Computer System Structures

1. Interrupt service routine--part of the operating system which takes action in response to the condition that cause the interrupt.

2. Modes of operation

2.1. User Mode -- User program executes in user mode. Certain areas of memory are protected from user access. Certain instructions may not be executed

2.2. Kernel mode -- Monitor executes in kernel mode. Privileged instructions may be executed. Protected areas of memory may be accessed

3. Hardware protection

3.1. I/O protection

3.1.1. All I/O instructions are privileged instructions; user program cannot issue I/O instructions directly.

3.2. Memory protection

3.2.1. Limit register – contains the size of the range

3.2.2. Base register – holds the smallest legal physical memory address.

3.3. CPU protection

3.3.1. Load-timer is a privileged instruction

4. A general-purpose computer system consists of one or more CPUs and a number of device controllers.

5. Interrupt Handling--The fixed location contains the starting address of the interrupt service routine.

6. Dual-mode

6.1. Mode bit is added to computer hardware to indicate the current mode: kernel (0) or user (1).

6.2. Privileged instructions can be issued only in monitor mode.