WDC1772 Drive/Track/Side Selection

11 November 2004

A new call-back is available for changing the drive buffer used by the emulation code whenever the track or side changes.

The drive and side select bits from the PSG were mapped to the FDC logic. All possibilities are supported as far as selection is concerned, but combining lines from the drives is not supported at the FDC logic level. In other words, while it is possible to select sides on multiple drives with one IO access, it is not possible for the FDC itself to drive both floppy disk drives at the same time (it cannot do that anyway due to the lines being combined).

On the real hardware it is possible to select both drives, in which case drive signals are pulled whenever any of the signals are pulled. However, since a “real” FDC is attached (unlike on an Amiga) there is no point supporting this possibility as the FDC logic would produce inconsistent working in this case anyway.

Now there is “only” the 1772 emulation left to do, all the Atari ST parts are complete apart from loading an image and getting the stream buffer for a track - all of those are IPF library specific calls.