M4: Uncovering Services Provided by the Operating System

Question 1

Control registers

Answer 1

Registers in the CPU that are used to store control

information on state of program execution, for example, the program counter and stack pointer.

Question 2

Data registers

Answer 2

Registers in the CPU that are used to store the intermediate data used when executing instructions.

Question 3

System call

Answer 3

executed in kernel mode and thus requires a context switch.
Provide an interface for the user to interact with the OS and request functionality.
Each system call has a call number, which is used by the system call dispatcher to lookup the code associated with the invoked system call.

Question 4

Blocked queue:

Answer 4

a process is placed on the blocked queue, maintained by the OS, if it is waiting for some condition to be true before it is able to proceed in its execution;
one example is a process waiting for a response from an I/O device that is not ready

Question 5

TRAP call

Answer 5

a series of assembly instructions leading to he privilege bit being flipped in the CPU to switch it from operating in user mode to kernel mode

Question 6

CPU cache

Answer 6

stores copies the data that is frequently accessed by this processor

Question 7

Disk operation

Answer 7

an operation requiring a response from the external disk memory

Question 8

Interrupt

Answer 8

a notification to the OS for an event that requires servicing; the interrupt may originate from a hardware device or software condition

Question 9

Interrupt identifier

Answer 9

used by the OS to map the interrupt notification to the

appropriate handler for the event

Question 10

Disk device driver:

Answer 10

device drivers are used as an interface between the

hardware and software to allow the operating system to ignore the specific details of the hardware component being used

Question 11

Interrupt handler

Answer 11

A function that is run whenever a particular type of interrupt is invoked and received by a process

Question 12

Segmentation faults (C programming):

Answer 12

a failure that occurs when the program
attempts to access a restricted portion or memory, or read/write illegally (e.g. write to a read-only location of memory). This is a means of notifying the OS of
the illegal action, and results in memory protection for the machine.

Question 13

Pointer (C programming):

Answer 13

store the addresses or memory locations of variables

Question 14

Dereferencing (C programming)

Answer 14

use the address value from the pointer to access the data in the memory location

Question 15

Hardware Interrupts

Answer 15

interrupts that are generated at the hardware level,

typically in response to an external event such as a clock tick or completion of a disk request.

Question 16

Clock pulse

Answer 16

All motherboards usually come with its own clock. Every clock tick generates a pulse which is delivered to the operating system in order to advance current time and invoke the scheduler.

Question 17

Illegal instruction

Answer 17

A hardware instruction that is disallowed, for example, an instruction that cannot be run due to lack of privileges or an instruction that touches memory location that is not available to the current process.

Question 18

Scheduler

Answer 18

a piece of software that resides in the scheduler that decides which process to run next.

Question 19

Interrupt Service Routine

Answer 19

whenever a hardware interrupt arrives, it is delivered
to the operating system where there is a piece of software called the interrupt service routine that decides how to handle the interrupt. This is analogous to the signal handler.

Question 20

Hardware interrupt ID:

Answer 20

a unique identifier assigned to each hardware interrupt.

The interrupt service handler uses this to decide how to handle the specific interrupt.

Question 21

Custom handlers

Answer 21

signal handlers that provided in user space and is

customized by the application for handling the particular signal.

Question 22

Default handlers:

Answer 22

signal handlers that are default in the operating system

Question 23

Signal relaying

Answer 23

sending one signal from one process to another through the kernel.

Question 24

Preemptible signal handlers

Answer 24

when a signal handler is running and a new
signal arrives, rather than complete running the signal, if the handler is “preemptible”, it means we can stop the current handler and deal with the signal first.

Question 25

Signal blocking

Answer 25

a signal being buffered and delayed at the operating system and not delivered to the process until it is explicitly unblocked.

Question 26

Example HW interrupts

Answer 26

1. Clock PUlse: Time is up
2. INput: e.g. Keyboard, HW signal
3. Illegal Instruction

Question 27

Signals for which Default Handlers cannot be overwritten

Answer 27

SIGKILL

SIGSTOP

Question 28

Are Signal Handlers Pre-emptible

Answer 28

Pre-emptible = signal handler A is running, new signal received, signal handler A stopped and new signal handler is run.
Pre-emption depends. If it is for the same signal type, not, signals processed one at a time per thread. If different and higher priority, then pre-emptible
To avoid preemption, use signal blocking in handler

Question 29

Are Signal Mask Process Dependent

Answer 29

Yes. OS needs to keep track, which signals for which process are currently allowed, not blocked. And if not blocked, shall custom or default handler take care

Question 30

Advantages of process abstranction

Answer 30

The process abstraction allows the operating system to isolate the resources and context required to perform different tasks

Question 31

Interrupts are only initiated by currently running processes.

Answer 31

Interrupts can be initiated by hardware, e.g. clock ticks, completion of I/O requests.