I am planning on building a 6502 SBC. My question is how hard would it be to add a VGA video chip to a 6502 bus? I really like the idea of using my high-resolution monitor. I am thinking of something that has it’s own video memory that I can write to through the chip’s memory mapped registers. If VGA is not an option then how about any good RGB video chips with external memory?

If you can find an eight bit VGA card it shouldn't be impossible but there are some things to consider.

The only 8 bit VGA cards I've got are 512k cards. This limits you to 640 x 480 in 64k colours or 800 x 600 in 256 colours which isn't too bad but the access time is slow. Even for 640 x 480 x 256 it will take a 2MHz 6502 over a second to write to every displayed pixel, and that's assuming fast 6502 code. The lower resolutions would be better, 320 x 240 in however many colours or even text modes shlould be at least reasonable.

The other downside of VGA is you will need the datasheet for the VGA chip on the card to anything more than the basic resolutions and possibly even to get the card to do anything at all as you won't be able to use the on board x86 BIOS.

An easier bet would be to use any of the general purpose on screen graphics chips from teletext to the menu chips used in sat boxes and video recorders. These often have a two or three wire interface, on board memory and can do straight graphics or genlock (overlay) on a video source.

I prefer the teletext chips as they often come on sub boards so you get a complete working board and don't have to mess with building the video section. You can see an example at ..

http://www.geocities.com/leeedavison/i2c/index.html

The only extra parts, apart from the i2c interface, on the board are a power socket (for the +12v), a video in socket (for genlocking) and an RGB out socket.

Here's a whacky idea:
Get an old 386, 486, or slow Pentium PC box - anything that supports the video card you want to use (maybe an old laptop?).
Connect your 6502 SBC to the PC via the serial or parallel port.
Write SW on the PC to process "requests" from the SBC to update the video. Note that you wouldn't have to repaint the whole screen on a regular basis - you only have to send the "updates".

This approach would give you the maximum functionality in the video, with the least headache and frustration. Your SBC can focus on doing something more suited to an 8-bit CPU.

Pete