6545 raster effects (Re: 6545 40/80 col register values)

From: Marko Mäkelä <msmakela_at_gmail.com>
Date: Thu, 26 Apr 2012 08:56:28 +0300
Message-ID: <20120426055628.GA3438@x220>
On Thu, Apr 26, 2012 at 07:36:34AM +0200, "André Fachat" wrote:
>On the other hand, changing the memory at the right time is tough. The 
>PET only has a single interrupt on the vertical blank, and from there 
>it is only counting cycles.

Can the 6522 VIA generate interrupts? Even if it cannot, you could use 
the VIA timers to simplify the cycle-counting.

The VIC-20 does not have any video interrupt whatsoever, but the raster 
line register is readable. This makes it possible to synchronize a 6522 
timer interrupt with the screen. You first need a loop that gets the 
rough timing right (some CPU cycles of jitter, corresponding to the 
difference between minimum and maximum number of cycles per 
instruction). Then you need a fine-tuning loop that would detect the 
exact cycle when the scan line is advancing.

On the PET, if there is no readable raster line counter in the 6545, I 
guess you would need a trick that uses the IRQ line for determining when 
exactly the vertical blank interrupt is being triggered. Then, you would 
set up some sort of a delay loop in the "main program" and wait for N 
more vertical-retrace interrupts to eliminate the horizontal jitter.

Best regards,


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Received on 2012-04-26 06:00:48

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