I would like to get a clear picture of what is in the ROM vs. what is in the OS. This will help me understand what those custom ROMs are all about.
2 Answers
Well, the OS is in the ROM, to start with.
You're suffering from a terminology problem: "ROM"s these days are all typically Flash memory, which is erasable and rewritable, but not byte-by-byte. So the OS may even live in the same physical ICs as the 'main' phone memory.
Basically, custom ROMs can enhance/modify/break EVERY aspect of your phone's operation. There is a separate 'radio rom' which includes the low-level drivers for the wifi/gps/3g/gsm/bluetooth functions, and these typically aren't 'customised' but you may still find newer versions available that can improve certain aspects of your phone.
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Thanks. Is there something that is analogous to "BIOS" on a PC?– HenryCommented Sep 21, 2010 at 0:54
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@Henry: Not really, as far as I know. But, Google for "Android Recovery ROMs" which are a little bit like a bios... On DOS PCs, the BIOS used to be a core part of the system run-time - but with Windows it's almost an anachronism. In most cases it just configures some low-level hardware and provides the first stages of the boot loader.– RoddyCommented Sep 21, 2010 at 9:46
ROMs are still mostly compiled official Android source code (or apps, etc. ripped from dumps of newer phones backported to older phones) with tweaks and some minor custom utilities or features that are not included in the stock ROM for a specific phone.
The biggest point of ROMs is getting newer versions of the Android operating system (or as much as the phone's hardware can support - see the G1 as limited device example when it comes to Android 2.1+) on older phones. This is perhaps the main point of the CyanogenMod.
ROMs also may be very similar to stock ROMs but are of different "themes" that change the colors of the notification bar, etc.
answered Sep 20, 2010 at 19:46
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1Thanks. So no clear delineation between firmware, OS, drivers, apps, etc?– HenryCommented Sep 21, 2010 at 0:56