A10.6 Types of operating system and virtual worksheet

Multi-tasking, multi-user, real-time, embedded and distributed systems, and a bytecode virtual machine that drives the robot.

Questions 5 marks in all

[1 mark]Which type of operating system is needed in a car's anti-lock braking system?

AReal-time
BMulti-user
CDistributed
DBatch

[1 mark]What is the defining feature of a real-time operating system?

AIt guarantees a response to an input within a fixed time limit
BIt is faster than any other operating system on average
CIt shows the time on the screen
DIt lets many users log in at once

[1 mark]What is a distributed operating system?

AOne operating system running across several networked computers that appear to the user as one
BAn operating system copied to many computers
CAn operating system downloaded from the internet
DAn operating system that lets several users share one computer

[1 mark]Which of these are uses of virtual machines?

Tick every answer that is true.

ATesting software on several operating systems from one computer
BRunning bytecode on any kind of processor
CRunning several virtual servers on one physical server
DMaking a program run faster than native machine code

[1 mark]What does this program print?

stack = []
for op in [('PUSH', 7), ('PUSH', 3), ('SUB',), ('PUSH', 4), ('MUL',)]:
    if op[0] == 'PUSH':
        stack.append(op[1])
    else:
        b = stack.pop()
        a = stack.pop()
        stack.append(a - b if op[0] == 'SUB' else a * b)
print(stack)

The task: a bytecode virtual machine

Finish the virtual machine so it runs bytecode, a list of tuples whose first item is the instruction and whose second item, when there is one, is a whole number. stack is a Python list whose end is the top, and pc is the index of the next instruction. The instructions are: | Instruction | Effect | |---|---| | PUSH n | push n | | DUP | push a copy of the top value (without popping it) | | ADD, SUB, MUL | pop b, then pop a, then push a + b, a - b or a × b | | FWD | pop a distance in cm and drive forward that far at speed 60 | | TURN | pop an angle in degrees and turn right that far at speed 30 | | BEEP | pop a frequency in Hz and play it for 0.2 seconds | | JZ a | pop a value; if it is 0, jump to instruction a | | JMP a | jump to instruction a | | HALT | stop | When HALT is reached, print stack at halt: <stack>, printing the list as Python prints it. The program drives a 20 cm square with a beep at each corner, and leaves one value on the stack. Every distance must come off the stack.

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

bytecode = [
    ("PUSH", 4), ("DUP",), ("JZ", 14), ("PUSH", 4), ("PUSH", 5), ("MUL",), ("FWD",),
    ("PUSH", 90), ("TURN",), ("PUSH", 660), ("BEEP",), ("PUSH", 1), ("SUB",), ("JMP", 1), ("HALT",),
]

stack = []
pc = 0
while True:
    instruction = bytecode[pc]
    op = instruction[0]
    if op == "PUSH":
        stack.append(instruction[1])
    elif op == "FWD":
        forward(60, distance=stack.pop())
    elif op == "TURN":
        turn_right(30, angle=stack.pop())
    elif op == "HALT":
        break
    else:
        raise ValueError("unknown instruction " + op)
    pc = pc + 1

Plan your program here, then type it in and press Run.

Do it on the robot
www.bugbotlab.com/learn/a10-6-types-of-os-and-virtual-machines/
The simulator checks it and tells you when it passes. Nothing to install, no account.

Challenges

Trace the stack for the first time round the loop, instruction by instruction. What is on the stack just before JZ each time?

Change the bytecode so the robot draws a triangle instead. Did you need to change the virtual machine?

Write a second virtual machine for the same bytecode that only prints what it would do, like machine 2 above. Why is this useful for testing?

A10.6 Types of operating system and virtual machines

What this lesson is about

Questions 5 marks in all

The task: a bytecode virtual machine

Challenges