26 Jan
2010
26 Jan
'10
10:54 a.m.
On 1/26/10, Dan Gahlinger <dgahling at hotmail.com> wrote:
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