Can't boot from ad0 anymore (Did the WDC AC2540H stop being supported?)

[Steve Willoughby]

Okay, here's my last attempt to get some useful work from an older system.
It's running 4.5-RELEASE now, which would be fine except that I'm told I
need to run a later release for the sound card driver I need.

The problem is that it looks like as I progress to later versions of 
FreeBSD, the kernel gets progressively less willing to believe in my
IDE controller.  I'm wondering now if FBSD just phased out support for
this, or if there's some simpler explanation available.  Here's the
behavior by FBSD version:

  4.5-RELEASE:
	When I install and/or boot 4.5R, everything works perfectly
	fine (as far as the disk and OS, anyway).  It recognizes ata0
	without complaint at all, finds disk ad0, boots it, 
	comes all the way up, and appears stable.

  4.7-RELEASE:
	The install boot works fine, just like 4.5R, no complaints,
	finds ad0 okay, and manages to install the OS image on it.
	However, when I try to boot the freshly-installed kernel,
	it finds "atapci0" (not ata0), gives a warning about the
	quality of the chipset, and refuses to acknowledge the 
	existence of any ad0 disk at all.  (Note that the loader
	loaded and booted the kernel from ad0s1a:/kernel okay, but
	that kernel couldn't use ad0 as a root device.)

	Boot messages include:
	
	atapci0: <RZ 100? ATA Controller !WARNING! buggy chip data loss
	possible> possible> port 0x3f4-0x3f7,0x1f0-0x1f7 at device 1.0 on pci0
	atapci0: Busmastering DMA not supported
	atapci0: Busmastering DMA disabled
	ata2: at 0x1f0 on atapci0
	ata2: unable to allocate interrupt

	...

	then I get "no such device 'ad'" when trying to specify the root
	device.

  5.0-RELEASE
	The installer kernel doesn't recognize it anymore now either.
	I just get the install program, but the disk slice editor can't
	see any hard drives at all.

Some info about the system:

Pentium 90MHz system from long, long ago (yes, this is its last chance
to avoid becoming a boat anchor)

AmiBIOS 1.00.09.BF0 (c) 1992
disk: WDC AC2540H
-- 
Steve Willoughby          | "The purpose of IT is to seamlessly and trans-
Intel DPG Eng. Computing  | parently provide the other nine-tenths of the
Application Development   | iceburg for people who need to work with chunks 
<steve at ichips.intel.com>  | of floating ice."           --Strata R. Chalup