Week 1

Question 1

Operating System

Answer 1

Software that acts as intermediary layer between a user and computer hardware

Question 2

How does OS control programs

Answer 2

Controls execution of user programs (CPU) and I/O Operation (disk drive)

Question 3

OS Kernel

Answer 3

Available to run at any time, stored in high memory, Used to boot program

Question 4

How does OS do resource allocation

Answer 4

Partitions RAM and provides uniform interface for hardware

Question 5

Explain early program systems

Answer 5

Batch mainframe system (hidden figures). Had a pool of jobs that were run sequentially. Minimal hardware and OS requirements.

Question 6

Multiprogram Systems

Answer 6

More than one program could occupy memory at the same time. Required more security and OS intervention.

Question 7

Hard-Real time

Answer 7

Hard limits that will make a system fail. I.e. Car engine not starting

Question 8

Soft-Real time

Answer 8

Missed deadline will make the system utility decrease. E.g. Video lag

Question 9

Interrupts

Answer 9

Hardware signal used to suspend execution of current running program. The program is suspended and CPU control is transferred to interrupt routine that is stored in the interrupt dispatch table.

Question 10

How does CPU determine interrupt source?

Answer 10

Polling or vectored interrupts

Question 11

Essential interrupt idea

Answer 11

THEY CAN OCCUR AT ANY TIME

Question 12

Trap

Answer 12

Signal generated by software similar to an interrupt. Requests a service from OS by the program. E.g. Error from division by zero. Accessed through the trap dispatch table

Question 13

How is kernel mode implemented

Answer 13

Bit in status register, 0 for not 1 for kernel mode

Question 14

User mode

Answer 14

Used for executing programs

Question 15

Kernel mode

Answer 15

Accessing hardware and CPU resources with privileged instructions. Processing traps and system requests.

Question 16

Example of switching between kernel mode

Answer 16

• Program runs
• Interrupt signal
• Program switches to kernel mode and OS memory
• Program location is saved on the stack stored in OS memory
• Interrupt routine is found and executed
• Return statement for interrupt is called which includes code that returns the status bit back to user

Question 17

Memory Base register

Answer 17

Points to beginning of allocated memory

Question 18

Memory limit register

Answer 18

Points to end of allocated memory

Question 19

What happens when you try to access outside of allocated program memory

Answer 19

Trap is thrown

Question 20

Cooperative Multitasking

Answer 20

Program calling the yield function if it knows it’ll be occupied. E.g. numerical calculations

Question 21

Program timers

Answer 21

Timers with allocated time that, when exceeded, throws an interrupt to yield the CPU from the current running program. Also used to update current date and time.

Question 22

Main OS activities

Answer 22

Create and destroy processes and their resources. Suspend and resume processes. Synchronize data sharing.

Question 23

I/O management

Answer 23

Hide devices behind drivers. Contains buffers, interfaces, and cache for devices

Question 24

Types of queues

Answer 24

Read queues, I/O queues

Question 25

Main memory

Answer 25

CPU accessed memory

Question 26

Secondary storage

Answer 26

Large and non-volatile. E.g. electronic disks and tapes

Question 27

Cache

Answer 27

High speed memory between CPU and main memory

Question 28

System Calls

Answer 28

Provide the user the capacity to invoke useful functionalities that they do not have the permission to use. Allows OS to take over using traps and execute command for them. E.g. file copy, command interpreter.

Question 29

Command Interpreter

Answer 29

Accepts and executes commands from user. Typically in scripting languages like bash with graphical interfaces.

Question 30

Program

Answer 30

Program in execution and all resources associated with this program, like memory, PC, input, open files, etc

Question 31

Process layout

Answer 31

Stack at the top (that grows downwards). Unallocated in the middle. Data segment after (that grows up). Contains global variables at the bottom and heap in the top half for malloc. Text segment at the bottom with the machine code.

Question 32

sbrk

Answer 32

Points to end of data segment

Question 33

Allocated stack and stack pointer

Answer 33

Point to end of and current position on the stack

Question 34

New State

Answer 34

When process first starts up. Loading the text and data segment into memory. Allocating stack, setting up terminal connection and I/O connection.

Question 35

Ready Queue

Answer 35

Ready to execute

Question 36

Running State

Answer 36

Process currently being executed by the CPU

Question 37

Wait Queue

Answer 37

Waiting for I/O device. Program will free up the CPU for other processes. Will be stuck on queue until request is serviced.

Question 38

Done State

Answer 38

Process finished running and resources are freed

Question 39

Process Control Block

Answer 39

PCB is allocated for each process and sometimes each thread. Repository of information that varies for each process.

Question 40

Elements in PCB

Answer 40

Queue Info, Process State, Schedule Info, Process ID, PC, Register states, Memory info, open files, accoutning

Question 41

Queue information in PCB

Answer 41

Link pointers that places process on master list and specialized lists.

Question 42

Process State in PCB

Answer 42

Integer to hold state. E.g. New, Ready, Wait

Question 43

Schedule Info PCB

Answer 43

Process priority and credits

Question 44

Process ID PCB

Answer 44

Integer that represents the process' UUID

Question 45

PC PCB

Answer 45

Value of PC when process was running saved in kernel stack

Question 46

Register PCB

Answer 46

Save register information at the time of interrupt

Question 47

Memory PCB

Answer 47

Allocated memory to that program

Question 48

Accounting PCB

Answer 48

Pay for use time saved

Question 49

Context switch

Answer 49

When CPU switches between processes after an interrupt routine

Question 50

Describe Context Switch

Answer 50

Process running. Interrupt occurs and interrupt service routine is entered. Save all PCB information (registers, time on CPU). Save PC onto kernel stack. Program put on ready or wait queue while interrupt routine occurs. Process 2's PCB gets pulled from the stack and it changes from ready to running. Restore process' state on the CPU and begin running.

Question 51

Overhead

Answer 51

When the CPU is doing no meaningful work. E.g. context switch

Question 52

I/O bound process

Answer 52

Interactive process that typically does mostly I/O. High priority

Question 53

CPU Bound process

Answer 53

Process that does mostly computation. Low priority.