OCR H446 1.2.1: Systems Software
The free BugBot lessons that teach this point, a robot to try it on, and the past-paper questions that test it.
What it covers
A processor runs one instruction after another. So how does it notice a key being pressed, a network packet arriving or a battery running flat, in the middle of whatever program it is running? It could keep checking every device, but that wastes almost all its time. Instead, devices interrupt it. This lesson follows an interrupt through the fetch-decode-execute cycle, and shows why the processor's registers must be saved on a stack first.
- What an interrupt is
- Polling or interrupts?
- Interrupts and the fetch-decode-execute cycle
- Why save the registers?
- Priorities and nesting
- A timer interrupt for a robot
Or do it in the simulator, with tasks that mark themselves
Lessons that teach it
- A9.7 Interrupts Computer architecture
Sources of interrupts, polling, the check at the end of each cycle, saving the volatile environment on a stack, and priorities. - A10.2 The operating system, BIOS and device drivers Operating systems, software and translators
Hiding the hardware, managing resources, booting from the BIOS, and a driver table for the robot. - A10.3 Memory management: paging, segmentation and virtual memory Operating systems, software and translators
Logical and physical addresses, page tables, segments, page faults and disk thrashing. - A10.4 Interrupts Operating systems, software and translators
Polling versus interrupts, priorities, the stack and interrupt service routines in the fetch-decode-execute cycle. - A10.5 Scheduling algorithms Operating systems, software and translators
First come first served, round robin, shortest job first, shortest remaining time and multi-level feedback queues, compared on one set of… - A10.6 Types of operating system and virtual machines Operating systems, software and translators
Multi-tasking, multi-user, real-time, embedded and distributed systems, and a bytecode virtual machine that drives the robot. - A10.9 Project: a tiny robot operating system Operating systems, software and translators
Share the robot between three tasks with round robin, drive it through device drivers, and stop the patrol with an interrupt.
Try it on a robot
The task from A9.7 Interrupts, running here. Change the program and press Run.
The specification is OCR's own document; this page says which of our lessons teach point 1.2.1 and is not endorsed by the board.