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FDC problems with laptop implementations
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Problems arise mostly from the newest FD controllers. The one's
I have data guides for I will speak to, the others well....
The two FDCs in discussion are the NS(National Semiconductor)8390
and the WD(Western Digital)37c65c.
One of the biggest problems is "autopower-down". What happens is
that the IBM code never reports any problems to the "end-user", all
problems are reported to the OS(MS-DOS or OS/2 in this case). They
in turn will retry as many as 5 times before quiting, and this is
only after the BIOS has tried 3 times on each request. This gives
the FDC 15 oppertunities to get on-line. It does come on-line
in approx. 12ms after the first read to the register.
The next problem is writing to what is supposed to be a
READ-only register, namely the MSR (Main Status Register). The
WD37c65c , usually the cheapest implementation, goes into a wierd
mode*,after a write to a read register.
Another problem of a minor order is that the OS may boot, off a
HD, before the BIOS has turned off the drive. Which may leave the
controller in a busy state. In MS-DOS the BIOS, while still in place,
will recover to this quietly. Even if the drive goes off the
controller may have a busy flag still up which requires servicing.
To which, I will "bang" the FDC with a bad command some time in the
future to start a reset of the FDC subsystem. BTW, some FDC data
guides imply the "bang" method to clear a stuck controller.
* I ignored the details while reading the data guide.
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BB - acronym for ball bearing
