Re: Some new 8032 Editor/Char roms

From: Marko Mäkelä <>
Date: Sat, 25 Apr 2015 14:54:35 +0300
Message-ID: <20150425115435.GA1696@x220>
On Sat, Apr 25, 2015 at 10:01:15AM +0200, Anders Carlsson wrote:
>Hm, does this mean the TI$ in BASIC is based on the simple timers (less 
>accurate?) found in both 6522 and 6526, and the TOD clock is something 

Yes. Actually, I made a mistake in my previous post by claiming that the 
PAL/NTSC detection does not affect anything. Now that I think more 
carefully, the CIA timer interrupts for RS-232, tape, and for the 
regular $ea31 interrupt must be at a fixed rate, so they must be adapted 
for the different 6510 clock frequency in the C64 (17734472/18 Hz on PAL 
and 14318181/14 Hz on NTSC). On the PET, the 6502 always runs at 1MHz.

The timer interrupt should be 60Hz. It is not 50Hz in PAL models, and I 
guess it is not synchronized with the 6567 screen updates in NTSC models 

AFAIU, the C64 KERNAL is very close to the VIC-20 one, and very little 
software depends on the ToD clock. GEOS is one example.

I wonder if anyone ever measured the inaccuracy of the TI$ clock, 
especially during disk, tape, and RS-232 operations. (And this should be 
without considering software that disables interrupts or changes the 
interrupt frequency.)


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Received on 2015-04-25 12:00:06

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