In the Atari VCS 2600 the playfield represents the background, or stage, in which the gameplay happens. The playfield is controlled by a 20 bit register divided in three addresses: PF0 (4 bits), PF1 (8 bits), and PF2 (also 8 bits).
Set the bits of the PF0, PF1, and PF2 registers to draw the background one line at the time. Race the beam by modifying these registers on each scanline to draw the complete screen background.