Back in the early days of my career, I saw MARK used once in
MACRO-11 to implement a switch statement. It was horrific and super
long compared to the code that replaced it a few months later. It was a
horrible abuse of the instruction, and the person who wrote the code
(not me) was just learning PDP-11 ropes at the time. The replacement
code was also faster. I doubt I'd ever have noticed if this code didn't
wind up in the hot path and attract the attention of the senior engineer
on the project. He was so horrified, he called us all together to go over
what the instruction actually did, and why it was such a bad idea.
Warner