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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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. A graphics 0 display is one row of characters. When Vertical scrolling is not set in the Display List the three Modes lines would naturally result in 24 displayed scan lines.
Most ahtic lists are less than bytes long. Blank Mode lines cannot be contained within a scrolling region, because Blank Mode line instructions inherently do not have a Vertical Scroll Modifier bit. 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.
ANTIC – Wikipedia
Even in this worst case example the CPU effort expended is a tiny fraction of the processing needed to bulk move screen data. So long as you retain the proper integer multiples, the horizontal shift will be avoided. Screen memory can be placed anywhere in the address space of the computer. Graphics mode 3 graphics mode Number of colors: However, the “new” information that has moved into the right side of the scrolling region was previously the first byte on the following line except for the last byte of the last line — that information had been completely off screen.
It is the responsibility of the DLI routine to save the state of the registers that will be used during the DLI routine, and then the last action is wntic restore the original values of the registers before exiting with an RTI instruction.
Antix Horizontal scrolling is enabled for a mode line ANTIC fetches the next size increment greater than the current screen width to provide the buffer of data subject to the ahtic scrolling control. With 1K of screen RAM, one thousand characters can be displayed on the screen.
However, use of this feature will produce interesting antid effects known as color artifacts.
Its only function is to provide a solution to a tricky problem: Each byte represents one character, allowing a choice of different characters in the character set. Second, each display image will be washed out because it only shows up one quarter of the time.
There are 24 display blocks being shown on the screen in Graphics 0. A Graphics 2 mode line is 16 horizontal scan lines high, while a Graphics 7 mode line is only two scan lines high. Since it can be located anywhere in the memory, there is little difficulty finding a sufficiently sized place in memory that does not cross a 1K boundary.
A program tightly integrated with the display cycle and aware of the electron beam position relative to the current display can bypass turning off Display List DMA. There are two jump instructions.
This value is useful during Display List Interrupts to identify the vertical screen position. For this reason the picture that the computer puts out must be somewhat smaller than the television can theoretically display.
It will, and the display will pop up on the TV. The Horizontal Fine Scrolling display list modifier causes ANTIC to fetch extra screen memory corresponding to the next larger Playfield width which provides the data needed to scroll into the display. Normally the anyic list specifies the ataari of the screen memory with the first display instruction—the initial LMS instruction.
Therefore, page flipping implemented by Display Lists that point to the next Display List in series will not operate as expected unless the Vertical Blank interrupt is disabled. The output is the graphics display. It is achieved by moving bytes of aari through a designated screen display area. A display block is a group of adjacent horizontal scan lines a group of scan lines all together, with no spacing between them which are all in the same graphics mode.
Display blocks do not have a fixed height.
The third important difference is that the screen RAM can be located qtari in the address space of the computer and xntic around while the program is running, while the other machines use fixed-screen RAM areas. Archived from the original on We design our displays around the available scan lines. For all normal color interpretations this is by color clock, not pixel even when pixels are larger or smaller than the color clocks.
Each byte in screen RAM specifies one character. The choice of color register is specified by the value of the screen data. Aantic, the display is actually a view port that is moved to look at a different areas of memory rather than moving the memory into a fixed screen map.
Graphics mode 5 graphics mode Number of colors: Jumping up to the top of the display list turns it into an infinite loop; waiting for vertical blank ensures that the infinite loop is synchronized to the display cycle of the television.
The same technique can be employed to display ahari more colours than seemingly allowed in any particular graphics mode, which can be mixed and altered at will.
The Atari will do all the constructing for you, and save you a lot of time. Both JMP and JVB are 3-byte instructions the first byte is the opcode, the second and third bytes are the address to jump to low then high. They are the ahari of one data element in display memory.