User Mode, Kernel Mode, Interrupts and System Calls

Question 1

What architecture do modern computers use? Where are the programs and data stored?

Answer 1

Von Newman. Stored in ram.

Question 2

How is data transferred from CPU, to RAM, ROM and devices?

Answer 2

Through the address and data bus.

Question 3

What can a service do in Kernel mode?

Answer 3

Can run any instruction, modify any location in memory, can access any register.

Question 4

What mode to OS services run in? Kernel or User?

Answer 4

Kernel, they need full control.

Question 5

What can a service do in User mode?

Answer 5

CPU can use limited instructions, limited sections of memory, limited registers.

Question 6

What happens with the modes when the OS boots?

Answer 6

It boots in Kernel mode to set up interrupt vectors and more. After starting the first process it moves to User mode.

Question 7

When to programs switch from User to Kernel mode?

Answer 7

When an interrupt arrives.

Question 8

Is more CPU time spent in User or kernel mode?

Answer 8

User

Question 9

Why are user and kernel mode separated?

Answer 9

Security, Robustness (crash in user doesn’t kill os), and Fairness

Question 10

If a bug occurs in Kernel mode can the os crash?

Answer 10

Yes

Question 11

What happens during an interrupt?

Answer 11

Hardware sets interrupt line to high, CPU jumps to interrupt handler, then returns to previous spot.

Question 12

What are some examples of interrupts?

Answer 12

move mouse
type key
ethernet packet

Question 13

What are the specific steps of serving an interrupt?

Answer 13

1. 2. 3. 4. 5. The CPU saves the Program Counter and
               registers in execution stack
               CPU looks up the corresponding interrupt
               handler in the interrupt vector.
               CPU jumps to interrupt handler and run it.
               CPU restores the registers and return back to the
               place in the program that was interrupted. The
               program continues execution as if nothing
               happened.
               In some cases it retries the instruction the
               instruction that was interrupted (E.g. Virtual
               memory page fault handlers).

Question 14

What is polling in the context of handling tasks?

Answer 14

when the OS waits in a busy loop instead of using interupts.

Question 15

What is Synchronous vs Asynchronous processing?

Answer 15

Polling is synchronous. Interrupt is asynchronous.

Question 16

What is the interrupt vector?

Answer 16

It is an array of pointers that point to the different interrupt handlers of the different
types of interrupts.

Question 17

What are a couple of interrupt handlers?

Answer 17

Hard Drive Interrupt handler
USB Interrupt handler (mouse, kbd)
Ethernet Card Interrupt handler
Page Fault Interrupt handler

Question 18

Why must interrupts be handled in Kernel mode?

Answer 18

Instructions needed only available in Kernel mode.

Question 19

What are the different types of interrupts?

Answer 19

Device Interrupts, Math exceptions, Page Faults, Software Interrupt

Question 20

What is a system call?

Answer 20

How programs request services from the OS.

Question 21

What are some examples of system calls?

Answer 21

◼ open(filename, mode)
◼ read(file, buffer, size)
◼ write(file, buffer, size)
◼ fork()
◼ execve(cmd, args);

Question 22

Why are software interrupts needed for system calls?

Answer 22

Need Kernel mode for OS services

Question 23

Do all parts of the system call run in Kernel mode?

Answer 23

No, ex: for malloc most of it runs in user mode until sbrk() is called to extend the heap.

Question 24

What library provides wrappers for system calls?

Answer 24

Libc

Question 25

What is checked in the sys call open(filename, mode) and by what?

Answer 25

The interrupt handler checks to make sure file exists, it also checks the permissions of the file being opened.

Question 26

Where can a list of system calls be found?

Answer 26

/usr/include/sys/syscall.h

Question 27

What happens when an error occurs in a system call?

Answer 27

OS sets global variable erno to number that gives reason for the error.

Question 28

What does perror(s) do?

Answer 28

Prints erno and preapends s.

Question 29

Explain each step of the call write(fd, buff, n)

Answer 29

he user program calls the write(fd, buff, n) system call to write to disk. The write wrapper in libc generates a software interrupt for the system call. The OS in the interrupt handler checks the arguments. It verifies that fd is a file descriptor for a file opened in write mode. And also that [buff, buff+n-1] is a valid memory range. If any of the checks fail write return -1 and sets errno to the error value. System Calls and Interrupts Example 4. The OS tells the hard drive to write the buffer in [buff, buff+n] to disk to the file specified by fd. 5. The OS puts the current process in wait state until the disk operation is complete. Meanwhile, the OS switches to another process. 6. The Disk completes the write operation and generates an interrupt. 7. The interrupt handler puts the process calling write into ready state so this process will be scheduled by the OS in the next chance.

Question 30

What is the Strace command?

Answer 30

It is a tool that prints the system calls that a program calls ex: strace ./hello