Mode 4 of CPCs (two bitplanes emulation)
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Mode 4 screen (two bitplanes emulation)
is a software technique to emulate two bitplanes on-screen at once. The main advantage is that you don't need to take care of the background while moving sprites around. Moreover, graphics need 50% less memory space.
Also known as 'Mode 3'.
Used in games like: Ghost'n'Goblins, ZTB-Mission Genocide etc..