== FDC Status Registers ==
The Main Status register can be always read through Port &FB7E. The other four Status Registers cannot be read directly, instead they are returned through the data register as result bytes in response to specific commands.
Main Status Register (Port &FB7E):
b0..3 DB FDD0..3 Busy (seek/recalib active, until succesful sense intstat)
b7 RQM Request For Master (1=ready for next byte) (see b6 for direction)
Status Register 0:
b0,1 US Unit Select (driveno during interrupt)
or senseint with no int occured, 3=aborted:disc removed etc.)
Status Register 1:
b0 MA Missing Address Mark (Sector_ID or DAM not found)
b7 EN End of Track (set past most read/write commands) (see IC)
Status Register 2:
b0 MD Missing Address Mark in Data Field (DAM not found)
b7 0 Not Used
Status Register 3:
b0,1 US Unit Select (pin 28,29 of FDC)