The border colour is managed by the ULA.
Changing the border colour is simply writing the required colour to the ULA whilst
storing a copy in the BORDCR
system variable at 0x5C48 so that the OS also knows what the colour is.
In this example we set the Border to black
Set the Spectrum Screen Border:
setBorder
LDA,0x00Set border to black
LD(BORDCR),ASet OS copy of border colour
OUT(&FE),ASet hardware border colour
RET