Re: branch address architecture, not interactive use, The Design of Design

John Levine wrote:

According to MitchAlsup1 <mitchalsup@aol.com>:

   JSR PC,@(SP)+
>
Popped the return address off the stack, pushed another return address
on
the stack and transfers control. This is how we did coroutines.

When I was teaching an operating system class in about 1977 I
challenged the class to come up with a minimal coroutine package. They
all found that pretty quickly.

It's not very good coroutines since it just switches the return
address, not any other stack context, but it can sometimes be useful.

The kinds of co-routines this is perfect for are those where each co-
routine is not reentrant so they can use their own data n their own memory to control their own state transition. One accesses these variables disp(PC)+ addressing. {almost as free as registers containing
those values {It was PDP-11 era...} I use this extensively in a RSTS controller applications.
But no-one would use co-routines this way with all our "accumulated" knowledge of how to write small simple efficient software these days.