[TUHS] Porting 2.11 BSD

[Oliver Lehmann]

Hi Nick,

first - thank you very much the time you spent writing!

Thing is - yes - there is a P8000 emulator but it has some bugs which may
render the assembler not to produce code and so on. I feel better to use
the original hardware and my SYSIII to compile the stuff I need.

Using a harddisk image made by simh is a great idea. Porting disklabel,
mkfs and so on first would be a huge task before it works. But with the
way you proposed I would see much earlier "something" running on the
system.

Getting data from and to the current SYSIII is not an issue as I have kermit
on it and can transfer files via serial without any problem.

Getting a filesystem to the "BSD disk" for the system should also be easy
as I already build a WDC-Controller emulation hardware which uses a SD-Card
or an IDE harddrive as datastorage but behaves 100% compatible to the P8000
like the original Controller.
Using an SDCard makes it kinda easy to just dd the filesystem to/from it.

The current SYSIII Firmware, Boot and Kernel sources are as good as 100%
available which I guess would it make it a lot easier to port it to boot
from a BSD Filesystem.

The current boot process on the SYSIII is "rather simple". The firmware has
some basic functionality to boot from the harddisk. It just loads block 0
of disk 1 with a length of 200 byte into Address %8000 and executes whatever
is on that address after it was loaded

https://github.com/OlliL/P8000/blob/master/firmware/MON16/p.boot.s
(See line 609-636 - sorry for the german comments)

I definitly do not want to change the firmware in any way as I want to make
a "drop in replacement" to the current SYSIII - so no firmware modifications.
I wonder this would mean to not use disklabels for example. I guess disklabels
are stored in block 0 which would make it incompatible with the current
boot0 loading which is expected to be in block 0 - right?


Then there is currently the boot0 loader which is currently on block 0 of
the harddisk which is now executed.

This one is also implemented in ASM and is now able to understand the
filesystem. It is configured by default to open again block 0, reads a
defined inode and searches for the secondary bootloader "boot".

https://github.com/OlliL/P8000/blob/master/WEGA/src/cmd/BOOT0/boot0.md.s
(See line 128 - 277)

The secondary bootloader is implemented in C and is the first program, able
to execute segmented code like the kernel:

https://github.com/OlliL/P8000/blob/master/WEGA/src/uts/conf/boot.c


I guess, I have to port the primary bootloader first to be able to understand
UFS/FFS as it is probably different than the current SYSIII-FS and the primary
bootloader needs to "understand" it to look for the secondary bootloader.
I plan to "reuse" the current primary bootloader and just change the
filesystem code. I guess this should be enough.

The secondary bootloader is hopefully also implemented in C in 2.11  
BSD (didn't
looked it up yet) So I can modify it to understand Zilogs s.out format instead
of BSDs a.out format.
This is vital as the whole toolchain needs to be taken over to BSD as there is
just no other toolchain available compiling, assembling and linking  
Z8001 code.
I also have important binaries with no source code which need to be able to
run in BSD as well as they did in SYSIII (Z80-RIO-Emulator, CP/M-Emulator,
Cross-Assemblers for Z8, Z80). So making BSD able to understand the s.out
format of SYSIII is vital as I'm only able to compile code in s.out format
the first place.

So to summarize it... The first thing would be:

0. get simh up and running
1. get a 2.11-BSD filesystem onto a (new) SD-Card
2. modify the primary bootloader to read the UFS-filesystem
3. write a secondary bootloader which only does "hello world"
4. place the secondary bootloader onto that filesystem
5. boot it
6. fix it until it works
7. create a /unix kernel which does a "hello world"
8. modify the secondary bootloader until it is able to load
    the kernel.

What do you think?

- Is a current FreeBSD still able to read/write 2.11 BSD disks?
- I wonder how much difference the SYSIII-FS and UFS has regarding to the
   primary bootloader (boot0.md.s lines 128-158 searches for "boot").

Regards, Oliver