amounts of RAM. Designing a display requires counting the scan lines of each Mode instruction and tracking the memory for each Mode line to prevent overrunning ANTIC or video standard limitations. (See the Limitations section). Mixing multiple graphics modes in one display is done simply by providing different character or map mode instructions for the desired position on screen. For example, the Atari Operating System graphics modes 1 through 8 offer full screen display, or an option for a four-line text window at the bottom of the screen. This split screen feature is accomplished using the inherent capabilities of ANTIC and its Display List without any CPU interrupts or custom manipulation of the video hardware. ANTIC has four types of instructions: Blank line - 8 instructions to display from 1 through 8 horizontal blank lines Jump instructions - 2 kinds of Jump instructions which reload ANTIC's program counter (3-byte instructions) Character mode - 6 instructions to display character data Map mode - 8 instructions to display colored pixels Map and Character mode instructions have additional modifiers that can be enabled with the instruction: Horizontal scrolling - Enables horizontal fine scrolling Vertical scrolling - Enables vertical fine scrolling Load Memory Scan (LMS) - Sets the starting address of graphics/character data (3-byte instruction) DLI - Display List Interrupt (called "raster interrupt" or "Horizontal Blank Interrupt" on other systems) Instruction Execution ANTIC begins executing the Display List pointed to by the 16-bit address in registers DLISTL/DLISTH ($D402-$D403hex/54274-54275dec). The address registers are incremented by ANTIC automatically as each instruction is executed. ANTIC can only update the lowest 10 bits of the address during the automatic increment limiting the display list to begin and end within a 1K address range. The registers are also updated during Display List execution by ANTIC's JMP (Jump) and JVB (Jump and wait for Vertical Blank). These instructions load the full 16-bits of address, so can be used to circumvent the 1K Display List limitation. The address is also updated by the Operating System's Vertical Blank Interrupt (VBI) routine using the values from Operation System shadow registers SDLSTL/SDLSTH ($0230-$0231hex/560-561dec). When the OS VBI routine is enabled direct updates to the ANTIC DLIST registers by the CPU or the ANTIC Jump instructions will be overwritten by the OS during the next Vertical Blank. Since VBI processing is the usual state of the system, most programs rely on this and only update