Working with the Unix OS

Introduction to kernel
...
C runtime library
open(char *name, int mode)
{
place parameters in registers.
execute trap instruction, switching to kernel code
return result of system call
}
Kernel
system call vector table
...
address of kernel close()
address of kernel open()
address of kernel write()
...
-----------------------------------
kernel system call code
kernel code for open()
{
manipulate kernel data structures
...
return to user code and user mode
}
The scheduler will not assign the CPU to another process during the execution of a system call. i.e. when a process
performs a system call, it cannot be "preempted".
System calls that make I/O requests from a device, may take time to complete. To avoid leaving the CPU idle, the
kernel puts the process to sleep and wakes it with a hardware interrupt signalling I/O completion.
Interrupts
highest
priority
0
1
2
3
4
Interrupt vector table
hardware errors
clock
disk I/O
keyboard
traps S/W interrupts
pointers to kernel
interrupt handlers
lowest
priority
current process suspend resume
----------------------------> - - - - - - - - - - - - - - - ------------------------------->
| |
| keyboard interrupt handler |
|----------------------------------------------->|
keyboard interrupt
completed
When an interrupt occurs, the current process is suspended and the kernel determines the source of the interrupt. It
then examines the interrupt vector table to find the location of the code to process the interrupt.
If a higher priority interrupt than the current arrives, the lower priority interrupt handler is suspended until the
higher priority interrupt completes.
Critical sections of kernel code protect themselves from interrupts by temporarily disabling interrupts.
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