Lesson 2: Syscalls and interrupts

Question 1

Privileged instructions

Answer 1

The way the OS talks with the hardware and storage. Only the OS can give these instructions and not a user.

Question 2

Interrupts

Answer 2

The wat the hardware speaks to the OS

Question 3

User mode

Answer 3

When the CPU is running the user’s code

Question 4

Kernel mode

Answer 4

When the CPU is running the OS’s code

Question 5

System calls

Answer 5

Moves from user mode to kernel mode

Question 6

Privileged mode

Answer 6

= Kernel mode

Question 7

How can user programs access or alter internal OS data structures or hardware?

Answer 7

Through system calls only (protection)

Question 8

Difference between system calls and other internal OS functions

Answer 8

System calls check the validity of the parameters, whereas other internal OS functions don’t.

Question 9

What’s the name of the process moving from the user to the kernel mode?

Answer 9

Trap

Question 10

Trap

Answer 10

The process of switching between user and kernel mode

Question 11

Mode bit

Answer 11

Provides ability to distinguish when system is running user code or kernel code. This way some instructions that are privileged can only be executed in the kernel mode.

Question 12

What causes the mode to change from user to kernel?

Answer 12

System call

Question 13

What causes the mode to change from kernel to user?

Answer 13

The return from the system call

Question 14

What can cause a transition from user to kernel mode?

Answer 14

1. A system call

2. An exception (e.g., divide by 0) - handled as part of the CPU architecture.

Question 15

Is “echo”, “grep” and other functions in the shell part of the OS?

Answer 15

No. They are applications that run above the OS but can be considered as OS services.

Question 16

What are the 6 layers of the computer system?

Answer 16

1. Apps
2. User interface - shell or windows
3. Libraries - the interface the OS gives us so we can use it with other applications
4. OS abstractions - file systems, jobs/processes/threads, sockets
5. Drivers - interface to hardware
6. Hardware

Question 17

How does the computer communicate with IO devices?

Answer 17

Via busses

Question 18

What are busses?

Answer 18

The way to connect to IO devices

Question 19

Advantages of busses

Answer 19

1. Versatility - easy to add new devices and move them between computer systems
2. Low cost - share a single set of ways in multiple ways

Question 20

Disadvantages of busses

Answer 20

1. Communication bottleneck - the bandwidth of the bus limits the maximum IO throughput
2. Max bus speed is limited by length of bus, number of devices on the bus, and the need to support a range of devices with varying latencies and data transfer rates.

Question 21

Interrupts (hardware interrupts)

פסיקה

Answer 21

Way that devices can communicate with OS.

For example: The way the screen tells the OS that it finished writing something, the disk finished reading a file, etc.

Question 22

IRQ

Answer 22

Interrupt request lines - special line that the interrupts travel through to get to the CPU.

Question 23

Interrupt vector table (interrupt description table)

Answer 23

Every interrupt has a code that says what needs to happen with the interrupt occurs. The table saves the address where this code resides.

Question 24

What does system calls cause?

Answer 24

A trap - aka software (or internal) interrupt.

Question 25

Software interrupts

Answer 25

Either a system call or an exception

Question 26

Exceptions are ____ interrupts

Answer 26

Exceptions are internal interrupts

Question 27

Periodic vs aperiodic interrupts

Answer 27

Periodic - the clock (part of the CPU) does an interrupt periodically so the control can pass over to the OS.

Aperiodic - all the other types of interrupts we spoke about

Question 28

Software vs hardware interrupts

Answer 28

Software: causes by a software, this includes traps and exceptions.

Hardware: caused by hardware, this includes interrupts from external devices.

Question 29

Internal vs external interrupts

Answer 29

Generally: Internal=Software, External=Hardware

Question 30

Synchronous vs. asynchronous interrupts

Answer 30

Synchronous: happens with the clock ticks (software, timer/clock interrupts).

Asynchronous: More tricky because you don’t know which command you need to go back to

Question 31

Maskable vs. non-maskable interrupts

Answer 31

Maskable: hardware interrupts that can be delayed if more important interrupts is currently handled.

Question 32

Where is there more overhead: in user mode vs kernel mode?

Answer 32

User, since you don’t have traps.

Question 33

What are advantages of having a smaller kernel?

Answer 33

1. Fewer traps –> less overhead –> higher throughput
2. Smaller memory footprint
3. More functions outside of kernel, so if they change, there’s no need to recompile the kernel.

Question 34

What are disadvantages of having a smaller kernel?

Answer 34

Less protection

Question 35

Monolithic kernel

Answer 35

The kernel has basic functions and you can load other modules into the memory when necessary

Question 36

Microkernels

Answer 36

A few different kernels that work together

Question 37

Difference between Monolithic kernels and Microkernels?

Answer 37

If they work on the same address space or not.

Single address space - monolithic.

Question 38

Are system calls privileged?

Answer 38

No.

Question 39

Interrupt controller

Answer 39

Serves a an intermediate between the hardware devices and the processor. it can take input from different devices, and alerts the processor.

Question 40

APC

Answer 40

Asynchronous procedure call

For example: an interrupt, because the program can’t control it.

Question 41

Why are interrupts characterized as APCs (Asynchronous procedure call)?

Answer 41

Because they aren’t part of the regular, pre-planned code. The program can’t control when it’ll be.

Question 42

Mechanism of interrupts

Answer 42

(Similar to functions)

1. Getting the interrupt
2. Saving current state
3. Transfer control & service the request (using an “interrupt vector”)
4. Previous state is restored
5. Return control (back to user mode)

Question 43

Internal interrupts

Answer 43

Internal interruption (exception) occurs when the processor detects an error condition while executing an instruction (unlike external interrupts, which occur at random times during the execution of a program)

Question 44

Examples of internal interrupts

Answer 44

Division by zero
Segmentation fault
Privileged instruction
Invalid instruction
Page fault

Question 45

What happens when an exception occures?

Answer 45

• When an exception occurs, the address of the instruction which generated the exception is saved.
• Then the OS gives the exception handler a chance to fix the problem.
• If there was an handler, the program is restarted at the address of the fault and continue normally
  Otherwise, the program is aborted.

Question 46

What type of interrupt can happen between clock cycles?

Answer 46

Only external interrupts (and not internal ones)

Question 47

Trap

Answer 47

A trap is a type of exception (internal interrupt), which occurs in the usual run of the program, but unlike it, it is not product of some error.
Trap causes switching to OS code and to kernel mode. Once in kernel mode, a trap handler is executed to service the request.

Question 48

Breakpoints in debuggers are created by using ____.

Answer 48

Breakpoints in debuggers are created by using traps.

Question 49

The system call is a wrapper function that runs in ____ mode which calls the interrupt which is handled in ____ mode.

Answer 49

The system call is a wrapper function that runs in user mode which calls the interrupt which is handled in kernel mode.