Which Raspberry Pi GPIO lines are free?

As per the subject, really, I need to know which of a Raspberry Pi B Rev 2’s GPIO lines are free for my own use and are not used by RISC OS.

I’ve started to turn a Raspberry Pi into my next central heating controller, only to discover that it has far less GPIO lines than I thought. I may well have to expand the I/O. I need something like 14 lines.

On the Rev 2 board there are 21 GPIO’s available, non of which are “used” by RISC OS.
If you have any third party add-on boards connected, they may be using some of them, especially the I2C lines.
Just because the board boots with GPIO lines set as other functions doesn’t mean they are in use.

There’s 17 off the main 26pin header, including the I2C lines for the clock, so 15 assuming you need those. My LED Cube used 14 GPIO lines happily.

Yes, thanks, both. I count 19 maximum, since the IIC lines are undoubtedly used. But that’s more than enough for my needs.

The bit that wasn’t clear to me is whether RISC OS makes any use of any of them, e.g. to flag activity. Other than IIC, which I know about, of course.

The bit that wasn’t clear to me is whether RISC OS makes any use of any of them, e.g. to flag activity. Other than IIC, which I know about, of course.

No, they’re all free for you to waggle up and down to your hearts content :-)