[Oberon] RISC.img format

[Paul Reed]

Dear Oberoners,

I've been asked a couple of questions about the RISC.img SD-Card image
downloadable from http://projectoberon.com for the FPGA RISC5 Oberon
systems.  (If you can, please post questions publically rather than
emailing me privately - otherwise I have to assume it's a private matter
which is more time-consuming to deal with - if I can ever get to it - and
doesn't benefit others.)

The RISC Oberon system boots from an SD-Card and uses it as its file
system.  This filesystem is much simpler than common contemporary
filesystems such as FAT, FAT32, NTFS, exFAT, EXT4 etc. and is not in any
way compatible.  (On the original Ceres, the backup floppy disk format was
very close to the FAT12 format, but with an altered directory to cope with
long filenames, which were not in those days supported on FAT).

It would have been easy to make the RISC5 Oberon filesystem (including its
reserved area for the bootfile) begin at offset 0 from the beginning of
the SD-Card.  This would have dedicated the SD-Card to Oberon use, but
simplified the bootloader.

Or alternatively, the Oberon filesystem could have perhaps been stored as
a file in the FAT filesystem on the SD-Card, making it much more
accessible to utilities running on other computers.  But this convenience
would have come at a high cost: it would have made the bootloader much
more complicated, and given many more possibilities for distracting
problems in development.

In the end, it was decided to place the Oberon filesystem at a fixed
offset from the beginning of the disk.  This was a trivial change to the
bootloader, keeping it easily-understandable and within one block ram
(BRAM) of the FPGA.

This meant that FAT-compatible data structures, and a reasonable-size and
perhaps useful (256MB) FAT partition could still exist at the beginning of
the disk.  If this FAT partition has nothing in it, it compresses very
well and therefore does not significantly increase the RISC.img download
time.

The MBR partition id for the Oberon filesystem was chosen as 0FFh, because
the ids are historically rather a free-for-all and the filesystem is not
exactly the same format as any that have gone before, even the ETH
PC-based Oberon filesystems.  0FFh is clearly an arbitrary value, and also
was only used by others for private data structures rather than for
filesystems which were interoperable amongst systems.

Note that a lot of the motivation for this was defensive.  For example it
makes it more difficult for the Oberon filesystem to be destroyed when the
SD-Card is inserted in a Windows system.

When submitting some early additions to Peter de Wachter's RISC emulator

https://github.com/pdewacht/oberon-risc-emu

for emulated RS232 file transfer which he kindly incorporated, I also
included a quick check on startup which allows the use of a disk image
which only contains the Oberon file system (called .OBERON.FS in the
Windows binary distribution of the emulator which I put on
http://projectoberon.com).

This check is for the magic number 9B1EA38DH (FileDir.DirMark) which is at
the beginning of the root directory sector.  Oberon uses a "parity" scheme
for sector numbers, making all of them divisible by 29, so the first
usable Oberon sector, number 29, begins at SD-Card sector offset 80002H. 
This slightly odd value comes from the fact that SD-Cards have 512-byte
sectors, but Oberon uses two of these (1K) for each Oberon sector, and
that sector 0 is not used.

So if you are using a nice operating system and you know what you are
doing (because it allows dangerous commands such as dd) you can extract
the Oberon file system from the image with a command like (262145 = 40001H
= 80002H * 512 DIV 1024)

  dd if=RISC.img of=.OBERON.FS  obs=1024 ibs=1024 skip=262145

and you can even (sudo) write it directly back to a real SD-Card with
something like

  dd if=.OBERON.FS of=/dev/disk1 obs=1024 ibs=1024 seek=262145

The name .OBERON.FS was chosen to make it look like a system file, which
would be totally destroyed if someone opened it and saved it with a text
editor, for example.

Hope that's useful!
Cheers,
Paul