StarRiderMainCpu
From DaphneWiki
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|0x4000-0x7FFF | |0x4000-0x7FFF | ||
| - | |ROM4 (U37, | + | |ROM4 (U37, not installed!) |
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|0x0000-0x3FFF | |0x0000-0x3FFF | ||
|ROM2 (U15) | |ROM2 (U15) | ||
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Revision as of 19:10, 31 January 2014
Contents |
Star Rider Main CPU Board
ROM program memory map
| Address Range | Description |
| 0xE000-0xFFFF | Main ROM program (U52) |
| 0xD800-0xDFFF | Board was designed to have a RAM or ROM chip here, but nothing actually was installed in production. |
| 0xD000-0xD7FF | RAM |
| 0xCC00-0xCFFF | NVRAM, 4-bit (upper 4 bits of data ignored) |
| 0xCBF0-0xCBFF | ? |
| 0xCBE0-0xCBEF | Color palette |
| 0xCBD0-0xCBDF | Video expander + background control |
| 0xCBC0-0xCBCF | ? |
| 0xCBB0-0xCBBF | DMA |
| 0xCBA0-0xCBAF | Vertical Control |
| 0xCB90-0xCB9F | Field |
| 0xCB80-0xCB8F | VGG U7 PIA control (80 is periph/dir for A, 81 is control for A, 82 is periph/dir for B, 83 is control for B, this pattern repeats). Port A goes to the video expander, port B goes to the PIF board (laserdisc player). |
| 0xCB00-0xCB7F | ? |
| 0xCA00-0xCAFF | ? |
| 0xC980-0xC9FF | CPU U10 PIA inteface is 0xC980-0xC983, CPU U20 PIA interface is 0xC984-0xC987, this repeats at 0xC990. |
| 0xC900-0xC97F | Watchdog |
| 0xC880-0xC8FF | LED on the main CPU board (writing anywhere to this address changes value displayed by LED). |
| 0xC800-0xC87F | Change active page (see page section). |
| 0xC000-0xC7FF | ? |
| 0xA000-0xBFFF | RAM |
| 0x0-0x9FFF | See Page section |
Page System
Star Rider employs a wacky paging system where it can modify what the address from 0-0x9FFF points to in real-time. This is most likely to workaround the 16-bit address limit of the 6809E CPU. Executable code can and is paged in and out, making reverse engineering more difficult.
The page value is 4 bits so has a theoretical range of 0-0xF however in practice it appears that there are only a few actual pages.
Page 0
The default page where most of the program ROM is accessible.
| Address Range | Description |
| 0x8000-0x9FFF | ROM5 (U45) |
| 0x4000-0x7FFF | ROM3 (U26) |
| 0x0000-0x3FFF | ROM1 (U8) |
Page 4
Loads an alternate ROM program.
| Address Range | Description |
| 0x8000-0x9FFF | Not connected |
| 0x4000-0x7FFF | ROM4 (U37, not installed!) |
| 0x0000-0x3FFF | ROM2 (U15) |
