Were there systems that wasted #framebuffer space in order to avoid needing hardware to implement the front and back porches for #VGA?
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I realized 256x240 would be an extremely convenient resolution, since on an 8 bit system you can convert X and Y to a framebuffer address without doing any math. Did a search for "256x240" and discovered this is the resolution the NES uses.
Turns out 10-bit counters are readily available, so to produce a 640x480x60Hz VGA signal, one can use the middle 8 bits on one for the low bits of the address, suppress the output when the high bit is on and reset when the top 5 bits are 11001. Not sure what the best way is to send the horizontal sync pulse; it needs to be 96 pixels wide, from pixel 656-791, which is 101001 through 101110 in the top 6 bits. I've read that monitors only care about the rising edge, so maybe I can send the horizontal sync pulse whenever the top 3 bits are 101?
Oh, actually, sending the horizontal sync pulse early is good, because otherwise the image will be on the left edge of the screen!
Vertical uses a very similar setup: middle 8 bits are the high bits of the address (each line is repeated twice), output suppressed when the high bit is set. Sync pulse is sent when the top 9 bits are 11110101. Apparently some systems don't like there to be any video signal in the horizontal blanking interval, but I don't know how much they care about the vertical blanking info. So it's possible that writing anything past row 240 will cause problems. The blitter can at least avoid doing that.
