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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Do not limit your imagination to this pattern; with the display list you can create any sequence of mode lines down the screen.

The pixel resolution is across but the color resolution half that, thus each group of two pixels will “blend” together to form a single color and four artifact colors may be generated this way.

The most common example is changing the background color. When horizontal scrolling is enabled ANTIC retrieves more screen memory bytes than displayed in order to show partially scrolled display bytes at the beginning and end of the line. Character displays are discussed in Section 3. The data following the second mode 0 line must be bit mapped format, ready for graphics 0 display. Everything above the interloper line is fine, but below it, the characters are shifted 20 spaces to the right.

This can be of value in setting up independent text windows. Some Write hardware registers do not have corresponding Shadow registers. It needs to know what to put on the very first scan line. In a standard byte line display, BASIC would position the characters onto the tenth line by skipping bytes from the beginning. One way to speed up calculations on the Atari is to ahtic off the display completely.

As this vertically mirrors the glyph data before it is used, the effect appears inconsistent for ANTIC Mode 3 descenders with glyph bytes 6 and 7 appearing at the bottom of the descender area.


Other platforms call this activity, ” Atar interrupts “, or ” Horizontal blank interrupts “. Next we will allocate display memory.

We are going to need some tables on how many bytes the various graphics modes consume. This specifies anntic page of the base address for the character set. If you absolutely must have it cross such a boundary, insert a JMP instruction just in front of the boundary.

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Displaying fewer than scan lines will cause no problems; indeed, it will decrease execution time by reducing the number of cycles stolen by ANTIC. However, a Blank Mode instruction can be used to end a scrolling region and is still subject to the expected scan line height changes per the vertical scroll value.

As it generates the display, ANTIC automatically increments its memory scan pointer from Mode line to Mode line to display memory contiguously.

Blank line afari cause ANTIC to display a number of horizontal scan lines with solid background color. Horizontal fine scrolling moves the scrolling region up to 16 color clocks.

If we atai a few more, we will get some bizarre displays you may want to try this out later on. The Atari will do all the constructing for you, and save you a lot of time. Presenting screen memory as long horizontal lines requires wntic LMS modifier for every Display List Text or Map Mode instruction in the scrolling region.

This kind of scrolling is called, “coarse scrolling”. Two bits per pixel allowing 4 colors inside one character matrix. You state the height of an image by specifying the number of horizontal scan lines it spans. In other words, the memory for the previous Mode line ends at the exact last byte of the 4K block and the next Mode line begins at the exact first byte of the next 4K block.

The color displayed comes from a color register. However, this atxri not impair the scrolling process and merely changes the value of addition and subtraction used to locate the start and end of each display line. They are stacked on top of one another. This page was last edited on 8 Decemberat Unlike many platforms Atari’s horizontal scrolling is visually consistent and free from color “strobing” artifacts due to Atari’s pixel size matching the color clock timing needed for accurate color.


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.

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They differ in the number of colors they display 2 vs 4the vertical size one mode line occupies 1 scan line 2, 4, or 8and the number of pixels that fit horizontally into one mode line 40, 80,or 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 antci screen contents anhic the right.

It should be possible to produce interesting effects with such lists. When ANTIC encounters such an instruction, it will load its memory scan counter with the following 2 bytes.

The beam starts at the top-left corner of the screen and traces horizontally across the screen. Retrieved from ” https: It saves a lot of time to mix graphics modes and not have to go through all the work of plotting your characters in high resolution dots. If you make the beam more intense, the spot in the screen that it strikes will glow brightly; if you make it less intense, the spot will glow dimly or not at all.