Creating your own CP/M disks

[haerfest]

My Electron has Dave Hitchin's ATI and a PiZero attached, happily running CP/M. But getting CP/M software from the web onto it was a bit of a puzzle. For others in the same boat (or myself in the future ) I thought I'd do a little writeup.

What we're dealing with:

• A single 400 Kb Acorn CP/M disc is stored in a container file across two 200 Kb .SSD discs, one containing the data for the top side, the other for the bottom side.
• These go, top and bottom, respectively in drives :0 and :2 (for A:) or :1 and :3 (for B:).
• The top disc must be a valid .SSD (DFS, 80 tracks, single sided); the bottom one can be garbage as CP/M overwrites it in its entirety.
• The top disc must contain one file called CPMDISC, filling up the entire disc.
• As described in J.G. Harston's AcornCPM, CP/M tracks are numbered 0-79 on the top disc, but 79-0 on the bottom one, so we'll need to do some reversing along the way.

I'm on a Mac and this is the software I used:

• Cpmtools (I used MacPorts to install).
• MMB_Utils.

To get e.g. Zork 1 onto the Electron:

1. Create an empty CPMDISC file of the right size:
   

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   $ dd if=/dev/zero of=CPMDISC bs=1 count=204288
   

2. Use MMB_Utils to create a new .SSD image named zork1.cpm, add the file to it and lock it if desired:
   

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   $ beeb blank_ssd zork1.cpm
   $ beeb putfile zork1.cpm CPMDISC
   $ beeb access zork1.cpm CPMDISC L
   

3. Extend it by the size of another .SSD image (for the bottom side), creating a single Acorn CP/M volume zork1.cpm:
   

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   $ dd if=/dev/zero bs=1 count=204800 >> zork1.cpm
   

4. With Cpmtools installed, edit /opt/local/share/diskdefs and add the acornsd format to the end, as described in AcornCPM:
   

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   diskdef acornsd
     seclen 512
     tracks 160
     sectrk 5
     blocksize 2048
     maxdir 128
     skewtab 0,2,4,1,3
     boottrk 3
     os 2.2 
   end
   

5. Format the disc for CP/M and copy the Zork 1 files for user zero:
   

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   $ mkfs.cpm -f acornsd zork1.cpm
   $ cpmcp -f acornsd zork1.cpm zork1.com 0:
   $ cpmcp -f acornsd zork1.cpm zork1.dat 0:
   

6. Split the file in two halves again and rename the split parts zork1-top.ssd and zork1-bottom.ssd respectively:
   

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   $ split -b 204800 zork1.cpm 
   $ mv xaa zork1-top.ssd
   $ mv xab zork1-bottom.ssd
   

7. Now the tricky part: we have to reverse the order of the tracks on the bottom disc from 0-79 to 79-0. A track is 2,560 bytes in size (10 sectors of 256 bytes), so we split the bottom .SSD in blocks of that size and concatenate them in reverse order, creating zork1-bottom-reversed.ssd:
   

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   $ split -b 2560 zork1-bottom.ssd
   $ cat `ls -1 x* | sort -r` > zork1-bottom-reversed.ssd
   

You now have two .SSD-like discs, zork1-top.ssd and zork1-bottom-reversed.ssd, that together make up one Acorn CP/M disc.

To use these with a working CP/M second processor setup, insert these in drives :1 and :3 respectively, making up a single CP/M drive B. For example, when using MMFS v1 with the discs in respectively slots 498 and 499:

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A>STAR DIN 1 498
A>STAR DIN 3 499
A>STAT B:*.*

 Recs  Bytes  Ext Acc
   68    10k    1 R/W B:ZORK1.COM
  664    84k    3 R/W B:ZORK1.DAT
Bytes Remaining On B: 294k

Getting data off of Acorn CP/M discs is essentially the process above in reverse:

1. Get the two .SSDs that make up one Acorn CP/M volume onto your modern computer, say zork1-top.ssd and zork1-bottom.ssd.
2. Realize that the bottom .SSD is reversed, so reverse its track order once again in exactly the same way as above, producing zork1-bottom-unreversed.ssd:
   

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   $ split -b 2560 zork1-bottom.ssd
   $ cat `ls -1 x* | sort -r` > zork1-bottom-unreversed.ssd
   

3. Combine both .SSDs together to create a single Acorn CP/M volume zork1.cpm:
   

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   $ cat zork1-top.ssd zork1-bottom-unreversed.ssd > zork1.cpm
   

4. Copy any files for user zero (for example) off of it:
   

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   $ cpmcp -f acornsd 0:zork1.sav zork1.sav
   

Enjoy some Zork or Turbo Pascal

- Wouter

[dominicbeesley]

Thanks for the write up. I remember struggling with this in the past

D

[danielj]

Top guide! Thank you

[shifters74]

Yup booked marked this post for future reference!

Thanks

Shifters