Darren New wrote:

> At the lowest lowest level, I'm pretty sure the libraries load up 
> arguments into registers and execute an INT instruction, which is a 
> "software interrupt", aka a trap, which causes a branch to an address 
> loaded into a specific area of memory. I.e., just like handling an 
> interrupt, or just like branching to a jump table in a library, except 
> that you get privilege bits flipped around.
> 
> I don't know of any modern multiuser OS that doesn't use that technique 
> to get into the kernel, as it's pretty much the only way to bypass 
> memory and execution permissions in a controlled way.

OK, that's what I wanted to know...

(PS. Is there more than one interrupt level available? Or does it decide 
which routine to call based on some other parameter?)

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