IDE Facts and Fallacies

[jdb8042@tamsun.tamu.edu (John Donald Baker)]

I grow tired of reading posts which demostrate a lack of understanding of
the nature of the IDE hard disk interface.  Thus, to educate to general
public, I have the following to say, based on my own research and experi-
mentation.

The IDE interface is an extension of the old Western Digital WD1002 series
of stand-alone disk controllers which were commonly used on machines like
the Kaypro 10, TeleVideo 8xxH, and Epson QX-10/QX-16 sytems.

IDE, like the WD1002s before them look like a group of 8 registers accessed
and interfaced in such a manner as to make the device look like just another
peripheral chip.  The registers are:

     addr. offset     name           operation
         0            DATA           read/write data
         1            ERROR/PRCMP    read error / set precomp info.
         2            SECTOR COUNT   set number of sectors to transfer
         3            SECTOR NUMBER  set starting sector of transfer
         4            CYLINDER LOW   low 8 bits of cylinder number
         5            CYLINDER HIGH  high 8 bits of cylinder number
         6            SIZE/DRIVE/HEAD  drive select, head select, sector size
         7            COMMAND/STATUS send command / read status

All registers except the DATA register are 8 bits wide.  On the older
WD1002, the DATA register is also 8 bits wide.  In IDE drives, the
data register is 16 bits wide.  This is the only relevant hardware difference
between the two systems.