Re: Hardware emulation of 6509 using 6502?

On Thu, Mar 15, 2018 at 12:55:29PM +0000, smf wrote:
> The SID and VIC are different because of analogue effects which are 
> different in the manufacturing processing. It would be hard to emulate 
> those differences accurately with an FPGA.

The 65xx SID has highly non-linear "opamps".  The 85xx fixes that.
Not everyone likes it, of course.

On later C64 machines the CS line to VIC goes active earlier, causing
the sparkles.  A little cap (from CS to ground) solves this.  There is
no difference between the 65xx and 85xx VIC here.

(You get the sparkles because the internal data bus on the VIC is
precharged; if CS is too early you get that value instead of the value
on the data bus written to the colour regs, for a little while that is,
half a pixel or so.  And the precharged value is 0b1111, light grey).


Segher