Flash memory chips work like that.
The hacked BIOS is 256K, or 1 meg, or some other odd ways of splitting it, if you're into that sort of thing.
To get the hacked BIOS onto your TSOP, you have to remove everything on the chip, and then write the complete image of the hacked BIOS to the chip. It came flashed from the factory. It's got that devil retail BIOS on it which won't let us play our backups/homebrew software. So, we reflash it to get around those constraints.