At the beginning of the compilation phase, the input buffer (an area of memory) is created to hold a record from the external file. The input buffer is created only when raw data is read, not when a SAS data set is read.
Program Data Vector
After the input buffer is created, the program data vector is created. The program data vector is the area of memory where SAS holds one observation at a time.
The program data vector contains two automatic variables that can be used for processing but which are not written to the data set as part of an observation.
_N_ counts the number of times that the DATA ...