Re: Screen memory layout
> Why was that so whacky?
The short and simple answer is that it saved memory.
At 256*192, a simple black and white display uses 6kb of memory. Conversely, a display using 8 bits of colour per pixel would use 48kb. Which wouldn't have left any memory to do anything else, unless Sinclair was willing to bump the RAM up to 64kb[*]!
Conversely, the colour overlay only used 768 bytes, since it only needs one byte per 8*8 block on screen.
And that meant that even the 16k Spectrum still had about 9kb of RAM free, which was enough to do something useful. Or to play games like Jetpac ;)
It's also worth noting that the ZX Spectrum wasn't the only computer to use this sort of technique, though most of it's rivals did offer other display modes.
https://en.wikipedia.org/wiki/Attribute_clash
Still, you gets what you pays for, and the ZX Spectrum was significantly cheaper than it's more flexible competition...
[*] 8 bit CPUs can generally only address up to 64kb of memory, which can be a mix of RAM and ROM. The basic Spectrum models had a 16kb ROM and then either 16kb or 48kb of memory. Conversely (and to somewhat simplify), the Commodore 64 had a 16kb ROM and 64kb of RAM, and the programmer could choose to either use the ROM + 48kb of RAM, or replace it with their own code and use the entire 64kb of RAM!
Biting the hand that feeds IT
