what is tool chain?
 

dont' know what it do.

when I need to work on the firmware right now, I need to install arm-tool-chain, then snapgear, than my user program

keep hearing this term comes up, what does the arm-tool-chain do and include.

I also remembered some conversation says it takes a long time to compile it? and why? :scratch:

http://www.gnuarm.com/

It takes a long time to compile because it includes the GNU Compiler Collection, which is not small.

I'd be tempted to download a binary; see above link.