Usually PMP does not have special instructions to manipulate registers; to access to the OPTION register the OPTION_REG register should be directly assigned.
Unfortunately, some old and / or small devices do not have OPTION_REG register, but a special instruction OPTION that is a write-only instruction.
The OPTION built-in procedure will automatically map into an OPTION_REG register assignment if such an addressable register exists or into an OPTION instruction otherwise.
As said before, OPTION is write-only: you cannot read back the value, so it should be saved in a global variable if you want to manipulate / mask individual bits.
<expression> is a numeric expression that gives a BYTE range.