a key on the CPU and the IR receiver gets the start bit - the NMI line is raised, the 8253 is programmed to time the clock bursts from the IR receiver, and the CPU reads out every bit of the scancode.

In software.

But honestly I wanted to learn the Rust programming language and I thought writing an emulator was a good a project to learn as any.

I have been working on a emulator in C++ in collaboration with a friend, it is also cycle-accurate. It is called XTCE-Blue.

From Wikipedia,

"Not long after its introduction, Creative Labs introduced its competing Sound Blaster card. The Sound Blaster was fully compatible with AdLib's hardware, and it also implemented two key features absent from the AdLib: a PCM audio channel and a game port. With additional features and better marketing, the Sound Blaster quickly overshadowed AdLib as the de facto standard in PC gaming audio. AdLib's slow response, the AdLib Gold, did not sell well enough to sustain the company."

If you have a CGA card, you're stuck with the code page of the ROM your card came with, which in most cases was code page 437, although a few variants were made.

EGA/VGA had software fonts so other code pages could be loaded.

Javascript says the key I pressed was keycode 220 (same number as 0xDC) or key "\".

I had Tandy 1000 as the selected machine. When I look at the onscreen keyboard, I don't see a backslash key underneath backspace, but I do see one next to left shift. For keyboards that have the alternate backslash there, it is listed as having a scancode of 0x56 and a VKEY code of VK_OEM_102 (0xE2). (I haven't actually checked information specific to the actual Tandy keyboard)