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@eobet the border isn't technically removed. There isn't any need to since you don't actually draw any graphics. Well, the top border is removed but that's just so that you can lock the raster as early as possible.

The fastest normal way you can change the background is 8 pixels horizontally. Conveniently it's 1 pixel per cycle, so the colour switch is done from a register to the background colour address. That instruction is 8 cycles.

However, it's also possible to use a 12 cycle instruction to offset the drawing by 8 pixels.

Note that if you want a switch in 8 cycles you need the source data to be a register, and since the destination address also needs to be in a register you are limited in the number of colours available per line. Also, there isn't enough time in the horizontal blanking area to change all the registers so if you want both high resolution and lots of colours you need be clever in the way you update the registers.