Mapping the Atari
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| Mapping the Atari | |
| Author | Ian Chadwick |
|---|---|
| Country | United States |
| Language | English |
| Subject(s) | Atari 8-bit family |
| Publisher | COMPUTE! Publications |
| Publication date | 1983, 1985 |
| ISBN | 0-87455-004-1 |
Mapping the Atari, written by Ian Chadwick in 1983, is a location-by-location explanation of the Atari 8-bit family's memory map. It was one of the "must have" references for programming the platform, although it was published somewhat late in the machine's timeline. An updated version covering changes to the operating system and newer machines like the 130XE followed in 1985.
Like most 8-bit machines, the Atari made extensive use of hardware registers that were mapped into the processor's memory, allowing these external devices to be controlled by reading and writing memory using the 6502's instructions. For instance, location 755 (2F3) stored the CHACT register of the ANTIC display processor, which controlled the text display and cursor. Normally set to 2, setting it to the value 6 would display the text upside down instead.
The book contained not only the memory locations, but often included details of how to use them as well. For instance, location 756 (2F4) CHBAS contained the location in memory of the data that told the ANTIC how to draw each character. The book goes into several pages of detail on how to use this feature to built custom character sets. One "problem" with the work was that the memory locations controlling a particular feature, character sets for instance, would be spread out over many pages of the text, each with their own little snippets of code.
These mappings might not be entirely obvious, for instance location 53760 was mapped to the POKEY controller and contained the value of the paddle controller when read, but set the frequency of the sound in channel one when written to. In other cases registers might be bitmapped, with each bit controlling a seemingly unrelated option.
