Games - ANTIC

	Games - ANTIC

: This article refers to the ANTIC display chip inside the 8-bit Atari
home computers. There was also a magazine of the same name (ANTIC
) for those computers.

ANTIC (Alpha-Numeric Television Interface Circuit) was an early video system chip used in the Atari 8-bit family of microcomputers as well as some of Atari's video game consoles of the 1980s.

ANTIC was a microprocessor dedicated to generating 2D computer graphics to be shown on a television screen or computer display. It was a true microprocessor, in that it had an instruction set to run programs (called display lists) to process data.

The display list and the display data were written into RAM by a 6502-compatible CPU. The ANTIC retrieved that information from RAM using a technique known as direct memory access (DMA). It processed the higher level instructions in the display list and translated these instructions into a real-time stream of simpler instructions to the CTIA chip, a combination providing for 12 graphics modes. With the more advanced GTIA, 16 modes were available.

ANTIC's most notable features were:

14 different graphics/text modes
Display modes can be mixed onscreen
Non-fixed screen RAM. Allowed screen RAM to be located almost anywhere in memory. This allowed for easy page-flipping and other effects.

ANTIC had four types of instructions:

Map mode - display colored pixels
Character mode - display character data
Blank line - display horizontal blank lines (solid color)
Jump instruction - reload ANTIC's program counter (3-byte instruction)

Each instruction had additional options by setting specific bits:

DLI - Display list interrupt
Load Memory Scan (LMS) - Loads address of graphics/character data (3-byte instruction)
Vertical scrolling - Enables vertical scrolling
Horizontal scrolling - Enables horizontal scrolling

Although ANTIC was a microprocessor, its program counter was only 10-bits long. This meant the display list needed a JMP (Jump) instruction to cross a 1K boundary. A LMS (Load Memory Scan) was needed for data crossing a 4K boundary. Finally, a JVB (Jump on Vertical Blank) instruction was placed at the end of the display list. The entire display had to equal 192 scan lines.

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