[TUHS] merry christmas

Nick Downing downing.nick at gmail.com
Wed Dec 28 22:24:20 AEST 2016


I will let you know when I get it working :) It's not a current focus,
but I will return to it someday. In the meantime, I'm putting it on
bitbucket, so others will be able to pick it up if they wish. However,
this also isn't my current focus, it's there, but it's not documented.

The IAR compiler on the Z180 supports a memory model similar to the
old "medium" memory model that we used to use with Microsoft or Turbo
C on DOS machines, that is, multiple code segments with a single data
segment. Yes, the Z180 compiled C code is larger than the PDP-11
compiled C code, but luckily you can have multiple code segments,
which you cannot (easily) have on the PDP-11.

Unfortunately code and data segments share the same 64 kbyte logical
address space, so what I did was to partition the address space into 4
kbytes (always mapped, used for interrupt handlers, bank switching
routines, IAR compiler helper routines, etc), 56 kbytes (kernel or
current process data and stack) and 4 kbytes (currently executing
function). The currently executing function couldn't be more than 4
kbytes and couldn't cross a physical 4 kbyte boundary due to the
hardware mapping granularity, but this was acceptable in practice.

I got the Unix V7 clone working OK under this model and then added the
networking, so although it was a bit of a dogs breakfast, it proves
the concept works. My memory management left a fair bit to be desired
(too much work to fix) however I think porting 2.11BSD would solve
this problem since it works in the PDP-11 under split I/D, which has
similar constraints except the 4 kbyte code constraint.

My understanding is 2.11BSD is actually a cut down 4.3BSD running on
the HAL from 2.xxBSD, I would like to audit each change from 4.3BSD to
make sure I agree with it, so essentially my project would be porting
4.3BSD rather than 2.11BSD. But I'd take the networking stack and
possibly a lot more code from 2.11BSD, since it is simplified, for
instance the networking stack does not use SYN cookies.

cheers, Nick

On Wed, Dec 28, 2016 at 6:14 PM, Peter Jeremy <peter at rulingia.com> wrote:
> On 2016-Dec-25 17:21:31 -0500, Steve Nickolas <usotsuki at buric.co> wrote:
>>On Mon, 26 Dec 2016, Nick Downing wrote:
>>> I became frustrated with the limitations of both UZI and NOS and decided to
>>> port 2.11BSD to the cash register as the next step, my goal was (a) make it
>>> cross compile from Linux to PDP-11, (b) check it can build an identical
>>> release tape through cross compilation, (c) port it to Z80 using my
>>> existing cross compiler.
>>
>>A Z180 is powerful enough to run 2.11BSD? o.o;
>
> I suspect shoe-horning 2.11BSD onto a Z180 would be difficult - 2.11BSD
> on a PDP-11 requires split I+D and has kernel and userland in separate
> address spaces.  Even with that, keeping the non-overlay part of the
> kernel in 64KB is difficult.  Equivalent Z180 code is going to be much
> larger than PDP-11 code.
>
> I'd be happy to be proved wrong.
>
> --
> Peter Jeremy


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