More Graphics Modes

Colin Boswell

Issue 1

Dec 82/Jan 83

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Graham Daubney of Atari recently mentioned at a BUG meeting that he had discovered, quite accidentally, some totally new Graphics modes. Intrigued by this, I went home and started messing around myself.

After much peeking and pokeing, I think I may have some answers. The register of interest is the graphics priority register at 53275 which is shadowed at GPRIOR, 623 decimal. This is used in setting the priority for Player-Missiles and the like. But the important part is that the GTIA chip uses the 2 most significant bits - 6 and 7 - to determine how it is to output the information.

Using some inspired guesswork, this is what I think may be happening. When GTIA gets a byte of Data, it first looks at the Priority register. If the 2 most significant bits are not set, then it outputs the data as normal for the mode in question i.e BASIC modes 0-8. But if either of the 2 bits are set then it splits the byte into 2 4-bit nibbles, which gives it 2 numbers between 1 and 16. It then outputs 2 pixels as shown in figure 1.

AtariLister - requires Java

Figure 1

Priority Registry Bit

7	6
0	1	Output color 4 at a luminescence given by (n)
1	0	Output the color stored in color register (n) which can be any color register or the Player-Missile color registers
1	1	Output the color whose number is (n) and whose luminesence is stored in color register 4

This is essentially what happens in Graphics modes 9-11. The operating system simply creates a Graphics 8 display list and then sets the bit in PRIOR. But what nobody seem to have thought of before is that this can be done for any mode, not just GR.8

The only mode I've tried it in so far is GR.0, but believe me - it's quite amazing! What happens is that instead of each character producing a letter, it produces two pixels in colour. Potentially this means that all the programs you had to produce in GR. 2 to get colour can now be expanded to a full 40 x 25 display in up to 16 colours giving a resolution equal to GR. 9-11.

I've included a quick demo in listing 1, which puts up a simple display and steps through the different priority settings. Somebody out there can do better than this. All I have done is some ground work, so get out your Atari and start writing some programs! Try some of the other Graphics modes too, I have only tried GR. 0.

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