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Dumping old Olivetti M20 Floppy Disks

Publish date: Apr 20, 2022
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The Olivetti L1 M20

Olivetti L1 M20

Back in the 1980’s my uncle owned a company for photo lithography. Eventually they needed a computer for adminstrative tasks and got a rather exotic Olivetti L1 M20. For the curious, the M20 has the following specs:

  • Zilog Z8001 16 Bit CPU
  • 128kb to 512kb RAM
  • PCOS Operating System
  • Built 1982-1984
  • 512x256 Graphics, 4 or 8 colors, depending on the available memory board in use. The M20 has no text mode and generates all graphics using pixels.

Years later, when the Olivetti was no longer used and I was asked if I would have “any use” for the machine, I of course said immeadialtely “yes”. Besides a few initial tests with the machine in the 1990’s, I never turned the machine on again (but maybe I will after this deep dive in M20 disk internals).

Alongside with the M20, I received some, now very aged, 5ΒΌ" disks. Since I also have a Kryoflux, let’s try to find out what they reveal.

My Olivetti L1 M20, condition is better than it looks like
The original monochrome monitor
The original printer PR1470 was also provided. Guess what is in that box above the printer ;)

Workflow

To dump the M20 disks, I use the following workflow:

                          Floppy Disk to raw disk image
                               conversion workflow

  +----------+          +----------+          +----------+           +----------+
  |  5.25"   [          |          |          |    3     |+          |          |
  |  Disk    |          | kryoflux |          |   RAW    ||+         |   RAW    |
  |    O     +--------->+  stream  +--------->+  images  |||-------->+  image   |
  |          | kryoflux |   files  | kryoflux |          |||   dd    |          |
  |          | dtc tool |          | dtc tool |          |||         |          |
  +----------+          +----------+          +----------+||         +----------+
                                               +---------+||
                                                +----------+

The steps in detail

Note that the Teac FD54B drive used here is a 40 Track drive. All dtc examples are shown for this drive. When using a 80 Track drive, e.g. a Teac FD55GFR, some of the commands need different parameters.

Calibrate the drive

First we need to calibrate the drive and check if the Kryoflux is working:

TEAC FD54B:

> dtc.exe -c2
CM: maxtrack=41

Create a Kryoflux stream file of the disk

To create a Kryoflux stream file, I use:

> dtc.exe -fM20_Assembler/dump -p -i0
  • -f<path>/<file> - target filename
  • -p - create paths
  • -i0 - create raw stream

Order of options is important, -ix must preceeded by other options.

Create the RAW image

We use the dtc tool to create a raw disk image containing only the sector data from the stream files:

$ dtc -m1 -fM20_Assembler/dump -i0 \
        -fM20_Assembler/M20_Assembler.raw \
        -s0 -e34 -os0 -oe34 -z1 -i4 \
        -l8 
KryoFlux DiskTool Console, v3.00_Linux_x86-64_LU, uiv.1, Feb 24 2022, 17:12:23
(c) 2009-2022 KryoFlux Products & Services Ltd.
Developed by The Software Preservation Society, www.softpres.org
Licensed for private, non-commercial use only.

Stream file: M20_Assembler/dump

00.0    : MFM: <unformatted>
00.1    : MFM: OK*, trk: 000, sec: 16, *H +10
01.0    : MFM: OK*, trk: 001, sec: 16, *H +10
01.1    : MFM: OK*, trk: 001, sec: 16, *H +14
...

Besided track 00.0 everything could be read. Let’s see if the image works …

Read files using m20 floppy tools

The image we just created can now be read with Christian Groesselers m20 floppy tools, that’s a first success:

$ m20  M20_Assembler/M20_Assembler.raw lsl

Volume name: ASSEMBLER
# of files: 15		free blocks: 8

NAME            	SIZE	BLOCKS	ALLBL	EXTENTS
--------------  	----	------	-----	-------
PCOS.SAV        	41634	163	303	1
asm.abs         	52436	205	206	1
asm.bin         	7996	32	33	1
asm.cmd         	999	4	5	1
edit.cmd        	28074	110	111	1
fkill.cmd       	1263	5	6	1
flist.cmd       	1702	7	8	1
hexdump.cmd     	786	4	5	1
link.cmd        	49939	196	197	1
mlib.cmd        	12273	48	49	1
pdebug.cmd      	10055	40	41	1
tdump.cmd       	18413	72	73	1
textdump.cmd    	6080	24	25	1
vlist.cmd       	3142	13	14	1
_tlocs          	2	1	4	1

Test the image with MAME

The famous “Multiple Arcade Machine Emulator” (MAME) has also an Olivetti L1 M20 emulation. Let’s try to see if our freshly created disk image can be booted there:

$ mame m20 -rompath . -bios 2 -flop0 M20_Assembler.raw
No luck booting the image in MAME

That did not work. The disk does not boot and is not readable in the emulator. So we have to dig a little bit deeper …

Anatomy of a M20 disk

Reading howto’s, reverse engineering existing images, and poking around with the Kryoflux dtc tool, I realized that the disk format is a rather unusual FM/MFM mixture of tracks: track 0, head 0 is FM formatted with 16 sectors * 128 bytes (=2048 Bytes/track) while the rest of the disk is MFM formatted with 16 Sectors * 256 bytes (=4096 bytes/track):

<Track>.<Side>Format
00.0FM, 16 Sectors * 128 Bytes
00.1MFM, 16 Sectors * 256 Bytes
(rest)MFM, 16 Sectors * 256 Bytes

In a raw disk image file for the MAME M20 emulator however, all sectors are assumed to be 16 * 256 bytes = 4096 bytes sized.

Create the RAW image, 2nd try

My new strategy is to convert the raw stream using different settings for track 00.0 and the rest of the disk. Unfortunately I could not figured out how to write the raw image with a single invocation of dtc. So I converted the stream in 3 chunks: Track 00.0 (2048 bytes), Track 00.1 (4096 bytes) and the remaining 34 tracks (34 * 2 * 4096 bytes). Next these 3 chunks are assembled to the final image with dd, but right padding the first chunk (track 00.0) with 2048 bytes of 0. In total, we now have an image of 35 * 2 * 16 * 512 bytes = 286720 bytes.

For convenience, I have provided a script here.

Final test with MAME

Let’s try to boot the disk we just created with MAME:

The M20 Assembler disk now boots fine in MAME

This time, it looks good, MAME successfully booted PCOS. The vlist command shows all files on the disk and programs can be started.

File listing with vlist command in booted PCOS. Happy Z8000 assembler hacking!

Summarizing, making bootable images of M20 floppy disks was a bit harder then what I experienced when i dumped floppy disks of the Sharp MZ-800, due to the mixed FM/MFM format. But equipped with this knowledge and some scripting, bootable images could be finally built.

References