The AT hard disk controller is incorporated onto the same adapter card as
its diskette controller.  Its I/O ports, commands, and status signals are
different from those of the XT controller.  See  XT Hard Disk Ports for
details.

  AT Hard disk controller #1 decodes ports 1f0H through 1f7H
  AT Hard disk controller #2 decodes ports 170H through 177H

The controller generates interrupt level 14 (IRQ 14) after every operation
(read, write, seek, etc.).  IRQ 14 is vectored to INT 76H where an action
flag bit is set at address 0:048e.

Port  Description
                                                                            
1f0H  Data register. Read/write data from/to controller sector buffer
                                                                            
1f1H  Write: Write Precompensation register.  Turn on write precompensation.
Read: Error register.  Contains bits for last error.
 7 6 5 4 3 2 1 0 
     0   0       
                  bit
                  0: Data Address Mark not found
                  1: Track 0 Error
                  2: Command was aborted
                  4: Sector ID not found
                  6: ECC Error: Uncorrectable data error
                  7: Bad block
                                                                            
1f2H  Sector count.  Read/Write count of sectors for operation
                                                                            
1f3H  Sector number.  Read/Write current/starting logical sector number
                                                                            
1f4H  Cylinder high (bits 0-1 are bits 8-9 of 10-bit cylinder number)
1f5H  Cylinder low (bits 0-7 of 10-bit cylinder number)
                                                                            
1f6H  Drive & Head.  Read/Write: bits indicate head, drive for operation
                                                                            
1f7H  Write: command register (Summary: see Tech Ref for full details)
1xH = Restore to cylinder 0 (low nibble is step rate)
7xH = Seek to cylinder (low nibble is step rate)
2xH = Read sector (low nibble indicates retry and ECC-read)
3xH = Write sector (low nibble indicates retry and ECC-read)
50H = Format track
4xH = verify read (low nibble indicates retry)
90H = diagnose
91H = set parameters (max heads,sectors) for selected drive

1f7H  Read:  status register
 7 6 5 4 3 2 1 0 
                 
                  bit
                  0: 1=previous command ended in an error.
                  1: Index: set to 1 while index hole is detected
                  2: Corrected data.  Error found, but ECC fixed it.
                  3: Data Request.  Buffer is waiting for data (busy)
                  4: Seek completed
                  5: Write fault
                  6: Drive ready for read/write/seek (unless bit 4=0)
                  7: Busy. 1=executing cmd (other bits are invalid)
                                                                            
See Also: INT 13H (disk I/O)
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AT Hard Disk Ports