Reorganise disk format documentation.

doc/disk-c64.md

@@ -0,0 +1,52 @@
+Disk: Commodore 64
+==================
+
+Commodore 64 disks come in two varieties: GCR, which are the overwhelming
+majority; and MFM, only used on the 1571 and 1581. The latter were (as far as
+I can tell) standard IBM PC format disks, so use `fe-readibm` to read them
+(and then [let me know if it
+worked](https://github.com/davidgiven/fluxengine/issues/new).
+
+The GCR disks are much more interesting. They could store 170kB on a
+single-sided disk (although later drives were double-sided), using a proprietary
+encoding and record scheme; like [Apple Macintosh disks](macintosh.html) they
+stored varying numbers of sectors per track to make the most of the physical
+disk area, although unlike them they did it by changing the bitrate rather than
+adjusting the motor speed.
+
+The drives were also intelligent and ran DOS on a CPU inside them. The
+computer itself knew nothing about file systems. You could even upload
+programs onto the drive and run them there, allowing all sorts of custom disk
+formats, although this was mostly used to compensate for the [cripplingly
+slow connection to the
+computer](https://ilesj.wordpress.com/2014/05/14/1541-why-so-complicated/) of
+300 bytes per second (!). (The drive itself could transfer data reasonably
+quickly.)
+
+A standard 1541 disk has 35 tracks of 17 to 20 sectors, each 256 bytes long.
+
+Reading discs
+-------------
+
+Just do:
+
+```
+.obj/fe-readc64
+```
+
+You should end up with an `c64.img` which is 187136 bytes long (for a normal
+1541 disk).
+
+**Big warning!** The image may not work in an emulator. Commodore 64 disk images are
+complicated due to the way the tracks are different sizes and the odd sector
+size. FluxEngine chooses to store them in a simple 256 x 20 x 35 layout,
+with holes where missing sectors should be. This was easiest. If anyone can
+suggest a better way, please [get in
+touch](https://github.com/davidgiven/fluxengine/issues/new).
+
+Useful references
+-----------------
+
+  - [Ruud's Commodore Site: 1541](http://www.baltissen.org/newhtm/1541c.htm):
+    documentation on the 1541 disk format.
+