The Execution Phase
The diagram below outlines the flow of execution for a typical data step that includes some statement that reads data.

Note: You write code within the data step to process a single, general observation. SAS automatically loops thru the code, executing the data step code repeatedly. The data step code is typically executed once per observation.

For more information see SL:R, Chapter 2 The DATA Step, DATA Step Processing and see SL:R Figure 2.1.