TUTORIAL: What's in a ROM?

[Reptile_16]

There's a lot in a ROM, here's the short-list:
- IPL,
- SPL,
- Radio File,
- Splash Screen,
- Extended ROM, and
- Operating System.

Ok, we can do the top four of these dead easy, they're a total snooze. IPL/SPL, Radio and the Splash screen consist of one file, and they do one job and it is IPL, SPL, Radio and splash screen respectively. There...

Now the interesting parts.

I'll leave the extended ROM to one side, because it represents another "drive" to use desktop parlance, from which to run programs and access files etc., that's all.

So...to the OS.nb.

Critics might say I'm putting the cart before the horse by describing what you find in a ROM before explaining how to find out what's in it - I say if I showed you how to open the thing up, you'll just end up saying "what's all this junk then?".

There is a pair of key file-types that would stick out if you extracted the contents of OS.nb into a folder. These are .rgu (Unicode Registry) and .dsm (I don't know what this abbreviation actually stands for but AFAIK the M is Manifest - it's a list of files, basically)

So what? The interesting thing about the .rgu and .dsm files is that they're paired (...ish. Where there's an .rgu there's always a corresponding .dsm, but .dsm's can live without .rgu's). They come with unfeasible names like "f50e9a40-aed9-497a-bdea-84451210e960", which isn't interesting at all in itself. What is interesting is that all these unfeasible names (they're called GUIDs - Globally Unique ID) are listed in another file 'packages.sof'.

So.....'packages.sof' has a list of GUIDs in, each GUID has it's own .dsm (with a list of files in) and these files describe [largely] every file in the ROM.

One other thing the .dsm files can contain is the name of a folder. Something nice and descriptive like "OS Files", or "OEMAPPS". So....put all that together and it's possible then to group practically every file extracted from OS.nb into a series of folders. Taking their name from the grandfather file, these folders are referred to as "Packages".

Using the above theory, the structure of the OS.nb can be (very broadly) summarised into the following areas:
- XIP,
- OEM, and
- SYS.

I'll leave XIP for now, because (even though it's possible to extract the files and a separate process is used) unless you're actually mental you won't really care about re-basing your XIP for a while until you've grasped how to reconstitute a working ROM.

Right, I'm back. We're down to OEM and SYS then. The OEM section contains typically about eight packages. These contain:
- Help files,
- the OEM dialer bits and some basic apps,
- a bunch of skinning and sound files,
- a series of provisionining XML (.provxml) files, and
- a really interesting file called "initflashfiles.dat". Keep an eye on this one.

The SYS section is where it gets really serious though. Everything is in here from your basic applications to your media players and office, through to all the bluetooth/gps drivers, input drivers and (crucially) registry files.

It's possible to extract a bunch of files from the OS.nb into one folder, change some of them (such as the backgrounds) and then re-package the whole thing back into a new OS.nb. Hurrah! Elementary cooking!

That's not what we're here for though, is it? "NO!" I hear you cry....

Onward to the tools then!

[bogigum]

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