I think I'm right to say, and I hope someone will correct me if not... The Sam, like most other micros, uses the PAL 50Hz half-resolution non-interlaced mode. There's a very slight timing difference in sync pulses that is used in transmitted video to signal whether the next frame contains odd or even frames. The TV judges exactly how far up to move the electron gun based on that and then scans downwards again.
The Sam doesn't do odd and even frames, it does them all with exactly the same timing. So the TV never scans half the on-screen scanlines and scans the rest at double the rate they would be for broadcast television. I have to admit to having no idea whatsoever whether that's an official allowed way to operate within the PAL spec, but I suspect it is simply by virtue of the way that odds and evens are signalled. On Thu, Oct 15, 2009 at 9:52 AM, Roger Jowett <[email protected]> wrote: > so its jsut the sam 192 line interrupt but due to the speed of the > processor you could only use 128 interrupts maximum didnt it say in > the technical manual with a 4¾mhz sam processor > unless in mode1 which is slower no or rom or external ram which might > be a bit quicker mebbe? > so the line drawing routine doesnt draw the odd or even scan line > pixels first then the remaining pixels as the screen is displayed in > two pal frames no? > > 2009/10/8 Colin Piggot <[email protected]>: >> Thomas wrote: >>> >>> (1) I'm vaguely aware that there's a "current scanline" hardware >>> counter somewhere - does that count actual PAL lines or only lines >>> with pixels on them? >> >> HPEN port (&01f8) will give the current scanline - for scanlines in the >> display area only (0-191 iirc) >> >>> And how long does the end-of-display interrupt signal last? My memory >>> is saying 24 cycles, but I'm not sure why. I guess since we're on a >>> system with no explicit interrupt acknowledgments, it's going to be >>> something tiny? >> >> If I also remember correctly from all the tinkering I've done, the interrupt >> pulse lasts for 128 clock cycles. >> >> Colin >> ===== >> Quazar : Hardware, Software, Spares and Repairs for the SAM Coupe >> 1995-2009 - Celebrating 15 Years of developing for the SAM Coupe >> Website: http://www.samcoupe.com/ >> >
