Attached is unfinished CHDK for sx520,firmware 100c. Based on the source code for the sx530.
The camera will probably have the same problems with some SD cards, as well as the sx530.
If you encounter a problem with the SD card, try using 'Firmware update method' or run CHDK pressing the POWER button longer than one second,using 'Bootable SD card method'.
It should be noted that I don't have this camera, so nothing was not tested.
Any use is at your own risk.
Use only with cameras that have firmware 100c.
EDIT:
Test build removed.