26 Jan
2010
26 Jan
'10
11:14 a.m.
On 1/26/2010 12:54 PM, Ethan Dicks wrote:
On 1/26/10, Dan Gahlinger<dgahling at
hotmail.com> wrote:
He's probably thinking of the earlier IEEE drives. Peddle designed them
and tried to be very clever in the design. They contained a 6502 and
6504 that shared the bus by running one on a (effectively) reversed
clock. THe 6502 got the bus for half a cycle, followed by the 6504, etc.
The 1541 (and some of the later IEEE drives) proved you didn't need that
level of sophistication to operate a floppy disk mechanism.
It cost Peddle, too, Jack was furious at the cost and design lag of the
IEEE drives. I'm not sure Chuck ever fully recovered.
Jim
--
Jim Brain, Brain Innovations (X)
brain at jbrain.com
Dabbling in WWW, Embedded Systems, Old CBM computers, and Good Times!
Home: http://www.jbrain.com
I keep thinking it was the 6520 or 6522 processor
in the 1541
which had the cool dual-clock ability.
1/2 a clock running as one mode, the other 1/2 as another mode,
technically running two instructions at once. so to speak.
The 6520 and 6522 are the PIA and VIA, respectively - parallel I/O and
timers, not processors.
There were tricks one could perform with the 6502 since it had
alternating fetch and execute cycles, so by doing something else with
the bus during the right portion of the clock phase, you could design
a board to "do twice as much" at 1Mhz, but I don't think the 1541 did
that.
It did have a pair of "processes" in its firmware - one to talk to the
host and parse DOS commands, and one to perform low-level "tasks"
(move the heads, read/write buffers, etc).
-ethan