Lecture 4: Processes 1

Question 1

Role of the OS:

1. Resource Abstraction
2. Resource Sharing

Supported in the these ways:

Answer 1

• Process and thread management (resource: CPU)
• Memory management (resource: Memory)
• File management (resource: Secondary Storage)
• I/O system management (resource: Devices)
• Protection system (resource: All)
• Command-interpreter system (resource: All)

Question 2

Shell (or Command-Line-Interpreter)

Answer 2

• A shell is a job control system which allows the programmer to create and manage a set of programs to do some task.
• it is a program that reads and interprets control statements.
• The “visible face” of an OS (sometimes with GUI (e.g.: Windows)

® Process creation and management
® I/O handling
® Secondary storage management
® Main memory management
® File system access
® Protection

Question 3

Control Statements of the Shell

Answer 3

User-driven commands to the OS (vs. event-driven commands such as interrupts)

Question 4

Process

Answer 4

-program in execution

Question 5

How Is a Process an Abstraction of the CPU?

Answer 5

• Gives an illusion of a uniprogramming environment

- Every process acts as though it has exclusive access to the CPU

Question 6

In Reality: Multiprogramming

Answer 6

1. CPU is time-sliced:
   processes take turns using the CPU
2. Memory is space-sliced:
   processes get partitions

Question 7

Threads

Answer 7

• Lightweight process (no isolation)
• Abstraction only of CPU
• One process can have many threads (multithreading), all share the same memory

Question 8

Difference between threads and processes

Answer 8

1. Threads: mostly cooperative. Processes: mostly independent –a thread communication is inexpensive (via shared memory)
   - a e.g., a user can type characters and have the speller running
2. Threads more appropriate for certain applications

Question 9

What makes up a process?

Answer 9

• Code section (text segment)
• Data section (data segment)
• Stack (stack segment)
• Registers (including Program Counter)

Question 10

Program vs Process

Answer 10

• Program is a passive entity
• an executable stored in disk

Process is a dynamic entity
-a program becomes a process when it is loaded into memory

Question 11

Process Control Block (PCB)

Answer 11

Processes Represented In Memory with Current “State”

Question 12

Context Switch

Answer 12

• process with access to CPU switched out (state copied to PCB)
• another ready process switched in (state copied from PCB)

Question 13

Process Scheduling: Multiprogramming Goal

Answer 13

• maximize CPU utilization

- have some process running (using the CPU) all time

Question 14

Process Scheduling: Time –sharing Goal

Answer 14

• users can interact with their running programs

- switch CPU among processes frequently enough

Question 15

Process Scheduler

Answer 15

it selects the

process for program execution on the CPU

Question 16

Job Queue

Answer 16

Contains all processes in system

Question 17

Ready Queue

Answer 17

It includes processes that are residing in main memory and are ready to execute

Question 18

Device Queues (Waiting) Queue Headers

Answer 18

It includes processes waiting for an I/O device

Question 19

Schedulers

Answer 19

-Select which processes to allocate to CPU
-Submitted processes can be spooled into disk for
later execution

Question 20

Short term scheduler

Answer 20

• selects one from memory and allocate the CPU to it
• Selects processes from the ready queue
• Will be invoked frequently (every few milliseconds) Þ needs to be fast!!

Question 21

Long term scheduler

Answer 21

-moves processes from disk to main memory

• Selects which processes should be brought into ready queue
• Controls degree of multiprogramming (# processes in memory)
• Invoked infrequently (e.g.: seconds or minutes), \ can be slow
• Needs to make a careful process mix between I/O vs CPU bound processes

Question 22

CPU Bound

Answer 22

spends more time doing computations

Question 23

I/O Bound

Answer 23

spends more time doing I/O

Question 24

I/O heavy mix

Answer 24

the ready queue will always be empty (no process is ready to use the CPU as they are in the device queue )

Question 25

CPU heavy mix

Answer 25

CPU contention, devices are unused and system is unbalanced

Question 26

Medium-Term Scheduler

Answer 26

• Can reduce the degree of multiprogramming by removing processes from memory (swapping) -> reduce CPU contention
• Can be used to improve the process mix or fix overcommitted memory requirements

Question 27

Process and Memory

Answer 27

• process also serves as an abstraction of memory
• Process has exclusive access to all memory that it can address
• Process has exclusive access to all memory it can reference
• Memory is partitioned amongst processes
• Memory consists of non-contiguous chunks

Question 28

Process and Memory Exclusivity

Answer 28

-all processes assigned a base register and limit register defining its address space (e.g.: 10,000 – 12,000 has base register = 10,000 and limit register = 2,000)

• base, limit registers are like status registers (process can’t touch )
• all addressing translated accordingly

Question 29

Virtual Memory

Answer 29

®
- (P’s addr space) can be larger than physical memory process P
Solution is “Virtual memory”
Idea: Some of process’ address space not in memory but on disk
-transparent to process (will see again later in course)

Question 30

Memory Mapped I/O

Answer 30

-Reserve addresses in address space to refer to device controller registers (e.g.: video controller, or disk)

-+ I/O registers are part of the regular memory address space.
+ uses the same address bus to address both memory and I/O devices
+ CPU instructions used to access the memory are also used for accessing
devices

Question 31

Segmentation

Answer 31

Store process’ address space as non-contiguous set of “contiguous chunks”