[TUHS] System III - TCP/IP

Nick Downing downing.nick at gmail.com
Tue Nov 10 08:36:04 AEST 2015

Well that looks to me like a standard split I/D setup where each process,
and the kernel, can have up to 64 kb of code and 64 kb of data. It will run
2.11bsd with a relatively straightforward porting effort -- direct
translation of each code fragment to do something equivalent on your device.

What I discussed as another option (that I did on Z180 which did not have
split I/D and hence had too small a logical address space to run 2.11bsd or
any other reasonable unix)... was to increase the code limit past 64 kb by
segmentation. Whilst this is difficult to do, it can be achieved without
(much) compiler support. Code pointers become 3 bytes instead of 2, but
luckily, code pointers are hardly used, and this limitation can be got
around by using stubs.

You are right though, in that a natively running system that can freely
utilize your 6 Mb or 8 Mb or whatever, certainly IS possible. The MS DOS C
compilers for PC/XT, PC/AT etc, supported this. The "large" memory model
had segmented code and data, any process could have multiple code and data
segments. Pointers were 4 bytes (equivalent to 3 bytes in your case, a
16-bit offset and a 7-bit segment). However, pointer arithmetic did not
carry into the segment field, so arrays and structs and the biggest
"malloc" were limited to 64 kbytes. It was somewhat slower than the "small"
memory model (2 byte pointers) and everything took more memory. The "huge"
memory model removed the 64 kbyte limitation altogether via a software
emulation of a flat address space (by doing carrying arithmetic whenever
pointers, arrays or structs were derefenced) but was of course MUCH slower.

If your compiler supports these things, go ahead and use them and port
4.3BSD across, it would be great. I thought you were talking about a
smaller system like 1 Mb, with 1 Mb you could support say 3 users and a
kernel @ 256 kbytes each using a 2.11bsd-like split I/D system and a small
memory model. They could run a reasonable toolchain consisting of a shell
spawning cc spawning a compiler or linker (for example) and an editor,
simultaneously, though it would be tight. In that case I would be concerned
about speed and size and would recommend a "small" memory model. But if you
have 6 Mb and your processor is relatively fast (say 8 MHz) and your
compiler efficient, then I guess a "large" model would be reasonable. I
like tiny unices, though :)

On 09/11/2015 8:40 PM, "Oliver Lehmann" <lehmann at ans-netz.de> wrote:

> http://cvs.laladev.org/index.html/P8000/WEGA/src/uts/sys/ureg.c?rev=1.9&content-type=text/x-cvsweb-markup
> 1.9 is the current version 1.1 is the plain SysIII file...
> Oliver Lehmann <lehmann at ans-netz.de> wrote:
> This is for example the ureg.c which shows the Segmentation Code for the
>> current SysIII Kernel I redid from original SysIII Sources + Disassembled
>> object-File analysis of the current SysIII Kernel running on the system:
>> http://cvs.laladev.org/index.html/P8000/WEGA/src/uts/sys/ureg.c?rev=1.1&content-type=text/x-cvsweb-markup
>> Oliver Lehmann <lehmann at ans-netz.de> wrote:
>> I hit the send button to fast :(
>>> Oliver Lehmann <lehmann at ans-netz.de> wrote:
>>> Nick Downing <downing.nick at gmail.com> wrote:
>>>> You can most likely implement a split I/D architecture by having code
>>>>> execute out of a different segment than data, although I haven't
>>>>> checked the datasheet so I don't know for sure if this is possible.
>>>> I have 3 different MMUs available. Stack, Code and Data it is called in
>>>> the Circuit diagram. I probably have to lookup the references but I
>>>> still
>>>> don't get why the kernel can only work with 64k of RAM as it is right
>>>> now
>>>> compiled as a segmented programm and uses more memory already with
>>>> SysIII.
>>>> Only the boot0-loader is compiled unsegmented and also some small
>>>> utilities. Every other portion of the system uses the segmentation
>>>> features
>>>> of the Z8001 and is not limited to 64k adressable space.
>>>> The system has right now MB of
>>> The system has right now around 6 MB of RAM but can be upgraded to 16MB
>>> minus 64KB (Due to a firmware bug) of RAM.
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