mac
====
## 400kB/800kB 3.5" GCR
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Macintosh disks come in two varieties: the newer 1440kB ones, which are
perfectly ordinary PC disks you should use the `ibm` profile to read them, and
the older 800kB disks (and 400kB for the single sides ones). They have 80
tracks and up to 12 sectors per track.

They are also completely insane.

It's not just the weird, custom GCR encoding. It's not just the utterly
bizarre additional encoding/checksum built on top of that where [every byte
is mutated according to the previous bytes in the
sector](https://www.bigmessowires.com/2011/10/02/crazy-disk-encoding-schemes/).
It's not just the odd way in which disks think they have four sides, two on one
side and two on the other, so that the track byte stores only the bottom 6 bits
of the track number. It's not just the way that Macintosh sectors are 524 bytes
long. No, it's the way the Macintosh drive changes speed depending on which
track it's looking at, so that each track contains a different amount of data.

The reason for this is actually quite sensible: the tracks towards the centre
of the disk are obviously moving more slowly, so you can't pack the bits in
quite as closely (due to limitations in the magnetic media). You can use a
higher bitrate at the edge of the disk than in the middle. Many platforms, for
example the Commodore 64 1541 drive, changed bitrate this way.

But Macintosh disks used a constant bitrate and changed the speed that the disk
spun instead to achieve the same effect...

_Anyway_: FluxEngine will read them fine on conventional drives.  Because it's
clever.

Macintosh computers never really used the twelve bytes of metadata and the
standard for disk images is to omit it. If you want them, specify that you want
524-byte sectors. The metadata will follow the 512 bytes of user data.

## Options

  - Format variants:
      - `400`: 400kB 80-track SSDD
      - `800`: 800kB 80-track DSDD
  - `metadata`: read/write 524 byte sectors

## Examples

To read:

  - `fluxengine read mac --400 -s drive:0 -o mac.dsk`
  - `fluxengine read mac --800 -s drive:0 -o mac.dsk`

To write:

  - `fluxengine write mac --400 -d drive:0 -i mac.dsk`
  - `fluxengine write mac --800 -d drive:0 -i mac.dsk`

## References

  - [MAME's ap_dsk35.cpp file](https://github.com/mamedev/mame/blob/4263a71e64377db11392c458b580c5ae83556bc7/src/lib/formats/ap_dsk35.cpp),
    without which I'd never have managed to do this

  - [Crazy Disk Encoding
    Schemes](https://www.bigmessowires.com/2011/10/02/crazy-disk-encoding-schemes/), which made
    me realise just how nuts the format is

  - [Les Disquettes et le drive Disk II](http://www.hackzapple.com/DISKII/DISKIITECH.HTM), an
    epicly detailed writeup of the Apple II disk format (which is closely related)

  - [The DiskCopy 4.2
        format](https://www.discferret.com/wiki/Apple_DiskCopy_4.2), described on
        the DiskFerret website.

