Week 1

Question 1

What does an OS do?

Answer 1

Virtualize resources, so every process can have the illusion of a dedicated machine.

Mange I/O so multiple processes can access devices (disk, network card, etc.). concurrently without interfering

Make the above easy - define simple unambiguous interfaces (syscalls).

Question 2

What were computers like in the early 1950s?

Answer 2

Were used for scientific computing and processing large datasets

Batch mode: submit a large computation,
wait for result, submit another one

Basically a very powerful calculator used by one operator at a time - no need for OS

Question 3

What did early machines have (1950s) that is now consider part of any OS.

Answer 3

Libraries of functions. The reason was convenience.

Still no virtualization (one program at a time). No mediation (any program could still do whatever they wanted).

Question 4

What were computers like in the 1960s?

Answer 4

There was separation. There was an idea to give the CPU two execution modes:

An unprivileged mode and a privileged mode.

Question 5

How do we implement separation? How do we switch between the two modes?

Answer 5

The CPU defines some type of trap. An instruction to switch into OS code and back.

The app can pass execution and parameters to the OS, but cannot define the code that the OS is going to execute.

Question 6

In separation there is the concept of the ring of fire. What are rings 0-3 used for?

Answer 6

Ring 0 (Most privileged) - Kernel
Ring 1 - Device Drivers
Ring 2 - Device Drivers
Ring 3 (Least privileged) - Applications

Question 7

True or false. The OS will allow multiple processes to write to the disk without coordination.

Answer 7

This is false.

Question 8

What was programming like in the 1970s?

Answer 8

Many processing tasks were still I/O bound

There was the introduction of multiprogramming which meant that you could let multiple processes run at the same time and switch between them when appropriate.

Question 9

How do we enable multiprogramming?

Answer 9

There are several things but the most important thing is memory protection.

The solution to this is virtual memory.

Question 10

What did UNIX provide as an OS?

Answer 10

UNIX was the first widely available OS which implemented storage (file system), time sharing (splitting CPU time), etc.

Question 11

What were computers like in the 1980s?

Answer 11

Companies like IBM and Apple started selling PCs.

PCs shipped with very simple OS’es which lacked proper protection and multiprogramming capabilities.

Example: MS DOS had no memory protection.

Question 12

What are computers like 1990s to today?

Answer 12

OSes improved from the 90s.

They now had true memory protection capabilities.

True multiprocessing/threading (the OS controls how processes share the CPU).

Robust, performant file systems.

Question 13

What is CPU virtualization?

Answer 13

This is when we “slice” a CPU

An OS needs to be able to run a program, and stop a program. It should decide which program should run at any given time.

Means that we divide CPU time across programs fairly.

Question 14

What is memory virtualization.

Answer 14

When we give every process the same virtual address space.

In other words each process gets the illusion of having memory to itself.

Mapping between virtual and physical pages can change.

Mapping is not necessarily 1:1. Resources are used by multiple processes.

Question 15

How is concurrency different from sharing CPU?

Answer 15

CPU virtualization just means that multiple processes can share the CPU.

However in concurrency, different parts of your program cooperate to complete a task.

Question 16

What are the requirements for concurrency?

Answer 16

The ability for different parts of the program to communicate.

The ability to synchronize access to shared data.

Question 17

What are two forms that a concurrent program can take?

Answer 17

Different communicating processes (separate address spaces)

Threads within the same process (shared address space).

Question 18

What do the following do?
pthread_create()
pthread_join()

Answer 18

pthread_create(): creates a new execution threads, pass the execution to a specified function.

pthread_join(): wait until a specified thread terminates.

Question 19

What is persistence?

Answer 19

Processes typically need to receive some pre-existing inputs and save some outputs

This is accomplished using persistent storage (disks, SSD)

No virtualization abstraction (it does not make sense to give each program the
abstraction of a “dedicated disk”, as programs often share data)

Instead, shared disk using files and directories (folders) abstraction

Question 20

What are the differences between virtual memory and the shared disk?

Answer 20

Virtual memory: works because processes mostly need their own memory space, and only occasionally need to share memory data

Shared disk: works because it is very common for programs to use or share disk data.