Also if I remember correctly most of the A500 motherboard revisions had a jumper that would allow you to manually select either a PAL or an NTSC mode as the default one.
Even after you move the jumper, you can still switch video modes via software, as the jumper only affects the default video mode at bootup.
At this moment I don't remember which of the board/chip revisions did not have this functionality, but I can look it up if you like.
