Be aware the G53 is NOT modal.
The use of machine coordinates via G53 only pertains to the block with the G53 in it.
To illustrate the point:
N1 G54 (this will set the WC system to G54)
N2 G40 (most Gcode references tell you to cancel cutter comp before doing G53 moves - see Smid's book for example)
N3 G53 X10 Y10 (this will move to 10,10 in machine coordinates)
N4 X10 Y10 (this will most likely result in additional movement as it will move to 10,10 in G54 WC!)
Dave
G53G0X600Y600 will move to 600 X and Y in machine coords.
Hood