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
To flash an older / modified version you have to let your phone reside in DFU mode (VROM->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). iTunes also must be pwned to accept the firmware file.