b0,1 US Unit Select (driveno during interrupt)
b2 HD Head Adress Address (head during interrupt)
b3 NR Not Ready (drive not ready or non-existing 2nd head selected)
b4 EC Equipment Check (drive failure or recalibrate failed (retry))
Status Register 1
b0 MA Missing Adress Address Mark (Sector_ID or DAM not found)
b1 NW Not Writeable (tried to write/format disc with wprot_tab=on)
b2 ND No Data (Sector_ID not found, CRC fail in ID_field)