Confused About How Roms Work

[spartan]

I installed OZ/3.5.4 with OPIE, then GPE, over my hard-worked Sharp 1.11JP ROM. 3.5.4 was too buggy, so I updated it back with the Cacko C3000 ROM. Somehow, everything is exactly as I left it, complete with scripts and installations and everything, minus icons and the japanese text support that was explicitly removed from the Cacko C3000 ROM.

If all my stuff is exactly where I left it last, how exactly do ROMs work if they don't overwrite my previous operating system?

[ThC]

Roms are made in three parts :
initrd.bin which is a jffs2 compressed file (max 5120Kb) wil be mounted read only on your / . this is the base OS
hdimage1.tgz which also contains hdimage2.tgz in it which mainly contains /usr/ dirs (so applications and everything)
.home_default.tar which is contained in initrd.bin (in /root/.home_default.tar) and will be used to repopulate /home if needed, /home being a 4Mb partition mounted rw where you can find your home dirs and your /etc dir

So you can see all your stuff staying around as your programs are probably still installed on hdd2 and /home isn't formated when you install a new OS ...

And for the "What happened to my 16Mb flash ?" question, there's also a 7Mb partition containing the base OS and flashing tools which you can see when booting with the D+B method