Clear interrupts

For some reason CLI popped into my head the other day.

CLI is the 808x instruction to clear maskable interrupts. If you’re writing a routine to service a hardware interrupt, you do a CLI early in your routine — to prevent another hardware interrupt from causing your routine to be re-entered. Neglecting this invites the most delightful form of bug, the intermittent bug.

Roland MPU–401

I spent a lot of time struggling with this stuff early in my coding career, writing a MIDI sequencer. The Roland MPU–401 MIDI interface would hit IRQ 8 when some MIDI bytes arrived, or when a timer ticked. I remember eagerly reading BYTE magazine articles and learning how write ISRs (interrupt service routines) by trial and error. Many trials and many errors. There was no internet search much less StackOverflow.com back then. (Also, we lived in a shoebox in the middle of the road.)

In real life we have interruptions. The worst is when you’re interrupted, then that interruption is interrupted. And so on. People don’t have stacks they can pop instantly to return to their prior context. Instead of popping a stack, the process is akin to flailing around with Google search, throwing around keywords and trying to sniff out the right track.

It’s too bad there’s no CLI instruction in real life.