the "background" color. COLPF1 always has the highest priority and cannot be obscured by Players or Missiles. The glyph/graphics pixels use the color component of highest priority color (Playfield, Player, or Missile), and the luminance component of COLPF1. Note that this behavior is also consistent where Player/Missile priority conflicts result in true black for the "background". In effect, the color value CTIA/GTIA finally uses for the "background" color "tints" the COLPF1 foreground glyph/graphics pixels. VDELAY $D01C Write Vertical Delay P/M Graphics This register is used to provide single scan line movement when Double Line Player/Missile resolution is enabled in ANTIC's DMACTL register. This works by masking ANTIC DMA updates to the GRAF* registers on even scan lines, causing the graphics pattern to shift down one scan line. Since Single Line resolution requires ANTIC DMA updates on each scan line and VDELAY masks the updates on even scan lines, then this bit reduces Single line Player/Missile resolution to Double line. GRACTL $D01D Write Graphics Control GRACTL controls CTIA/GTIA's receipt of Player/Missile DMA data from ANTIC and toggles the mode of Joystick trigger input. Receipt of Player/Missile DMA data requires CTIA/GTIA be configured to receive the data. This is done with a pair of bits in GRACTL that match a pair of bits in ANTIC's DMACTL register that direct ANTIC to send Player data and Missile data. GRACTL's Bit 0 corresponds to DMACTL's Bit 2, enabling transfer of Missile data. GRACTL's Bit 1 corresponds to DMACTL's Bit 3, enabling transfer of Player data. These bits must be set for GTIA to receive Player/Missile data from ANTIC via DMA. When Player/Missile graphics are being operated directly by the CPU then these bits must be off. The joystick trigger registers report the pressed/not pressed state in real-time. If a program's input polling may not be frequent enough to catch momentary joystick button presses, then the triggers can be set to lock in the closed/pressed state and remain in that state even after the button is released. Setting GRACTL Bit 2 enables the latching of all triggers. Clearing the bit returns the triggers to the unlatched, real-time behavior. HITCLR $D01E Write Clear Collisions Any write to this register clears all the Player/Missile collision detection bits. Other CTIA/GTIA Functions Joystick Triggers TRIG0 $D010 Read SHADOW: STRIG0 $0284 Joystick 0 trigger TRIG1 $D011 Read SHADOW: STRIG1 $0285 Joystick 1 trigger. TRIG2 $D012 Read SHADOW: STRIG2 $0286 Joystick