SETACTIVEAREA area

Sets the active area for printing and saving pictures. The area input must be a list of the form: [XLow YLow XHigh YHigh]. All members must be integers.

You can achieve the same effect by using Bitmap - Active Area from the menu.

BITBLOCK 100 100
SETACTIVEAREA [0 0 50 50]
BITSAVE "myfile.bmp
CLEARSCREEN
BITLOAD "myfile.bmp

See how only the part of the rectangle that was within the active area is restored.