Chapter 2 of De Re Atari: ANTIC AND THE DISPLAY LIST. Retrocomputing podcast about the Atari 8-bit line of personal computers. Vol. 1 – No. 1. Antic issue Vol. 1 – No. 1. April – (PDF). Vol. 1 – No. 2. Antic issue Vol. 1 – No. 2. June – (PDF). Vol. 1 – No. 3. Antic issue Vol. 1 – No. 3.

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This is done by writing machine language routines to change the values of hardware registers at different vertical positions on the screen. After all the data from the first full screen has been shifted off the display the scrolling application can rebuild the first screen to contain a known transition point so that the scrolling action can jump back atwri the first address in the block with no apparent stutter or obvious interruption in the scrolling.

Atari 8-bit Forever by Bostjan Gorisek

In the low-resolution graphics mode, the video hardware reaches into screen memory and interprets it differently. The DLI modifier bit is available for blank line instructions.

When the fine scrolling limit is reached, scrolling continues by resetting the VSCROL value and performing a coarse vertical scroll of the scrolling region. Graphics mode 0 text mode Number of colors: The choice of color register is specified by the value of the screen data.

The main processor of the Atari is called arari Since programmer time is becoming the most expensive factor in owning a computer, this sort of time saving is very important. There are also four special options that can sometimes be specified by setting a designated bit in the ANTIC instruction.

The third important difference agari that the screen RAM can be located anywhere in the address space of the computer and moved around while the program is running, while the other machines use fixed-screen RAM areas.

Vertical Fine Atsri allows single scan line movement of the scrolling region for Text and Map modes which are greater than one scan line tall. Only even values should be used to ensure correct scrolling. Graphics mode 6 graphics mode Number afari colors: The beam starts at the top-left corner of the screen and traces horizontally across the screen.

Operating System Shadow registers are copied from RAM to the hardware registers during the vertical blank. Therefore, a display can be “moved” by merely updating the initial LMS modifier’s address; adding the number of bytes used for a line in the current Text or Map Mode shifts the screen contents up one line while subtracting the same amount moves the screen down.


Video Inverse and Video Atark enabled together result in reverse video characters displayed as an inverse blank space. That 40 bytes must immediately follow the 20 bytes of mode 2 data in memory, because that is where ANTIC will be looking for them.

ANTIC has no capacity for writing back computed values to memory, it merely reads data from memory and processes it for output to the screen, therefore it is not Turing complete. 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. Horizontal scrolling requires the same Display List implementation as stari worst case example described for Vertical Coarse Scrolling.

Rather than atrai repeated LMS modifiers, a quirk of vertical scrolling can be exploited to create a anyic scan-line mode that displays pixels using the hi-res or GTIA color interpretation modes.

ANTIC – Wikipedia

Most television sets are designed with “overscan”; that means they spread the image out so the picture edges are off the edge of the television tube. The display list must not cross a 1K address boundary. They are the height of one data element in display memory. There are actually sweeps across the screen in an alternating system known as “interlace.

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.

Since fine scrolling is based on scan lines not pixel height, and scrolling region movement is done simply by skipping scan lines from the first Mode line and adding scan lines to the last Mode line, it is possible to use different graphics Modes within the scrolling region. Thus, some map modes give better resolution; these will of course require more screen RAM. 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.

One of them is mixing graphics modes on the screen. There are many examples of demo programs available on the internet, displaying these and other features.


Graphics mode 14 graphics mode Number of colors: Clearly, you have to map out your displays in advance and also map out your display memory.

This is because graphics 8 display blocks are only one scan line high. Keep track of the scan line count of your display by looking up the scan line requirements of the various modes in Figure The and ANTIC share memory, and the computer is designed to keep them from trying to use the same memory at the same time. The video display system was designed with careful consideration of NTSC display methods and limitations.

This makes a load memory scan command. In the high-resolution graphics mode each bit in screen memory is mapped to a single pixel. Only display list manipulations gain the user access to these modes.

Often, modes that display well ataei European systems display awfully on US systems, or vice versa. Think of them as long, thin horizontal bars extending the full width of the screen. Second, if you wish to use any of the operating system screen routines you must abide by the conventions the OS uses.

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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. It consults the display list again for what to do with the next display block, then grabs another 40 bytes from display memory using the Graphics 0 display block, right below the first 40, and plots another line of graphics 0 characters, and continues.

It stretches horizontally all the way across the screen. Certain combinations antlc values in the Display List can produce unexpected results.

ANTIC The Atari 8-bit Podcast

Eight pixels can be stored in one screen data byte. Horizontal Fine Scrolling allows single color clock movement of the scrolling region for Text and Map modes. Graphics mode 8 graphics mode Number of colors: Programs should avoid reading the hardware register directly unless the program is certain the register is read at a time insuring the value is valid.