When the processor constructs an array, a logical file, or a code segment for a process in memory, this action is referred to as a presence-bit operation. An initial presence-bit operation is one that creates an array or a code segment because the related procedure has just been invoked. A noninitial presence-bit operation is one that copies an array or a code segment back into main memory from disk.

The number of initial presence-bit operations performed by a process, and the processor time they take, are relatively stable from one run of a program to the next (provided that each run results in the same sequence of procedure entrances). However, the number of noninitial presence-bit operations performed by a process depends to a large extent on how much memory is being used by all the other processes in the mix. When memory is crowded, more noninitial presence-bit operations are performed.

You can monitor the number of presence-bit operations for a process, and the processor time spent on them, by using the TI (Times) system command, by interrogating task attributes, or by reading system log entries. Refer to the discussion of processor usage accounting in Controlling Processor Usage.