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I have a question about how an actual system call is made. I know that the magic of system call (like read etc.) is done in C library but don’t understand the exact mechanism. My main issues are

  1. The c library routine is in user address space; then how can it get the address of the interrupt service routines. Are interrupt service routines predefined(on boot up) in physical memory?.

  2. Even if somehow the ISR routine is called how does the address space change? I mean before we start the execution of ISR how will the 'page table base register' change to point to kernel's page table. If the 'C' routine does it then how does it know the address of Kernel's page table?

  3. How are parameters copied from user space to kernel space?

Please excuse me if my questions are too basic but I am new to this. :)

Thanks Rohit

Rohit
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2 Answers2

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On most systems, there's an instruction that can be executed by user code to invoke a user-defined interrupt (for example, int on x86 and swi on ARM will request a "software interrupt").

The CPU, executing in user mode, will switch to kernel mode upon seeing one of these instructions, and will jump to a predefined ISR location for that particular interrupt. The interrupt number is typically fixed, and the corresponding ISR is the system call handler for the kernel.

The kernel can inspect the user-mode registers and stack which were present at the time the interrupt was called (in a manner similar to saving all registers on the stack during a context switch), and obtain the system call arguments from there.

nneonneo
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Ok i think i found the answer (at least i think so) at questions about kernel space

1.The c library routine is in user address space; then how can it get the address of the interrupt service routines. Are interrupt service routines predefined(on boot up) in physical memory?.

The ISR location is predefined as answered by nneonneo above.

2.Even if somehow the ISR routine is called how does the address space change? I mean before we start the execution of ISR how will the 'page table base register' change to point to kernel's page table. If the 'C' routine does it then how does it know the address of Kernel's page table?

There is no change in address space as the kernel space is essentially same as users (just the difference in protection level)

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Rohit
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