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From: mitchalsup@aol.com (MitchAlsup1)
Newsgroups: comp.arch
Subject: Re: branch address architecture, not interactive use, The Design of Design
Date: Sat, 25 May 2024 20:29:20 +0000
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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.