mx
====
## Soviet-era PDP-11 clone
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The DVK (in Russian, 沾7d65
Computing Complex) was a late 1970s Soviet personal computer, a cut-down
version of the professional SM EVM (ⵁ241c
--- literally System of Mini Computers), which _itself_ was an unlicensed
clone of the PDP-11. The MX board was an early floppy drive controller board
for it.

<div style="text-align: center">
<a href="http://www.leningrad.su/museum/show_big.php?n=1006"><img src="dvk3m.jpg" style="max-width: 60%" alt="A DVK computer"></a>
</div>

The MX format is interesting in that it has to be read a track at a time. The
format contains the usual ID prologue at the beginning of the track, then
eleven data blocks and checksums, then the epilogue, then it stops. The
actual encoding is normal FM. There were four different disk variants, in all
combinations of single- and double-sided and 40- and 80-tracked; but every
track contained eleven 256-byte sectors.

The format varies subtly depending on whether you're using the 'new' driver
or the 'old' driver. FluxEngine should read both.

A track is:

  * 8 x 0x0000 words (FM encoded as 01010101...)
  * 1 x 0x00F3 --- start of track
  * 1 x 0xnnnn --- track number
  * 11 of:
    * 128 words (256 bytes) of data
    * 16 bit checksum
  * **if 'new' format:**
    * 3 x 0x83nn --- `n = (track_number<<1) + side_number`
  * **if 'old' format:**
    * 3 x 0x8301

The checksum is just the unsigned integer sum of all the words in the sector.
Words are all stored little-endian.

## Options

  - Format variants:
      - `110`: 110kB 5.25" 40-track SSSD
      - `220ds`: 220kB 5.25" 40-track DSSD
      - `220ss`: 220kB 5.25" 80-track SSSD
      - `440`: 440kB 5.25" 80-track DSSD

## Examples

To read:

  - `fluxengine read mx --110 -s drive:0 -o mx.img`
  - `fluxengine read mx --220ds -s drive:0 -o mx.img`
  - `fluxengine read mx --220ss -s drive:0 -o mx.img`
  - `fluxengine read mx --440 -s drive:0 -o mx.img`

## References

  - [The Soviet Digital Electronics
    Museum](http://www.leningrad.su/museum/main.php) (source of the image
    above)

  - [a random post on the HxC2001 support
    forum](http://torlus.com/floppy/forum/viewtopic.php?t=1384) with lots of
    information on the format

