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Subject: Re: Answers to Paul Suades Questions
From: Björn Stenberg (bjorn_at_haxx.se)
Date: 2001-12-09


Andrew Jamieson wrote:

> What does the mod file do? How is it used to patch the code?

I don't think it patches the code at all, I'm pretty sure the mod file is the
complete code. If you look at my notes page, you'll see the mod file contains
a complete exception vector table and fully runnable code.

If the mod file was in fact a patch file, it wouldn't look like that. There
would be a number of patch segments, and they would be difficult to make sense
of.

Another thing that speaks against the patch theory is that everybody gets
different ROM versions when they buy their Archoses. Mine is v3.18 and I've
seen in posts on the yahoo forum that people now get v5.05 in their ROMs. Yet
for all these versions, the same MOD file works just fine. That would be
almost impossible with a patch solution.

> So, the mod must be stored in the DRAM - does this mean that all the code
> is copied to the DRAM at boot, and then the mod is overlaid to patch
> required bits?

How about this theory:

- The unit boots from ROM, with just the stack in (on-chip?) RAM
- After boot, the ROM code looks for a MOD file on the harddisk. If it exists,
it's loaded into DRAM and executed.

/Björn



Page was last modified "Jan 10 2012" The Rockbox Crew
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