comsci

Question 1

define interrupts

Answer 1

-CPU is fast, IO device and other services are slow
-CPU doesnt stop and wait
-When IO services are finished use interrupt to get CPUS attention

Question 2

How is the modern OS interrupt Driven?

Answer 2

-With the interrupt vector.
-With the interrupt handler
- with soft interrupts like; traps and system calls

Question 3

Why cant user interact with HW directly?

Answer 3

because of Mode bit and system calls

Question 4

What is Mode bit?

Answer 4

-motivation
-user operation and kernel operation seperate but difficult to distinguish user or kernel
-Hardware support for system protection
-1 kernel operation
-0 user operation

Question 5

What is a system call?

Answer 5

Interface between user and kernel
- All IO, Hardware, System calls are handled by the kernel
-Users must make systems calls to access any system resources

Question 6

What is the 5 process transition diagram?

Answer 6

• transition from waiting-> running, possible?
• transition from running -> waiting?
  -transition from running -> ready?

Question 7

What are the two types of concurrency?

Answer 7

-interloped which involves time shearing
-parallel which involves multiple CPUs

Question 8

What is the PCB? WHere is it located?

Answer 8

Process control block located in the kernel

Question 9

What does the PCB contain/handle?

Answer 9

-process ID
-Register values
-scheduling related info
-page table
-opened file info

Question 10

What are the two types of scheduling?

Answer 10

Preemptive and non-preemptive

Question 11

What is preemptive scheduling?

Answer 11

A process can be paused by the scheduler at any time during its execution

Question 12

What is non-preemptive scheduling?

Answer 12

• The process itself decides to give up the CPU when its computations are in a
  stable state
• Scheduler doesn’t stop/pause process

Question 13

What is the duty of the dispatcher?

Answer 13

switch between 2 user processes

Question 14

What are the 5 scheduling algorithms?

Answer 14

FCFS, SJF, SRFJ, RR, Priority

Question 15

What is race condition?

Answer 15

Race condition happens when two or more processes access and manipulate the same data concurrently and the outcome depends on the order of access (this is not desirable)

Question 16

What is spin lock?

Answer 16

Also known as busy waiting, using polling to detect a certain condition even though the process doesnt do a meaningful job it is still in the running state, CPU is busy

Question 17

what could cause internal and external fragmentation respectivly?

Answer 17

external- “holes” in dynamic contiguous memory allocation(solution is compaction:)
internal - fixed contigous memory allocation, paging internal fragment < page size

Question 18

What is the dynamic contigous memory allocation strategy?

Answer 18

-first fit(fast)
-Best fit(try to maximize the utilization of free space, but easily create small holes)
-worst fit( try to reduce the small holes)

Question 19

How do we manage paging?

Answer 19

We keep a page table for each process.

Question 20

Where is the page table?

Answer 20

In the kernel within the PCB

Question 21

Does paging have external fragments?

Answer 21

No, paging is a solution to fragmentation.

Question 22

Does paging have internal fragments

Answer 22

Yes, but it is minimal.

Question 23

How does the OS keep track of free frames?

Answer 23

with the TlB(translation look aside buffer)

Question 24

Why does the paging system have a speed issue?

Answer 24

Sometimes it can grow to be too large.

Question 25

What is the formula to find logical address?

Answer 25

Pgae number + offsets