Re: sys/dev/isa/fd.c FDUNIT/FDTYPE

[Tom Spindler <dogcow%babymeat.com@localhost>]

> } >      That must have been fun.  As I recall, this was basically the
> } > Apple ][ disk controller squished into one chip. It was hard to call
> } > that thing a disk controller since it wasn't much more then a TTL
> } > driver and a shift register.
> } 
> } And a state machine as provided by one of the PROMs. A terribly complete
> } analysis can be found in "Understanding the Apple II" by James F Sather.
> 
>      The PROMs on the Apple ][ only handled booting.  They didn't have
> anything to do with the drives after booting.  The one other thing I
> didn't mention was some kind of data discriminator to seperate clock
> pulses from data pulses.  The hardware controller didn't even do the
> GCR encoding/decoding, that was done in software.

You're incorrect; one of the PROMs was the 6502 code, yes, but the
other acted as the state machine that interfaced to the shift register
to indicate when a valid byte was received (to oversimplify.) See:
  
http://mirrors.apple2.org.za/ground.icaen.uiowa.edu/MiscInfo/Hardware/p5p6.p5ap6a
  
http://mirrors.apple2.org.za/ground.icaen.uiowa.edu/Mirrors/uni-kl/hardware/diskIIcontroller_hardware
and around page 9-14 of UtA2, or p244 on
  
http://www.scribd.com/doc/201423/Understanding-the-Apple-II-by-Jim-Sather-1983Quality-Software
 
URL for Beneath Apple DOS: 
http://www.scribd.com/doc/200679/Beneath-Apple-DOS-By-Don-Worth-and-Pieter-Lechner