The display pixel format
The screen has a fixed resolution of 256x192 pixels or 32x24 characters. Colour is restricted to 8x8 pixel squares equivalent to each character.
It is located in memory at address 0x4000…0x5AFF and is split into two parts:
|5800||DISPLAYATTR||Screen Colour data|
The 128K Spectrum's have a second screen in RAM Bank #7 which can be used as an alternate to the default one which is in RAM Bank #5. The memory layout is the same except for where it appears in the memory map.
This section will refer to the default memory location as used by all Spectrum's.
If you want to use the alternate screen then you need to change the base address to that of the correct bank. These are shown in the memory map's:
The display attributes specifying colour information