Post #2849080

Thinking I adapt the Beeb’s hardware design a tad, ditching the shadow RAM concept in favour of making the video memory just a special case of sideways RAM. Reducing the screen to 200 logical lines will make it fit within 16kB. Line doubling will make it 400, within a 640x480 at 25MHz VGA. Line doubling how exactly? Well… I could alternate between character codes and character definitions on even and odd lines, using an 80 byte buffer. No need for badlines that freeze the CPU, provided I run the CPU and VDC at 3.125MHz and the memory at 6.25MHz. NMI response times protected. The added beauty is that an all-points-addressable bitmapped graphics mode is actually a special type of character mode in which every character is 1 logical line in height and whose definition equals its code. The identity function. So, all I’d need to do is support a configurable character height. The 256 x 1 byte identity will fit within the 384 bytes free in the 16kB after the (640/8)*200 ‘character codes’ have had their fill.