When an interrupt happens on a PDP-10, as on many other architectures you get the program counter pushed onto the stack. Actually, the program counter includes many status bits in the upper half of the word, similar to early ARMs.
One of these bits is the "First Part Done" bit, which apparently may be set to 1 or 0 depending on if something was done or not.
My question is about that bit. What is the First Part of executing an instruction? Why service an interrupt after that thing, instead of after the entire instruction?