Boot Section Lock (BOOTLOCK) and Application Code Section Write Protection (APCWP) are located in the NVMCTRL.CTRLB register and are used for run-time write protection.
BOOTLOCK prevents read access and code execution from BOOT. This bit can
only be set by code executed from BOOT, and will activate when code execution moves out
of BOOT. When BOOT is locked, any attempt to read from BOOT will return
0x00, and any instruction executed from BOOT will be a No Operation
(NOP) instruction.
APCWP controls write access to APPCODE. When set, any attempt to write to this section will result in a write error.