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We're almost ready to write a keyboard driver in Rust! But first, we need to deal with two obstacles: setting up the PIC, and handling interrupts without crashing. This is one of the most frustrating steps, as Julia Evans explains in her hilarious and very helpful post After 5 days, my OS doesn't crash when I press a key:
- Turn interrupts on (sti).
- The OS AGAIN crashes every time i press a key. Read “I Can’t Get Interrupts Working” again. This is called “I’m receiving EXC9 instead of IRQ1 when striking a key?!” Feel on top of this.
- Remap the PIC so that interrupt
igets mapped toi + 32, because of an Intel design bug. This basically looks like just typing in a bunch of random numbers, but it works.- 12. THE OS IS STILL CRASHING WHEN I PRESS A KEY. This continues for 2 days.
We're going to follow Julia Evans' roadmap. (She saved me several days of suffering.) And once we're past these next few obstacles, things will get easier. Let's talk to the PIC first.
The 8295/8295A Programmable Interrupt Controller
We're going to with the retro approach here, and handle interrupts using the 8295 PIC. You can read all about it on the OSDev wiki, as usual. The PIC works fine in 64-bit mode, but someday, if we want to support multiple processors and I/O, we'll eventually need to support the newer APIC and IOAPIC. But for now, let's keep it simple.
Technically, the x86 architecture has two PIC chips, usually known as PIC1 and PIC2. PIC1 handles external interrupts 0–7, and PIC2 handles 8–15. PIC2 is actually chained into interrupt 2 on PIC1, which means that we'll frequently need to talk to them as a pair.
Unfortunately, the modern x86 architecture reserves CPU interrupts 0-31 for processor exceptions. This means that when we press a key, the CPU will think it just received the "EXC9" mentioned by Julia Evans, which the Intel manual tells me is "Coprocessor-Segment-Overrun Exception." So we need to tell our PIC that, no, McGyver and Miami Vice are no longer cutting-edge television, that there's this new-fangled thing called 386 Protected Mode, and that it needs to start mapping interrupts at offset 32.