Chapter2: COMPUTER-SYSTEM STRUCTURES

1. CPU Protection

1.1. Timer

1.1.1. interrupts computer after specified period to ensure operating system maintains control over CPU.

1.1.2. When timer interrupts, control transfers to the operating system.

1.1.3. Load-timer is a privileged instruction.

1.1.4. Example: -Every time the clock ticks, the counter is decremented. -When timer reaches the value 0, an interrupt occurs.

2. Synchronous I/O Structure

2.1. After I/O starts, control returns to user program only upon I/O completion.

2.1.1. Wait instruction idles the CPU until the next interrupt

2.1.2. Wait loop (contention for memory access).

2.1.3. At most one I/O request is outstanding at a time, no simultaneous I/O processing.