It is a useful, memory-saving technique for multiprogrammed

timesharing systems. A Reentrant Procedure is one in which

multiple users can share a single copy of a program during

the same period. Reentrancy has 2 key aspects: The program

code cannot modify itself, and the local data for each user

process must be stored separately. Thus, the permanent part

is the code, and the temporary part is the pointer back to

the calling program and local variables used by that

program. Each execution instance is called activation. It

executes the code in the permanent part, but has its own

copy of local variables/parameters. The temporary part

associated with each activation is the activation record.

Generally, the activation record is kept on the stack.