Case 1 and Case 2 provide unsecuring
considerations for zones with and without code security.
- Case 1: Zone With Code
Security
A zone with
code security must have a predetermined password stored in the password
locations of that zone. The following are steps to unsecure any secure zone:
- Perform a dummy read
of the password locations of that zone.
- Write the password
into the CSMKEY registers.
- If the password is
correct, the zone becomes unsecure; otherwise, the zone stays
secure.
- Case 2: Zone Without Code
Security
A zone without
code security must have 0x FFFF FFFF FFFF FFFF FFFF FFFF FFFF FFFF (128 bits
of all ones) stored in the password locations. The following are steps to
use this zone:
- At reset, the CSM
locks memory regions protected by the CSM.
- Perform a dummy read
of the password locations.
- Since the password is
all ones, this unlocks the zone and all the secure memories
dedicated to that zone are fully accessible immediately after this
operation is completed.
Note: Even if a zone
is not protected with a password (all password locations all ones), the CSM
locks at reset. Thus, a dummy read operation must still be performed on
these zones prior to reading, writing, or programming secure memory if the
code performing the access is executing from outside of the CSM protected
memory region. The Boot ROM code does this dummy read for
convenience.