How to get a PARM from the EXEC statement

I’m studying to use Natural in batch mode. How can I, in the program, get the contents of the PARM? For example, if I have a PARM=‘UPD=YES’, how can I access the ‘UPD=YES’ text? And how do I know the informed parm length? Thanks !

Check out USR4004N in library SYSEXT.