CPUs Microcode Protection and Protection Modes

Question 1

When CPU runs in Kernel mode, it can:-

• run any ___
• modify any ___ in ___
• access and modify any ___ in CPU and devices
• full control of the computer

Answer 1

instruction; location, memory; register

Question 2

The OS services runs in ___ mode.

Kernel/User

Answer 2

Kernel

Question 3

When CPU runs in User mode, it can:-

• use a limited set of ___
• modify only sections of ___ assigned to the process running the program
• access only ___, and cannot access ___ in ___
• limited access to the resources of the computer

Answer 3

instructions;
memory;
a subset of registers in CPU, registers, devices

Question 4

The user programs run in ___ mode.

Kernel/User

Answer 4

User

Question 5

Kernel/User Mode:

1. When the OS boots, it starts in ___ mode.

Answer 5

kernel

Question 6

Kernel/User Mode:

2. In ___ mode, the OS sets up the ___ and ___ all the devices.

Answer 6

kernel, interrupt vector, initializes

Question 7

Kernel/User Mode:

3. Then it starts the first process and switches to ___ mode.

Answer 7

user

Question 8

Kernel/User Mode:

4. In ___ mode, the OS runs all the ___.

Answer 8

user, background system processes (daemons)

Question 9

Kernel/User Mode:

5. Then it runs the ___ or ___.

Answer 9

user shell, windows manager

Question 10

Kernel/User Mode:

6. While running in ___ mode, the program switches to ___ mode to request OS services (___).

Answer 10

user, kernel, system calls

Question 11

Kernel/User Mode:

7. User programs also switch to ___ mode when ___ arrives.

Answer 11

kernel, interrupt

Question 12

The interrupts are executed in kernel mode.

True/False

Answer 12

True

Question 13

The interrupt vector can be modified in both kernel and user mode.
True/False

Answer 13

False. Only in kernel mode

Question 14

Most of the CPU time is spent in __ mode.

Answer 14

User

Question 15

Separation of user/kernel mode is for:-
___ (make sure user have enough privileges to run the call)
___ (Killing bugs and crashing)
___ (OS calling in kernel mode for fair access)

Answer 15

Security; Robustness; Fairness

Question 16

If a process that tries to write to an invalid memory location, the OS will ___ the program, but the OS ___.

Answer 16

kill, continue to run

Question 17

Which of the following is false?
A. A crash in the process will not crash the OS
B. A bug in user mode causes program to crash, OS will also crash
C. A bug in kernel mode may cause OS and system to crash

Answer 17

B.

A bug in user mode causes program to crash, but OS will still run.

Question 18

An interrupt is an event that requires ___.

Little/moderate/immediate attention

Answer 18

immediate attention

Question 19

In hardware, a device sets the interrupt line to ___.

low/medium/high

Answer 19

high

Question 20

When an interrupt is received, the CPU will stop whatever it is doing and jump to the ___ that handles that specific interrupt.

Answer 20

Interrupt handler

Question 21

After executing the handler, the CPU ___ and the program continues.

Answer 21

return to the same place where the interrupt happened

Question 22

Arrange the steps of servicing an interrupt.

1. CPU jumps to interrupt handler and run it
2. CPU restores the registers and return back to the place in the program that was interrupted. The program continues execution as if nothing happened
3. CPU saves the program counter and registers in execution stack
4. CPU looks up the corresponding interrupt handler in the interrupt vector

Answer 22

3, 4, 1, 2

Question 23

Interrupts allow CPU and devices to run in parallel without waiting for each other.
True/False

Answer 23

True

Question 24

Which of the following about poling is false?
A. Poling is also called “busy waiting”.
B. During poling, OS decides not to use interrupts for some devices and wait in a busy loop until completion.
C. Poling is used to print debug messages in the kernel.
D. Poling saves CPU cycles.

Answer 24

D.

Poling wastes a lot of CPU cycles.

Question 25

Poling is called ___ processing since the execution of the device is ___ with the program.

Answer 25

synchronous, synchronous

Question 26

An interrupt is also called ___ processing because the execution of the device is not ____ with the execution of the program.

Answer 26

asynchronous, synchronized

Question 27

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

Answer 27

Interrupt vector

Question 28

Interrupts run in kernel mode. Why?

Answer 28

An interrupt handler must read device/CPU registers and execute instructions only available in kernel mode.

Question 29

Interrupt vector can be modified only in kernel mode.

True/False

Answer 29

True

Question 30

Interrupt vector is initialized when drivers added to the system.
True/False

Answer 30

False.

Interrupt vector is initialized on bootup, and modified when drivers are added to the system.

Question 31

Types of interrupts:-

____ is generated by devices when a request is complete or an event that requires CPU attention happens.

Answer 31

Devices interrupts

Question 32

Types of interrupts:-

___ generated by the CPU when there is a math error.

Answer 32

Math exceptions

Question 33

Types of interrupts:-

___ generated by the MMU (___) that converts virtual memory addresses to physical memory addresses.

Answer 33

Page faults; Memory Management Units

Question 34

Types of interrupts:-

___ generated by software with a special assembly instruction.

Answer 34

Software interrupt

Question 35

Match the reasons to their description for the MMU interrupt:-
___ : interrupt prompts a SEGV signal to the process.
___ : Access to a valid address but there is not page in memory. This causes the CPU to load the page from disk.
___ : (trying to write on a read only page) causes a SEGV signal to the process.

Answer 35

Invalid address;
Page not resident;
Invalid permission

Question 36

System calls use ___.

Answer 36

software interrupts

Question 37

Why do we use software interrupts for syscalls instead of function calls?

• Software interrupts will switch into ___ mode.
• OS services need to run in ____ mode because they need ___, accessing devices and ____, they need to enforce ___.

Answer 37

kernel; kernel; privileged instructions; kernel data structures; security in kernel mode

Question 38

Only operations that need to be executed by the OS in kernel mode are part of the system calls.
True/False

Answer 38

True

Question 39

Libc calls system calls directly.

True/False

Answer 39

False.

Libc provices wrappers for the system calls that eventually generate the system calls.

Question 40

Which of the following about system calls is false?
A. The software interrupt handler for system calls has entries for all system calls.
B. The handler checks that the arguments are valid and that the operation can be executed.
C. The arguments of the syscall are not checked for quicker operations.

Answer 40

C.

The arguments of the syscall are checked to enforce the security and protections.

Question 41

When an error in a system call occurs, the OS sets a global variable called ___ defined in libc.so with the ___ of the error that gives the reason for failure.

Answer 41

errno; number

Question 42

Arrange the sequence of system calls and interrupt handler:-

1. 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 [buff, buff+n] is a valid memory range. If any of the checks fail, write return -1 and sets errno to the error value.
2. The interrupt handler puts the process calling write into ready state.
3. The OS puts the current process in wait state until the disk operation is complete. The OS switches to another process.
4. The User program calls the write(fd, buff, n) system call to write to disk.
5. The Disk completes the write operation and generates an interrupt.
6. The OS tells the hard drive to write the buffer in [buff, buff+n] to disk to the file specified by fd.
7. The write wrapper in libc generates a software interrupt for the system call.

Answer 42

4, 7, 1, 6, 3, 5, 2