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* HELIOS DISK PORTS AND BIT EQUATES
*
DSTAT EQU 0F0H  Disk status port
DTCOM EQU 0F1H  Disk Transfer Command port
DTLL  EQU 0F3H  Disk Transfer Length low byte
DTLH  EQU 0F4H  Disk Transfer Length high byte
DTADL EQU 0F5H  Disk Transfer Address low byte
*                           (this also clears status register)
DTADH EQU 0F6H  Disk Transfer Address high byte
DCOM  EQU 0F7H  Drive Command port
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*  Status Port (F0) Bit Assignments
*
TC  EQU 1  Bit 0 = Transfer Complete (high)
SR  EQU 2  Bit 1 = Status Ready (high)
AB  EQU 4  Bit 2 = Abort error (high)
CE  EQU 8  Bit 3 = CRC Error (high)
CC  EQU 10H  Bit 4 = CRC Check Completed (high)
DR  EQU 20H  Bit 5 = Disk Ready (low)
SC  EQU 40H  Bit 6 = Seek Completed (low)
IX  EQU 80H  Bit 7 = Index Hole present (low)
*
*  Transfer Port (F1) Bit Assignments
*
NER  EQU 1  Bit 0 = Erase (low)
RD  EQU 2  Bit 1 = Read (high)/Write (low)
TRH  EQU 4  Bit 2 = Transfer: Header (high)/Data (low)
*
*  Drive Command Port (F7) Bit Assignments
*
NSP  EQU 1  Bit 0 = Step (low)/No Step (high)
DO   EQU 2  Bit 1 = Direction Out (high)
DS1  EQU 4  Bit 2 = Drive Select (left hand drive)
DS2  EQU 8  Bit 3 = Drive Select (left hand drive)
NRS  EQU 10H  Bit 4 = Restore (low=return to T 0)
NH0  EQU 20H  Bit 5 = Do not load head of disk 0
NH1  EQU 40H  Bit 6 = Do not load head of disk 1
SD0  EQU 80H  Bit 7 = Select Disk 0=high/1=low
*