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Chapter 5 Input/Output Interfaces
Compaq Deskpro 4000N and 4000S Personal Computers
First Edition – September 1997
5-4
The contents of the Error register are interpreted as a diagnostic status byte after the execution of
a diagnostic command or when the system is initialized.
Bit Function
7 Bad Block Mark Detected in Requested Sector ID Field (if set)
6 Non-correctable Data Error (if set)
5 Reserved
4 Requested Sector ID Field Not Found (if set)
3 Reserved
2 Requested Command Aborted Due To Invalid Hard Drive
Status or Invalid Command Code (if set)
1 Track 0 Not Found During Re-calibration Command (if set)
0 Data Address Mark Not Found After Correct ID Field (if set)
Set Features Register, I/O Port 1F1h/171h (Write Only)
This register is command-specific and may be used to enable and disable features of the interface.
Sector Count Register, I/O Port 1F2h/172h
This register defines either:
♦
the number of sectors of data to be read or written
or
♦
the number of sectors per track for format commands
If the value in this register is zero, a count of 256 sectors is specified. The sector count is
decremented as each sector is accessed, so that the value indicates the number of sectors left to
access when an error occurs in a multi-sector operation. During the Initialize Drive Parameters
command, this register contains the number of sectors per track.
Sector Number Register, I/O Port 1F3h/173h
The Sector Number register contains the starting sector number for a hard drive access.
Cylinder Low, Cylinder High Registers, I/O Port 1F4h, 1F5h/174h, 175h
These registers contain the starting cylinder number for each hard drive access. The three most-
significant bits of the value are held in byte address 1F5h (bits <2..0>) while the remaining bits
are held in location 1F4h.