From Hardware Register Listing 7.0:
-------+-----+-----------------------------------------+---
IDE Controller (Falcon, ST-Book, IDE cards)
-------+-----+-----------------------------------------+---
$F00000|long |Data Register                            |R/W
$F00005|byte |Error Register       BIT 7 6 5 4 3 2 1 0 |R
       |     |Bad block mark ----------' | | | | | | | |
       |     |Uncorrectable error -------' | | | | | | |
       |     |Media change ----------------' | | | | | |
       |     |ID-Field not found ------------' | | | | |
       |     |Media change requested ----------' | | | |
       |     |Command aborted -------------------' | | |
       |     |Track 0 not found -------------------' | |
       |     |DAM not found -------------------------' |
$F00009|byte |Sector Count Register                    |W
$F0000D|byte |Sector Number Register                   |W
$F00011|byte |Cylinder Low Register                    |W
$F00015|byte |Cylinder High Register                   |W
$F00019|byte |Drive Head Register                      |W
$F0001D|byte |Status Register                          |R
$F0001D|byte |Command Register                         |W
$F00039|byte |Alternate Status Register                |R
$F00039|byte |Data Output Register                     |W
-------+-----+-----------------------------------------+---