General System Variables
!CPU: Controls the state of the IDL thread pool.
!DEBUG_PROCESS_EVENTS: Allow processing of widget events while stopped.
!DIR: Path to the main IDL directory.
!DLM_PATH: List of search paths for DLM files.
!EDIT_INPUT: Controls command-line editing.
!HELP_PATH: List of search paths for HELP files.
!JOURNAL: Contains the LUN for journal output.
!MAKE_DLL: Configure options for MAKE_DLL and CALL_EXTERNAL.
!MORE: Controls pagination for long help output.
!PATH: List of search paths for IDL routines.
!PROMPT: The text string for the prompt.
!THEME:Tells what display theme IDL widgets are using.
!QUIET: Print or suppress informational messages.
!VERSION: Information about the running IDL process.
Constant System Variables
!COLOR: Contains standard RGB color triplets.
!CONST: A list of physical constants.
!DPI: Double-precision value of pi.
!DTOR: Convert degrees to radians.
!FALSE: Boolean value of false (0).
!MAP: Direct Graphics map information.
!NULL: A variable of type Undefined.
!PI: Single-precision value of pi.
!RADEG: Convert radians to degrees.
!TRUE: Boolean value of true (1).
!VALUES: Contains IEEE values for Infinity and NaN.
Error Handling System Variables
!ERROR_STATE: The state of the last error message.
!EXCEPT: Controls how math exceptions are handled.
!MOUSE: The mouse status from the last cursor read operation.
!WARN: Controls warnings about obsolete routines and use of parentheses.
Direct Graphics System Variables
!D: State of the Direct Graphics device.
!ORDER: Controls whether Direct Graphics images are ordered top-to-bottom.
!P: State of the Direct Graphics plotting system.
!X: State of the Direct Graphics x-axis
!Y: State of the Direct Graphics y-axis
!Z: State of the Direct Graphics z-axis