AMSDOS is designed for single sided disc drives, and the disc drive in the CPC664, CPC6128, CPC6128+, DDI-1 and FD-1 are single sided. However, it is possible to use both sides of a 3" floppy disc by turning it over.
But, if you have a 3.5" second drive you will only be able to use one side of a 3.5" floppy disc because it is not possible to turn over a 3.5" disc and put it into the drive!
If you want to use both sides with AMSDOS, you will need a side switch.
The side switch is designed so that you can use:
Note, that the CPC floppy disc controller does support double sided disc drives, and both sides can be used if you use another DOS. (e.g. C/PM, Parados, Romdos etc).
The switch has two positions.
When the switch is in position 1:
When the switch is in position 2:
The switch is a SPST (Single-Pole, Single-Throw) toggle type. The switch is locking in both directions, and has 3 contacts.
This switch can be fitted internally or externally.
Cable SIDE 1 SEL | | GND | | o o \ SPST switch \ o | | | Side 1 Sel FDD
The side select signal is generated by the FDC and is active low.
When the switch is in position 1, the side select signal is not changed:
Therefore the drive will access either side 0 or side 1, depending on the "/SIDE SELECT" output from the FDC. When used with AMSDOS, side 1 will always be accessed. When used with another DOS that supports double-sided discs, side 1 or side 2 can be used.
When the switch is in position 2, the signal is forced to "0":
Therefore the drive will always access side 2.