Curiosity

[Ron Natalie]

MARK is part of a bozo subroutine instruction linkage that no one
uses.

You push it on the stack after all the parameters to a subroutine
and then call the subroutine.  When you return from the subroutine
rather that return to the called address you return to the MARK
instruction, which pops everything back of the stack and then
continues execution with the address that was pushed by the JSR.

What it really does (from the Processor Handbook) is:

sp <- pc + 2 + 2n
pc <- r5
r5 <- (sp)+

Great? Right.  A thousand and one uses.