All bbc machines are currently tagged with MACHINE_IMPERFECT_GRAPHICS due to incorrect rendering when anything unusual is written to the 6845. To improve this I'm using bbcb_flop:froggerr as a test case as it really pushes the bbc video system to it's limits by using the 6845 in innovative ways. See it in action at http://bbcmicro.co.uk//jsbeeb/play.php?autoboot&disc=http://bbcmicro.co.uk//gameimg/discs/Disc108-FroggerRSCB.ssd
Many games change the screen size to reduce video RAM usage, and this game runs at 224x256. It doesn't set the 6845 to this size though, it defines multiple 'screens' as the screen is being updated. So R4 is set to 0x00 (1 character row/8 scanlines) and R12/13 point to RAM, and these are updated during HSYNC? This allows the same area of RAM to be repeated in different parts of the overall screen.
The bbc currently uses MCFG_MC6845_UPDATE_ROW_CB for rendering but due to all these 'screens' being defined at 224x8 then we only see the first 8 scanlines stretched full height of our output. How should I handle this?
I'm thinking I need to use a fixed size canvas and introduce MC6845_RECONFIGURE to stop the 6845 from resizing my screen, then rewrite MC6845_UPDATE_ROW to render in the active area only.
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