How is the kernel entered or invoked

Kernel is invoked as a result of :

1. External interrupt - such as a device (printer, disk drive, kettle) or a timer (clock).
2. Internal interrupt - error condition (such as divide by zero, memory protection violation, page fault, execution of an illegal or previledged instruction etc).
3. Kernel call from a running process.

  
Figure:

Kernel entry switches processor into previledged mode

Access to :

• Kernel general purpose registers
• Program status word
• Memory management registers
• I/O instructions (on some processors)
• Enable and disable of interrupts
• Special instructions (such as halt)

Note: some system processes may need to execute in previledged mode (such as device handlers).

How is kernel entered from a user processes

1. Procedure Calls
2. Special Instructions (such as Supervisor Call (SVC) or trap)
3. Message passing (as in Mach)

For procedure call, Kernel resides in shared address space (or ROM) and

a) Kernel procedures are linked into process code
b) Single entry point into kernel, and parameters indicate kernel operation

The special instructions are similar to hardware interrupts, and register parameters determines operation.

In a message passing case, each process has default ports for sending and receiving messages to or from the Kernel.

Where are the kernel parameters stored ?

On the Kernel stack, which is used for variable and parameter passing within the Kernel.

1. Kernel has a stack like any other process - kernel registers
2. Two stacks are per process - a user mode and a kernel mode stack, however, this required extra memory (Unix)
3. Kernel runs on a user stack (as done in monitors), but in this case it is possible that the data may become corrupted