Flippy Disks

Several 8-bit home computers used 5.25" drives with a single read head but used both sides of the disk to store data, requiring the user to flip the disk to access side B. Double-sided drives with two heads cannot read both sides in one pass, for two reasons:

1. The second drive head is offset inwards by several tracks, such that the outer tracks on side B of a flippy disk are inaccessible.
2. The side-B tracks which the double-sided drive can read will appear to be recorded backwards.

The obvious solution is to physically flip the disk, as in the original system. However if you try this you will receive the error Command Failed: GetFluxStatus: No Index. This is because the index hole in the disk jacket is in the wrong position for the drive sensor when the disk is flipped. There are a few possible solutions, which are also summarised on page 7 of the Kryoflux flippy-drive manual:

1. Punch a second index hole in the disk jacket in the correct flipped position. This will require removal of the inner disk media from its jacket.
2. Fake the index pulse signal using Greaseweazle. This requires a drive which will spin up in the absence of index pulses (many will not).
3. Modify a drive to include a second index sensor (see PDF guides here and here). This allows to select the correct index sensor based on a switch setting on the modified drive.
4. Obtain a flippy-modded disk drive which can read both sides of a flippy disk in a single pass.

Faking the index pulse

First measure the precise rotation speed of your drive by placing a disk in the drive and running gw rpm --nr 10. The RPM values should be relative constant for every measurement.

Now you can flip the floppy disk in your drive as you would do on the original hardware and use the following command prefix: gw read --fake-index --tracks h=0 .... For example, to read a raw single-sided image of side B:

gw read --fake-index 300 --tracks h=0 --drive 0 --raw test.scp

Flippy-modded disk drive

This requires a specially-modified drive as originally specified by the Kryoflux project. You can then read flippy disks in a single pass, for example (including C64 half tracks):

gw read c=0-79:h=0,1:h1.off=-8 test.scp

Note that side B will be read backwards in this case. Conversion software will need to be aware of this.