Re: NTSC VIC-II timing

Date: Wed, 21 Jun 2017 20:12:41 +0200
Message-Id: <>
> On 2017-06-21, at 19:44, Gerrit Heitsch <> wrote:
>> ... that I never used LP IRQs but thought that it would trigger only when the button line is grounded within a specific timing window so some kind of synchronisation with raster position would be required for the IRQ to happen. But that's a different subject anyway.
> No, the LP IRQ just latches the current X and Y position of the electron beam when you get the high to low transition on the LP line. And you also get an IRQ to notify you of that event.

> On 2017-06-21, at 19:46, smf <> wrote:

> The VIC just latches the x & y position & fires an IRQ when it is triggered, because it assumes that if the light pen can see the gun then it must be over the current pixel.
> Adding something to VIC to ignore pulses outside the visible portion of the display would be a waste of space.

But that means that I can basically trigger valid LP IRQs using joystick and pushing its fire button..

Well, as I mentioned, I never wrote a single instruction to serve any lightpen support.

SD! -

       Message was sent through the cbm-hackers mailing list
Received on 2017-06-21 19:00:03

Archive generated by hypermail 2.2.0.