PC/AT Disk Controllers: What's Available???

[Robert Wolpert]

My IBM PC/AT's F/H disk controller bit the dust;  is it possible
to replace it with something better (and usable both under
Xenix and DOS), e.g.

1)  A controller that supports 1.44Mb, preferably as A:
    (to do so it would have to have on-board POST rom to
    replace the AT's default boot sequence, wouldn't it?);           *OR*
2)  A controller with on-board cache, possibly enabling 1:1
    interleave;                                                      *OR*
3)  A RLL controller that packs more onto my Bell B/86 drive
    (actually a Toshiba 86Mb unformatted, ~70 Mb drive), originally
    running with IBM's MFM controller.

Is it possible to use a RLL controller with a disk intended for MFM??
I don't know whether that's a feature of the controller only, or of
both controller and disk.  Thanks---- Please reply by e-mail to
    rlw at cs.duke.edu                            /or/
    {decvax | ihnp4 | ... }.duke!rlw
--Thanx