[angersock]

If I'm not mistaken, the keyboard should raise high an interrupt pin on the CPU, which should cause an interrupt service routine to be called.

That routine should then mask lower-priority interrupts, poll the appropriate region of memory (assuming memory-mapped IO) for the byte or bytes held down, push those onto the buffer for key inputs or into the STDIN equivalent, unmask lower-priorty interrupts, and return.

It is then up to the user program to read in from the buffer and do the needful.

[kragen]

The Hack CPU doesn't have interrupts, IIRC.

[angersock]

I was actually working on implementing a simple little VM library in C as a fun exercise, and deciding how to handle interrupts was where I got caught up.

[kragen]

Most VMs don't have interrupts. As a less-complex alternative, you might consider implementing an I/O thread, something rarely done in hardware because an extra CPU is a lot more hardware than an interrupt mechanism.

One interesting exception is the Unix VM, whose interrupts are called "signals".