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Doing your own is a solution of course, but I would love to see some generic improvements to the 6845 so that all drivers benefit.

I'm going to put that harder: fix the 6845 to work. Abandoning it is both cowardice and poor documentation value, and I will not look kindly upon any change lists that do so.

I'm not saying there's anything wrong with the current 6845, the real device doesn't output the video yet we have MC6845_UPDATE_ROW to do just that. The BBC has a VideoULA to output video RGB which takes the various 6845 outputs MA, RA, CURSOR, DE to generate video for the whole screen, not just the last area the 6845 was configured for. The amstrad driver uses the 6845 in a similar way and doesn't use MC6845_UPDATE_ROW, is that wrong?

I need to do some more testing but will probably submit current progress with borders and fixed blanking using MC6845_UPDATE_ROW. I'm not expecting this to break anything (apart from artwork) and will certainly improve many games that simply reduce the screen size to save video RAM.

Could it be related to this display bug in the CPC driver? -> http://mametesters.org/view.php?id=6864

I think that's related more to setting up for differing frame rates, where the driver just bases the frame rate on a machine config setting.

If this is referring to improving mc6845_device::screen_update then I agree.

Out of curiosity, why would Machine Information report different Video resolutions depending on whether MCFG_MC6845_SHOW_BORDER_AREA is true or false?
false:

true:

because it shows the current visible area size, which is obviously different if borders are showing..

Obviously, was a case of typing before thought.

Do any other drivers handle changing of the pixel clock during a scanline? This will be required to handle split modes vertically rather than the more commonly used horizontally split.

So the 6845 currently has a scanline_timer but I really need a pixel_timer. Would it makes sense to implement this in my MC6845_UPDATE_ROW or would a new MC6845_UPDATE_PIXEL be the way to go?

Again just thinking out loud for guidance.

MAME doesn't really support "horizontal partial updates" at the moment, and it definitely doesn't support changing resolution within a line. If you've got horizontal resolution changes within a line, you need to render at lowest common multiple. A per-pixel timer would kill performance. If you need to render per-pixel, you'd be better off making the video chip implement device_execute_interface and letting the scheduler deal with it.

Yeah, I need to get back to the horizontal partial update thing after my current large project lands. Gotta get those French Touch demos perfect.