The VROM, or Virtual Read Only Memory, is said to be the earliest significant code that runs on the S5L8900. It is mapped to 0x20000000, and is believed to be copied from ROM and put there. It either boots the device or runs DFU. On 1.x, it was jumped to for RSA checking, although now in 2.x onward, iBoot and friends do that on their own.
The pwnage exploit resides here.