From mboxrd@z Thu Jan  1 00:00:00 1970
From: jnc at mercury.lcs.mit.edu (Noel Chiappa)
Date: Fri, 27 Nov 2020 06:54:44 -0500 (EST)
Subject: [TUHS] Apple IIe Unix?
Message-ID: <20201127115444.E581018C091@mercury.lcs.mit.edu>

    > From: Steve Nickolas

    > there's no easy way to do preemptive multitasking without extra
    > hardware.

Perhaps you're using some idiosyncratic definition of "preemptive" and
"multitasking", but to me that statement's not accurate.

Let's take the "multitasking" part first: that just means 'two or more
computations can run at the same time, sharing the machine' - and that's not
hard to do, without special hardware, provided there's some way (in the
organization of the software) to save the state of one when the other is
running. Many simple systems do this; e.g. the MOS system that I used on
LSI-11's, BITD.

"Preemptive" is a bit trickier, because things have to be organized so that
one 'task' can be temporarily stopped arbitrarily (i.e. without it explicitly
giving up the processor, which is what e.g.MOS did) to let another run. There
does need to be some asynchronous way of inciting the second 'task' to run,
but interrupts (either clock, or device) do that, and pretty much every
machine has those. MINI-UNIX, for example, has premptive multitasking.

The thing that takes special hardware is _protecting_ one task from a bug in
another - a bug which could trash the first tasks's (or the system's!)
memory. One has to have memory management of some kind to do that.


    > From: Dave Horsfall

    > I would start with something like Mini-Unix

MINI-UNIX would be a good place to start if one wanted to bring up a system on
a machine without memory management; there's nothing in the kernel which is
PDP-11 dependent that I can think of (unlike V6, which had a fairly heavy
dependency on the PDP-11 memory management hardware - although one could of
course rip that all out, as MINI-UNIX did).

However, one's still looking at a fair amount of work, both to get rid of any
traces of PDP-11isms (e.g. stack growth direction), and translate the
assembler part (startup, and access to non-C operations). Something like FUZIX
might be an easier option.

       Noel