Module 4: Uncovering Services Provided by the OS

Question 1

control registers

Answer 1

registers in the CPU that are used to store control information on state of program execution (e.g. 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 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 devices that is not ready

Question 5

TRAP call

Answer 5

a series of assembly instructions leading to the 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 of 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 devices 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

segmentation faults

Answer 10

a failure that occurs when the program attempts to access a restricted portion of 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 11

disk device driver

Answer 11

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 12

interrupt handler

Answer 12

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

Question 13

pointer

Answer 13

store the addresses or memory locations of variables

Question 14

dereferencing

Answer 14

use the address value from a 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 OS 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 OS that decides which process to run next

Question 19

interrupt service routine

Answer 19

whenever a hardware interrupt arrives, it is delivered to the OS 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

custom handlers

Answer 20

signal handlers that are provided in user space and are customized by the application for handling the particular signal

Question 21

default handlers

Answer 21

signal handlers that are default in the OS

Question 22

signal relaying

Answer 22

sending one signal from one process to another through the kernel

Question 23

preemptible signal handlers

Answer 23

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 24

signal blocking

Answer 24

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