the OS shadow registers (SDLSTL/SDLSTH $0230-$0231hex/560-561dec) to set the Display List. Instruction Bytes This legend applies to the instruction bit diagrams below: Instruction Bits: Bits 7:4 are modifiers for Playfield Mode instructions in bits 3:0. Bit value 1 Enables the modifier, and 0 disables the modifier. Bit 3:Bit 0 - Playfield Mode Instruction. Values , and are special instructions. Mode values through specify Playfield Character and Map modes. Bit 4 - - Horizontal Scroll. Bit 5 - - Vertical Scroll. Bit 6 - - Load Memory Scan. Bit 7 - - Display List Interrupt. Mode 0 Instruction - Blank Lines When Mode bits are all zero ANTIC performs no playfield DMA and displays one or more blank scan line(s) of the background color. For normal color interpretation modes the "background" is color register COLBK (D01Ahex/53274dec) OS Shadow COLOR4 (02C8hex/712dec), though this differs for the GTIA color interpretation modes. See discussion of COLBK on the GTIA page for more information. The number of blank scan lines are specified by the value of Bits 4 through Bits 6 allowing a range from 0 through 7. ANTIC adds one to this value and displays the resulting number of scan lines. Instruction Values: Blank lines are useful for delaying the start of the screen display until the electron beam has left the vertical overscan area at the top of the display. The Atari Operating System creates all its graphics modes beginning with three 70hex/112dec instructions (or 24 blank scan lines) for this purpose. Blank lines are also useful for partitioning parts of a custom display with different purposes (i.e. a border between a status display and the main graphics.) Blank Mode lines cannot be contained within a scrolling region, because Blank Mode line instructions inherently do not have Fine Scrolling Modifier bits. However, a Blank Mode instruction can be used to end a vertical scrolling region and is still subject to the expected scan line height changes per the vertical scroll value. The DLI modifier bit is available for blank line instructions. When the DLI bit is enabled a Display List Interrupt will be triggered during the last blank scan line of the instruction. Since playfield DMA does not occur during blank lines (only Display list DMA and Player/Missile DMA (if enabled) occurs) the blank lines can be used where Playfield graphics will not be needed giving more time for Display List Interrupt routines