ANTIC

Under the direction of Jay Miner, the chip was designed in 1977–1978 by Joe Decuir, Francois Michel, and Steve Smith[2] for the Atari 8-bit computers first released in 1979.

ANTIC provides 15 Read/Write registers controlling Playfield display parameters, DMA for Player/Missile graphics, fine scrolling, light pen input, and interrupts.

As HSCROL increases more color clocks from the end (right side) of the buffered data are added to the left edge of the display causing the fine scroll shift to move the screen contents to the right.

The shadow register is the recommended source for reading this information, since it will be updated during the vertical blank guaranteeing consistent and reliable results.

The Operating System does not use DLIs, so the default address for the DLI vector (VDSLST, $0200hex/512dec) points to an RTI instruction in ROM.

Alternatively, a Vertical Blank Interrupt routine can be used to reset the VDSLST address to guarantee the DLIs remain in sync with the screen.

User programs can insert execution of custom code either before (Immediate) or after (Deferred) the Operating System's Vertical Blank Interrupt routines.

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.

Since VBI processing is the usual state of the system, most programs rely on this and only update the OS shadow registers (SDLSTL/SDLSTH $0230-$0231hex/560-561dec) to set the Display List.

If ANTIC register NMIEN (D40Ehex/54286dec) has Bit7 ($80) set then on the last scanline of the Mode line an interrupt routine will be triggered which is vectored through address VDSLST (200hex/512dec).

ANTIC Playfield modes are exposed to users through Atari BASIC via the "GRAPHICS" command, and to some other languages, via similar system calls.

Any graphics mode in the default GTIA color interpretation can be freely mixed without CPU intervention by changing instructions in the Display List.

Mode F was not widely used by software due to its high memory requirements (8k) and inconsistent artifact colors that did not appear the same on every model of Atari 8-bit computer.

Games that utilize it include Sierra graphical adventures such as Troll's Tale and Wizard and the Princess, Lode Runner, and the Ultima series.

Most computers including the Atari 8-bits accomplish coarse scrolling as described above — common examples are viewing a long BASIC program listing or directory of files.

However, ANTIC provides direct hardware support for vertical and horizontal, coarse and fine scrolling taking the concept of a "view port moving over data" from a metaphorical illusion to literal implementation.

Since ANTIC's memory scan counter cannot automatically increment over a 4K boundary the limit of vertical scrolling in this simple manner is up to 4K of movement.

Even in this worst case example the CPU effort expended is a tiny fraction of the processing needed to bulk move screen data.

Presenting screen memory as long horizontal lines requires an LMS modifier for every Display List Text or Map Mode instruction in the scrolling region.

Using memory actually organized as a long, horizontal, contiguous series of bytes requires an LMS modifier for every Display List Text or Map Mode instruction in the scrolling region.

The usual purpose is to change display-related values at a known location so transitions are visually precise or occur where they will not conflict with display activity.

Since the DLI is 6502 machine code executed by the CPU, any kind of processing work is possible provided it is short enough to not conflict with other activity and does not overrun a subsequent Display List Interrupt.

Properly launching the interrupt requires the following conditions: When updating display control values care must be taken to ensure ANTIC is not currently using them.

If ANTIC is processing the Display List containing an Instruction with the DLI modifier, then an interrupt could be dispatched while changing the VDSLST address, potentially crashing the system.

This direct memory access capability is leveraged to read data on behalf of CTIA/GTIA and update the GRAF* graphics pattern registers as each display scan line is generated.

The DMACTL bit values for Player/Missile resolution: CTIA/GTIA must also be specifically enabled to receive the Player/Missile graphics data via DMA to correctly present the objects on screen.

This produces a lower vertical resolution where each pixel is 2 or more scan lines tall, but has higher DMA overhead due to ANTIC needing to re-read the same bytes from screen memory.

However, if Vertical Scrolling is enabled for the Mode F Display List instruction, and the VSCROL register is set to 13, then ANTIC's line counter will count from 13 to 0 (four scan lines—13, 14, 15, 0).

In any case, this is easy to avoid, since this is not a practical Display List arrangement as this places the Mode line in the overscan area where it is not ordinarily visible.