"The Datasheets can be found on the AMD site"
Each manufacturer will have their own datasheets on their own site ;-)

"The flash that runs code cannot be erased nor written!"

Some Flash devices feature "Sector Erase," whereby you can erase just part of the device.
Thus, so long as you don't erase or write the "Sector" in which your code is actually running at the time, you can erase/write other parts of the device.