Role of Interrupts
Interrupts are
signals sent to the CPU by external devices, normally I/O devices. They tell
the CPU to stop its current activities and execute the appropriate part of the
operating system.
1.Hardware Interupts
are generated by hardware devices to signal that they need some attention from
the OS.
2.Software Interupts
are generated by programs when they want to request a system
call to be performed by the operating system.
3.Traps are generated
by the CPU itself to indicate that some error or condition occured for which
assistance from the operating system is needed.
CPU
Execution Mode
There are two modes
of execution,known as user mode and kernel or supervisor mode.
User mode is
restricted in thatcertain instructions cannot be executed, certain registers
cannot be accessed, and I/O devices can not be accessed.
Kernel mode has
none of these restrictions. A system call will set the CPU to kernel mode, as
will traps and interrupts. Application programs cannot do this.
Supervisor mode
»Can execute all machine instructions
»Can reference all memory locations
User mode
»Can only execute a subset of instructions
»Can only reference a subset of
memory locations
CPU Response to Interrupts
The CPU performs the following actions
in response to an interrupt:
Using the pointer to the current
process control block, the state and all register values for the process are
saved for use when the process is later restarted.
The CPU mode bit is switched to supervisory
mode.
Using the pointer to the
interrupt handler table and the interrupt vector, the location of the kernel
code to execute is determined. The interrupt vector is the IRQ for hardware
interrupts
•Reference :
•Stallings, William, Operating
Systems, Second Edition, Prentice-Hall
International Editions, Englewood Cliffs, New Jersey,
1995.
•Tanenbaum, Andrew S., Modern
Operating Systems, Prentice-Hall
International Editions,
Englewood Cliffs, New Jersey,
1992.
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