Seperate HD & FD controllers?

[Thaddeus P. Floryan]

In article <528 at trux.UUCP> car at trux.UUCP (Chris Rende) writes:
>I've noticed that the 3b1 can only access one disk at a time. I.e., during
>floppy disk accesses the hard disk is idle - and vice versa. (Especially
>evident during floppy formatting).
>
>However, I've seen references to seperate HD and FD controller chips.
>
>I suspect that there is some common circuitry between the HD and FD interfaces
>which prevents them from operating independantly. (IBM PC clones have seperate
>controllers which can work independantly - 3b1's don't seem to have that
>luxury).
>
>Is this true?

The "Disk Bus Interface Unit" (both floppy and HD) are on the same DMA per
the info I just read in the UNIXPC Reference Manual.  Expansion boards and
the 68010 itself are on different DMA channels.  Reading pages 2-43 thru
2-44 indicates that one or the other disk can be transferring data at one
time through the same DMA circuitry, so there's your "common circuitry" and
the reason for the "problem."  If some (external) disk controller could be
plugged into an expansion slot, it'd be on its own DMA channel and, thus,
not block the other disk(s); I'm not aware of any such external disk
controller cards.  The separate DMA channels also explains why expansion
card operations don't (appear to) block disk access (fortunately! :-)

Thad Floryan [ thad at btr.com (OR) {decwrl, mips, fernwood}!btr!thad ]