S5L8900

This is the Application Processor shared between the iPhone, iPod touch, and the iPhone 3G. Not much is known about it through official sources.

Exploits

 * Restore Mode
 * LibTIFF
 * Symlinks
 * Ramdisk Exploit
 * Pwnage 1.0 (Ramdisk + AppleImage2NORAccess)
 * Diags Exploit
 * Pwnage 2.0 (DFU + Malformed Signature)

Boot Chain
VROM->LLB->iBoot->Kernel->System Software

One of the iPhoneLinux goals are to replace that Boot Chain after iBoot : VROM->OpeniBoot->Linux Kernel->X Server->Window Manager

Restore mode
The common upgrade process chain is VROM->DFU->WTF->iBoot->Kernel->Ramdisk->Restore, also called restore mode.

DFU mode
See full article here. To flash an older version of the iPhone software you have to let your phone reside in DFU. In iTunes you have to press the Alt-Key (Mac) or the shift-key (Windows) when pressing 'Restore' to be able to manually chose an update file (ipsw file).

Boot Chain
VROM-->DFU