(Long - this is one of my favourite subjects!) On Fri, 25 Aug 2000, Nicolas Welte wrote: > Anyway, it would be very interesting to know more about that test bit. I am > not sure if the RAM refresh is corrupted if the test bit is set, because > raster lines are skipped completely, so the refresh counter probably also > gets affected. I guess there are two situations; setting TEST during the 5 cycle period where the raster line state machine is refreshing RAM and setting it at all other times. It's possible that the state machine continues refreshing; maybe incrementing the counter, maybe not. However, I strongly suspect that it just reverts to cycle 0 (out of the 63) and does that access each time. In which case we have to assume DRAM does not get refreshed. Does setting TEST cause the machine to crash in BASIC? If so, that might be your explanation. I've just realised what this hack can be used for - making 60Hz PAL mode games! Many games consoles can be hacked to run in 60Hz and I've even seen ST and Amiga games which give you the option of running in 60Hz because the gameplay is so much better. I experimented with 60Hz PAL modes on the plus/4 when I was playing around with expanding the screen matrix, and came to the conclusion that most PAL displays could cope with a 60Hz 262 line (524) display quite happily. I also experimented with a program for NTSC mode which stretched the screen out to 312 lines (624) 50Hz, but without a "normal" NTSC television I couldn't verify its operation. The nice thing about these psuedo-modes was that they worked in colour, even though the line scanning circuits of the TV would lock onto the timing of the 'other' TV standard (ie. PAL colour, NTSC-size display). However, that was of course the plus/4; useful with its weird 2 badline FLI routines and expanded screen matrix routines, but not relevant to the C64. Here is an opportunity to do a similar thing on the C128 (or C128 in 64 mode). At first I thought it would only be useful for PAL users to steal lines and get a 60Hz NTSC display (as has evidently been done, although I haven't seen the program myself - anyone know a good C64 warez site? ;) ). You'd steal a few lines from the top border and a few from the bottom the same way as on the plus/4 and you'd get a 262 line display. 50 lines altogether, of course. You might have to be careful that each of the 128 DRAM refresh rows (256 on a 41256-expanded machine) get refreshed, which may or may not be a problem depending on whether TEST resets the DRAM refresh counter or not. Then I realised it might be possible to cause the line counter to reset mid-frame (as you do directly on the plus/4 to change the screen size), but without generating frame sync pulses as there is no raster line state machine operation during TEST bit set. Thus you could perhaps create "extra" bits of border in an NTSC machine, emulating a 312 line 50 Hz PAL display. Bingo - 50Hz NTSC for the NTSC games people to try :) > There's also some difference in behaviour between PAL and NTSC chips, the > NTSC 8564 somehow shows a ghost picture shifted 20 characters to the right > if the test bit is activated in the end of the lower border. I think the > ghost picture is only present in every second frame, but I don't know. I > tested this on a 8564R4 and a 8564R6, so I don't think it is a problem of > an old chip revision. How ghostly is this picture? It's important to distinguish between an artifact of the TV display, caused by *very* non-standard signals at its input (which may vary completely between TV sets) and an actual VIC-IIe effect. This also makes me wonder why the original author stopped at 64Hz; was that an estimate of the greatest out-of-spec signal "most" TVs would accept? Finally - about the PAL 2 line thing. I think it's fairly obvious that this is due to PAL phase alternations being locked to the LSB of the vertical line counter, so the colour circuit always produces one kind of line (eg. U) on odd lines and the other (-U) on even lines. However, the colour burst is also generated by that state machine, so I suspect any attempt to shift the whole screen 1 line to get extra colours won't work (at least, it will produce odd display-dependent effects such as all out loss of colour) because the colour subcarrier PLL inside the TV decoder will simply lock onto the "new" alignment of the PAL lines during the altered parts of the frame (it will take a few lines to re-synchronise) and then lock back onto the normal alignment at the end of the altered parts of the frame. You have to always maintain an even number of lines in each frame because the VIC always reverts to the same PAL line type (eg. U) at the top of each frame, if it's anything like the TED. Finally, we can look upon the VIC-IIe as a truly _usefully_ expanded version of the VIC-II. Hooray for the C128! Richard -- Richard Atkinson Software Engineer Tenison Technology EDA Ltd http://www.tenisontech.com/ - This message was sent through the cbm-hackers mailing list. To unsubscribe: echo unsubscribe | mail firstname.lastname@example.org.
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