Let me scratch my head and think back to those computer science classes I sat through:

I think the problem with global variables is primarily that they have to be reserved and are never released. That hogs RAM. Local variables only need to be reserved while that routing is active (or while it is on the stack). In some degree the selection of global versus local variable passes on information to the compiler regarding options it can use for efficient RAM use. This is not to say that all compilers do this level of sophistication.

There are other issues that can come into play but from compiler design, that's the biggy.

By the way, I got half way through a masters in computer science before I ran out of money and had to rejoin the workforce. I stayed in graduate university earning degrees in the early '90s while I lived off my stock profits buying in at the bottom of the '87 stock market crash. It lasted about 5 years on campus. :)

aka j