Cirrus Logic CS485 Manual de usuario descargar pdf (Pagina 41)

CS485G Spring 2015 41

(d) When the kernel is ready to return from the exception:

i. The CPU changes to its previous mode (typically user mode)

and its previous location.

ii. The saved information is popped from the stack.

45 Example: opening a ﬁle in Linux x86

1. Lecture 25, 4/1/2015

2. In C, a program invokes open(filename, options).

3. The C library handles this function by executing an interrupt instruc-

tion: int $0x80.

4. The Linux kernel runs a procedure called sys open(). Any error it

encounters causes it to return a speciﬁc negative number indicating

the error, such as EFAULT.

(a) verify that the ﬁlename is in a valid location in memory.

(b) verify that the ﬁle exists.

th manner speciﬁed by the options.

(d) build a kernel data structure files struct.

(e) return the index of that structure within the kernel’s per-process

fd array.

5. The C library sees if the return value R is negative. If so, it stores R

in the global variable errno and returns –1. Otherwise, it returns R.

46 Example: Page fault

1. A program tries to access a memory address that is in its virtual

space but which is not currently available.

2. The MMU (memory management unit) notices the problem and gen-

erates a page fault.

3. The Linux kernel executes a procedure called fault(). It takes pos-

sibly complex action to make sure the memory address is available.

4. The kernel then returns to the same instruction that caused the fault.

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Cirrus Logic CS485 Manual de usuario Pagina 41