At runtime, the following data objects are always present, and do not have to be declared:

The data object SPACE is a constant with type C. It is one byte long, and contains a space character. SPACE is a constant, and as such, cannot be changed.

SY is a structure with the ABAP Dictionary data type SYST. The components of SY are known as system fields. System fields contain values that provide information about the current state of the system. They are automatically filled and updated by the ABAP runtime environment. Examples of system fields:

• SY-SUBRC: Return code for ABAP statements
  (zero if a statement is executed successfully)
• SY-UNAME: logon name of the user
• SY-REPID: Current ABAP program
• SY-TCODE: current transaction
• SY-INDEX: Number of the current loop pass

System fields are variables and can be changed by the program. However, you should only do so where the system documentation recommends it for a specific field (for example, you can change the value of SY-LSIND to navigate between detail lists). In all other cases, you should treat system fields as though they were constants, and only read them. If you change their values, important information for the flow of the program may be lost.

For a complete list of all system fields with notes on their use, refer to