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System Calls

05/27/2009

Today I asked to my self the obvious question of how system calls are actually made.

As usual, I brain-stormed my self for a few minutes before going to the books. I imagined each process code containing a copy of some kernel interface: set of functions which relative addresses were well known by the Linker so they could be called from user code. These little functions responded by switching the CPU into Kernel mode before serving the request.

Technical littérateur told me later a different tale. In Linux, user processes make system calls by raising interrupt 0x80 which is handled by the Kernel. Application programmers, however, use wrapper C functions so they don't deal with the interrupts directly.

That tale is, of course, the short answer to my question. I expended longer reading about this and other related topics. And it was while reading about "real things" than my crazy idea of a "hardware-isolated kernel" arose once gain, making better sense to me now than before. However I don't want to workout this bizarre idea for now.

System calls took me to interrupts which in turn turned me to User/Kernel modes switching. Thinking about this I came with the surprising idea that, in my machine (an thanks to the Multi-Dimensional Matrix idea) I can treat Kernel code with less reverence than usual... as pointed out in the following note: