Boot process: Difference between revisions
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In general, the boot process is something like this:<br> | In general, the boot process is something like this:<br> | ||
# Dreamcast runs BootROM code and either lands at the BIOS page or tries reading a disc | # Dreamcast runs BootROM code and either lands at the BIOS page or tries reading a disc | ||
# IP.BIN is loaded to 0x8c008000 | # [[IP.BIN]] is loaded to 0x8c008000 | ||
# 1ST_READ.BIN (or whatever file is named directly in IP.BIN) is loaded to 0x8c010000 | # 1ST_READ.BIN (or whatever file is named directly in IP.BIN) is loaded to 0x8c010000 | ||
# SH4 jumps to IP.BIN's entry point and starts executing code | # SH4 jumps to IP.BIN's entry point and starts executing code | ||
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As a form of copy protection, the 1ST_READ.BIN file is generally scrambled according to an obfuscated formula, and the Dreamcast unscrambles it when loading 1ST_READ.BIN into memory. Loading an unscrambled binary ends up having the effect of scrambling the binary and destroying the program.<br> | As a form of copy protection, the 1ST_READ.BIN file is generally scrambled according to an obfuscated formula, and the Dreamcast unscrambles it when loading 1ST_READ.BIN into memory. Loading an unscrambled binary ends up having the effect of scrambling the binary and destroying the program.<br> | ||
<br> | <br> | ||
Using a file loader such as DCLOAD-IP or DCLOAD-SERIAL bypasses this entirely and allows enterprising developers to load programs directly onto the the console.<br> | Using a file loader such as [[DCLOAD-IP]] or [[DCLOAD-SERIAL]] bypasses this entirely and allows enterprising developers to load programs directly onto the the console.<br> | ||
<br> | <br> | ||
Other details: | Other details: | ||
Revision as of 00:05, 10 January 2020
In general, the boot process is something like this:
- Dreamcast runs BootROM code and either lands at the BIOS page or tries reading a disc
- IP.BIN is loaded to 0x8c008000
- 1ST_READ.BIN (or whatever file is named directly in IP.BIN) is loaded to 0x8c010000
- SH4 jumps to IP.BIN's entry point and starts executing code
- IP.BIN jumps to 1ST_READ.BIN and excutes the loaded program
As a form of copy protection, the 1ST_READ.BIN file is generally scrambled according to an obfuscated formula, and the Dreamcast unscrambles it when loading 1ST_READ.BIN into memory. Loading an unscrambled binary ends up having the effect of scrambling the binary and destroying the program.
Using a file loader such as DCLOAD-IP or DCLOAD-SERIAL bypasses this entirely and allows enterprising developers to load programs directly onto the the console.
Other details:
- 0x8c000000 is the base of the system's 16MB of RAM, accessed in privileged mode due to the SH4's 29-bit addressing method
- Dreamcast system calls and bios font are stored in the region 0x8c000000 - 0x8c004000
- The region 0x8c004000 - 0x8c008000 is used by multi-disc games to store persistent information across disc changes. This region is also used by persistent programs such as DCLOAD-IP/DCLOAD-SERIAL so that terminating programs can return to those loaders.
- 0x8c008000 - 0x8c010000, which is used by IP.BIN, doesn't really matter beyond initial loading unless a program absolutely needs to refer to IP.BIN. The DCLOAD-IP/DCLOAD-SERIAL loaders overwrite most of IP.BIN after they've been loaded, for example, as they don't need any of it.