Operating Systems

Question 1

Key Roles of an OS

Answer 1

1) Hide hardware complexity (e.g. no need to think about how to interact with hdd)
2) manage underlying HW resources (allocates memory for apps)
3) provides isolation and protection (apps shouldn’t access other app’s memory)

Question 2

What is an OS? (3 things)

Answer 2

A layer of software systems that:

1) directly has priviliged access to underlying HW
2) manages hw on behalf of 1 or more apps according to predefined policies
3) ensures that apps are isolated and protected from one another

Question 3

List abstractions:

Answer 3

1) process, thread, file, socket memory page

Question 4

List mechanisms:

Answer 4

2) create, schedule, open, write, allocate

Question 5

policies:

Answer 5

used to determine exactly how the mechanism will be used to manage underlying hw (LRU)

Question 6

Give an example of abstractions, mechanism and policies:

Answer 6

Abstraction: memory page
Mechanism: “allocate”
policy: determines when to move page from DRAMto disk (LRU)

Question 7

Separation of Mechanism and policy:

Answer 7

memory management mechanisms should be generalized to these different policies (LRU, LFU for memory)

Question 8

optimize for common case

Answer 8

understanding the common case helps OS implement correct policy basedon generalized mechanism

Question 9

user-mode

Answer 9

unprivileged

where most apps run

Question 10

kernel mode:

Answer 10

OS operates in kenernel mode with direct access to HW

privileged

Question 11

user kernel mode crossing

Answer 11

1) app wants direct acess to HW so it uses a system call
2) control is switched to OS which can flip a bit in cpu to allow app access to HW
3) if privileged instructions are encountered in non-privileged execution, the app becomes trapped at which point the OS can grant access or terminate

Question 12

trapped

Answer 12

when a privileged instruction is encountered by HW in non-privileged execution

Question 13

system call definition:

Answer 13

an interface that can be invoked by app to allow privileged access of HW resources for the app on behalf of OS (e.g. open, send, mmap)

Question 14

System call flow:

Answer 14

1) app needs access to HW so makes system call
2) control is passed to OS which accesses HW
3) execution control and data is passed back to app process from OS

Question 15

Monolithic kernel Pros and Cons:

Answer 15

Pros: everything included
inlining, compile time optimizations
Cons: no customization
not portable
large memory footprint that may impact performance

Question 16

Modular kernel pros and cons:

Answer 16

Pros: maintainability, smaller footprint than monolithic, less resources needed
Cons: may reduce optimization (but probably not)
maintenance may be issue

Question 17

microkernel pros and cons:

Answer 17

pros: size, verifiability
cons: bad portability due to customized to underlying HW
expensive cost of frequent user/kernel crossing