When you submit a DATA step, SAS processes the DATA step and
then creates a new SAS data set

Compilation Phase
Execution Phase

creation of input buffer and PDV

1)Compilation Phase

When you submit a DATA step,it reads the input statements 
it creates an input buffer and brings the variables and 
observations.it is a logical memory area.
and pdv brings the observations at a time from input buffer 
and checks the errors.
PDV contains 2 automatic variables _n_ & _error_ ,these 
checks the erros in observations.
_n_ : indicates the no of obs.
_error_: 1 if error occured
         0 if no error
After that it assigns the datavalues to appropriate 
variable and build a sas dataset.

2)Execution Phase

SAS CHECKS FOR SYNTAX ERRORS

Compilation phase happens first in data step processing.
during compilation phase, each statement is scanned for  
syntax errors.