> The CPU isn't really fast enough to fine-scroll large sections of the screen.
That's fine; it is not how you'd do it on the C64. Nor was VSP very common.
> The there is a useful "rotate memory left/right" instruction which takes 7 cycles. An unrolled loop of these could shift about 2000 bytes of memory left/right per frame. A 160x200 2bit color bitmap mode screen takes up 8000 bytes, so at the very least you can shift ΒΌ of the screen.
Nobody used bitmaps to for scrollers on the C64 (or indeed for most games). A game screen using character mode takes at most 2000 byes if you need to scroll the entire screen, including color RAM, which you wouldn't be doing as you'd spend some space on other things, like score displays etc., and putting more static decorations was indeed a common way of cutting down on the amount actually scrolled. Another way was to do level designs so that you did not have to scroll everything every time. E.g. by keeping some lines using the same colour, or fill part of the screen with sprites instead of characters.
To scroll the first 7 pixels in any direction then only requires updating a single register to shift the screen. This applies for bitmaps too, so there really are no circumstances where you'd bit-shift bitmaps to scroll.
For the 8th pixel you need to copy, but you don't need to complete the copy in one frame; you just need to outpace the raster-beam. So the moment it has rendered the first full line of text, you can scroll it; as long as you take care to not speed past the rendering, then you can continue following past. If you do this you have nearly two full frames to complete the copy, for about 1000 byes per frame.
That's fine; it is not how you'd do it on the C64. Nor was VSP very common.
> The there is a useful "rotate memory left/right" instruction which takes 7 cycles. An unrolled loop of these could shift about 2000 bytes of memory left/right per frame. A 160x200 2bit color bitmap mode screen takes up 8000 bytes, so at the very least you can shift ΒΌ of the screen.
Nobody used bitmaps to for scrollers on the C64 (or indeed for most games). A game screen using character mode takes at most 2000 byes if you need to scroll the entire screen, including color RAM, which you wouldn't be doing as you'd spend some space on other things, like score displays etc., and putting more static decorations was indeed a common way of cutting down on the amount actually scrolled. Another way was to do level designs so that you did not have to scroll everything every time. E.g. by keeping some lines using the same colour, or fill part of the screen with sprites instead of characters.
To scroll the first 7 pixels in any direction then only requires updating a single register to shift the screen. This applies for bitmaps too, so there really are no circumstances where you'd bit-shift bitmaps to scroll.
For the 8th pixel you need to copy, but you don't need to complete the copy in one frame; you just need to outpace the raster-beam. So the moment it has rendered the first full line of text, you can scroll it; as long as you take care to not speed past the rendering, then you can continue following past. If you do this you have nearly two full frames to complete the copy, for about 1000 byes per frame.