hi brandon Thanks very much for your reply. The problem I have is when i start my IDL application (I mean i use the .sav), and check the memory in the window task,  it takes about 1.2 G (idlrt.exe *32 showing memory at 1,107,804 k). But for the same application with the workbench, the function help, /memory find the current memory to 472 Mb. The last value seems (472 Mb) seems to me logical because my application load 4 big images (3x7216x5412) that suppose to take 3x7216x5412x4 = 468635904 k  =468 mb of memory. But why after compile the code in a .sav, the task manager indicate the memory use is rather 1.2 G that is really more 472 MB. Maybe i am confused about what happen. If you have an idea about this Thank very much for your answer aurelien