The A720IS (and the A650IS--I believe) are Canon "Dry OS" cameras. These cameras do not have the "firmware update" function that the other cameras have (where you save the PS.FIR file, turn the camera on in "play" mode, then scroll down the menu to "update firmware"--names are approximate--I have an A720IS).
Of course--CHDK does not update the camera's firmware--in the other Canon cameras, it is just a convient way to get the "CHDK application" to load and run (in Windriver OS cameras).
So, for the A720 and A650 you will only find a DISKBOOT.BIN file, there is no PS.FIR file (the very first release of CHDK did have a PS.FIR file--but it would not do anything with the A720).
So, to get the A720IS code to work:
Format your SD/SDHC card with the Camera or with a program that can format FAT16 format. By the way, if your card is 4GB or larger, you probably need a SDHC compatable external USB adapter... And you will need to type (this is Windows):
Format x: /fs:fat (where x: is replaced with your drive letter)
For 4GB SDHC cards, this will give a non-standard FAT16 table and may not read with some programs... But it does work fine with my A720 and my Sandisk SDHC adapter. And for any cards >4GB, the card will be formated to only 4GB and the rest of the card is not accessible (due to the FAT16 limitations). There may be other ways of using 8GB+ cards--but I am not sure that works right now--or at least makes things much more confusing for a newbie (like myself).
After you have formated the card, run the "bootable.exe" program (in Windows--or follow the HEX Editing instructions for other OS's). The bootable program just sets a bit in the SD/SDHC card that tells the camera that it is OK to boot from this card.
Now, copy your DISKBOOT.BIN file to the "root" or top directory on your x: card. Pay attention that you have the correct DISKBOOT.BIN for your camera and Camera's version (and that you have AllBest or which ever CHDK version you want to try).
Lastly, pull the card from your computer and slide the switch to "write protect" and place in your A720IS (the above instructions work for all CHDK compatible cameras).
Place the card in your camera and turn it on (play or shoot mode does not matter). And you should see the blue CHDK splash. And you are now ready to go.
Hope it helps!
-Bill
