Subject: Re: Happy PDP-5 day

From: [email-masked] (Charles Lasner)
Date: 4 Sep 1993 02:20:33 GMT

In article <[email-masked]>,
Paul Repacholi <[email-masked]> wrote:
>In article <bqt.745685867@Minsk>, [email-masked] (Johnny Billquist) writes:
>> In <[email-masked]> [email-masked] (Charles Lasner) writes:

I didn't say that; watch the attribution!

>Hey Charles, do you know the Zigglet story?

No, please enlighten us.

cjl

---

From: [email-masked] (Charles Lasner)
Date: 4 Sep 1993 02:31:15 GMT

In article <[email-masked]>,
 <[email-masked]> wrote:
>In article <[email-masked]>, [email-masked] (Martin Minow) writes:
>> 
>> RT11 was done by many of the same people who had worked on OS/8. It
>> used some of the OS/8 ideas, but wasn't very similar inside. Its
>
>One difference that seemed significant to me at the time I used these things, 
>was that OS/8 turned interrupt of, RT11 of course had it on.

This is quite true, and is why OS/8 can get away with what was once known
as "the glorious lack of overlap" which allows handlers the right to depend
on the use of the data-break locations in 07750-07756 to be used as temporaries
from OS/8 system handlers for other devices.  If a handler were running on
these devices, the locations could be in the state of being wiped out by
those handlers, etc. unbeknownst to the system handler.

Of course this also means that for all programs other than the likes of F4, 
which has its own interrupt system while active, you cannot get overlapped
I/O on unit record stuff like the LPT or 03/04 console, etc.

However, if you want to write a high-performance real-time system of your
own design, the non-interrupt nature of OS/8 is such that it's totally out
of your way, since you don't have to go around patching interrupt handlers
to point to your code, and hooked to other routines from other areas of the
system as well.  Real-time applications and overly clumsy interrupt structures
don't mix.  Witness MS-DOS for an "interesting" example of this, etc.

When every cycle matters, give me a system that gets out of my way, as opposed
to something where everything is "built-in" but you have to conform to the
structure, which can inevitable work against you.  (Anyone ever met people
who counted instructions on LSI-11-based real-time code?)

cjl