ANTIC (Alpha-Numeric Television Interface Circuit or Alpha-Numeric Television Interface Controller) is an early video system chip used in the Atari 8-bit family of microcomputers as well as the Atari 5200 in the 1980s.
ANTIC's most notable features are:
ANTIC is a microprocessor dedicated to generating 2D computer graphics to be shown on a television screen or computer display. It is a true microprocessor, in that it has an instruction set to run programs (called display lists) to process data.
The display list and the display data are written into RAM by a 6502-compatible CPU. The ANTIC retrieves that information from RAM using a technique known as direct memory access (DMA). It processes the higher level instructions in the display list and translates 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 are available.
ANTIC has four types of instructions:
One of the jump instructions is a simple JMP - it transfers the display list execution to another place within the 64k address space. The other one, JVB (Jump on Vertical Blank) instruction is placed at the end of the display list. Its role is to get the display list execution in sync with the Vertical Blanking. The JVB's argument usually points to the beginning of the same display list, but it of course can also point to another display list, so that a chain of display lists, executed after consecutive blanking pulses, is formed.
Each instruction has additional options by setting specific bits:
Combining the bitmap mode instruction with LMS attribute makes it possible to set the screen memory address freely within the 64K address space independently for each display line. In other words, the screen memory does not have to be a solid memory area scanned sequentially towards higher addresses - only a single line must cover a continuous area of memory, usually consisting of 32, 40 or 48 bytes.
Although ANTIC's program counter is 16-bit, only 10 bits are changed during normal (i.e. sequential) execution of the display list. This means the display list need a JMP (Jump) instruction to cross a 1K boundary. This is not a serious limitation, because the size of a single display list usually varies from 32 to 202 bytes, and virtually never exceeds 720 bytes. Since it can be located anywhere in the memory, there is no trouble in finding a place for it, that does not cross a 1K boundary.
Also the Memory Scan Register, a register addressing the data stored in the screen memory, is 16-bit, but only 12 bits are changed when ANTIC is sequentially scanning the video memory. Thus an LMS (Load Memory Scan) instruction is needed for data crossing a 4K boundary. High resolution graphic modes usually need more than one LMS instruction within the display list to be displayed properly.
The character generator can be located anywhere in the memory, but, depending on the display mode used, it has to be aligned to a 512-byte or a 1K boundary.
Type the following program at the BASIC prompt (typically the word "READY"), terminating each line by pressing the Enter (or Return) key. Run it by typing "RUN" and pressing Enter. When the program has finished running, type "PRINT USR(30720)" and hit Enter. The program should display a green pixel in BASIC graphics mode 3, and lines in BASIC mode 0.
10 FOR N=30720 TO 30829
20 READ D
30 POKE N,D
40 NEXT N
50 DATA 169,112,141,0,128,141,1,128
60 DATA 141,2,128,169,66,141,3,128
70 DATA 169,0,141,4,128,169,130
80 DATA 141,5,128,169,2,141,6,128
90 DATA 141,7,128,169,72,141,8,128
100 DATA 169,0,141,9,128,169,140
110 DATA 141,10,128,169,8,141,11,128
120 DATA 141,12,128,141,13,128,141,14
130 DATA 128,169,65,141,15,128,169,0
140 DATA 141,16,128,169,128,141,17,128
150 DATA 169,0,141,48,2,169,128,141
160 DATA 5,141,6,150,169,150,141,7,150
170 DATA 5,141,6,150,169,150,141,7,150
180 DATA 169,2,141,11,140,5,150