Re: JIffyDOS Coding

From: Richard Atkinson (
Date: 2001-03-27 13:05:59

On Tue, 27 Mar 2001 wrote:

> Not knowing enough about either the VIC20 or the Plus4:
> 1) are the port locations of DATA and CLOCK signals as convenient as they
> are on the C64/C128?

On the plus/4, DATA is output from bit 0 of the 8501 I/O port (inverted)  
and CLK from bit 1. However, DATA is read in from bit 7 whereas CLK is on
bit 6 - ie. the order of the signals is reversed.

> 2) the Plus4 runs at two speeds (?), is this controllable (since you
> wouldn't want to be switching speeds during such critical open-loop timing)

Yes, you can tell the plus/4 to run at the slower speed and this is
constant except for TED chip badlines. However bear in mind that this is
somewhat slower than the C64's near 1MHz, so if the routines rely on the
drive processor (1MHz) remaining roughly in sync with the computer
processor for a certain minimum number of cycles then you may have


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