... Atari programming requires racing the beam, updating graphics registers just before the instant they’ll be used to paint the next pixels on the current scan line. With only 76 CPU cycles per scan line, there just isn’t enough time for the poor 6502 to do very much. Want to update the foreground color multiple times at different horizontal positions? OK, but there might not be enough time remaining to also update the pixel bitmap during the scan line, or set the sprite positions. It’s a series of difficult tradeoffs and code optimization puzzles.
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Activision’s Pitfall was one of the most popular games for the Atari 2600. For the sequel Pitfall II, expectations were high, and the game’s designer David Crane did something that had never been done before: put a full-blown coprocessor inside the game cartridge. Crane’s DPC (Display Processor Chip) added two additional hardware sound channels, a music sequencing capability, a hardware random number generator, and a graphics streaming capability with built-in options for masking, reversing, or swizzling the bits.
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The DPC provides some extensions to speed up this process. From my study of the chip, it works like this:

Writing to a special pair of addresses in the cartridge’s address space will set up a graphics stream pointer
Reading from a special address will return the next byte from the stream pointer, and increment the pointer
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This reduces the total number of CPU cycles for every repetition from 10 down to 7, enabling the pixel bitmap to be updated every 21 pixels instead of only every 30 pixels – quite a nice improvement.
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