On 26 April 2011 13:15, Thomas Harte <[email protected]> wrote:
> Apart from a desire for part uncontended memory (ala the Spectrum) and > a hardware scroll, a simplified blitter would have been advantageous > (eg, give it start address, end address, length, tell it to go and > then it replaces the z80 on the bus until the copy is complete; even > with the CPU having to do a lot of lifting around the outside and run > some other logic that'd give you a 25fps scroll if that's what you > wanted it for, or filled vectors if you prefer). > You end up having to contend with the video during the move so you're still talking about significant slowdown. You might get 25fps scroll but that would take most of your bus time, leaving you not much time to do anything else. Quite apart from the significant amount of hardware you'd have to add of course. Filled vectors are going to be not a huge amount quicker unless you make the hardware a lot smarter - you'd spend most of your time setting up the external registers to fill objects one line at a time. Failing that, I really don't think video adjust registers could have > cost that much. So you've a horizontal register and a vertical > register, each capable of taking values in the range 0 to 7 and will > offset the pixel output by that amount on the screen. They're timing > delays on the video output, essentially, but they buy you up to 8 > frames of scrolling with very limited redraw costs, and that gives you > enough time to prepare the next major offset on a secondary screen. If > memory serves, you get a similar thing for free in the 6845 that > appears in the BBC, Amstrad and VGA adaptors. > > What's the advantage over the writable screen-address idea? As far as I can see that involves more hardware and is way more complex to code for. It's only really useful in mode 4 if you need pixel-by-pixel horizontal scrolling. I guess it's useful on the Beeb (and indeed in Sam Mode 2) because there are 4-colour (and 2-colour) modes so byte-by-byte would be 8 pixels at a time. But Mode 2 is just such a horrible hack I pretend it doesn't exist :) G
