The CMD-Technologies CMD640 IDE chip is used on many common 486 and
Pentium motherboards, usually in combination with a "Neptune" or
"SiS" chipset. Unfortunately, it has a number of rather nasty
design flaws that can cause severe data corruption under many common
conditions. Say Y here to include code which tries to automatically
detect and correct the problems under Linux. This option also
enables access to the secondary IDE ports in some CMD640 based
systems.

This driver will work automatically in PCI based systems (most new
systems have PCI slots). But if your system uses VESA local bus
(VLB) instead of PCI, you must also supply a kernel boot parameter
to enable the CMD640 bugfix/support: "cmd640.probe_vlb". (Try "man
bootparam" or see the documentation of your boot loader about how to
pass options to the kernel.)

The CMD640 chip is also used on add-in cards by Acculogic, and on
the "CSA-6400E PCI to IDE controller" that some people have. For
details, read Documentation/ide/ide.txt.

source code:

drivers/ide/cmd640.c