3 - Basic OS (Hardware, Booting, Cache, Kernel)

Question 1

Describe the CPU

Answer 1

The ‘brain’ of the computer

• On board registers for fast computation
• GPRs for data & addresses
• Special Purpose Registers for PC, SP and status register
• FPR etc.

Question 2

What are the operations for a basic CPU cycle?

Answer 2

Fetch
Decode
Execute

Question 3

What is the bottleneck of a CPU cycle?

Answer 3

Fetching from memory

Question 4

What is a solution to the performance hit of fetching from memory?

Answer 4

Pipeline

Caches

Question 5

What are the advantages and disadvantages of a pipeline?

Answer 5

Benefits:
 - The cpu can execute more than one instruction at a time
 - 'hide' the memory access time
Cons:
 - increased complexity

Question 6

What is a superscaler cpu

Answer 6

Multiple pipelines (Fetch and Decode) feeding into a buffer with multiple execute units at the output

Question 7

What are 3 ideal memory characteristics?

Answer 7

Fast
Large
Cheap

In practice, we can only ever get 2/3 of these

Question 8

What is a typical memory hierarchy?

Answer 8

Registers
Cache
Main memory
Disk

Question 9

What is the typical access time and capacity of registers?

Answer 9

1 nsec

<1KB

Question 10

What is the typical access time and capacity of caches?

Answer 10

2 nsec

4MB

Question 11

What is the typical access time and capacity of main memory?

Answer 11

10 nsec

1-8GB

Question 12

What is the typical access time and capacity of disk?

Answer 12

10 msec

1 - 4TB

Question 13

Describe main memory

Answer 13

RAM - random access memory
An array of words each with its own unique address
Operations - Load/Store
Slow compared to CPU

Question 14

What is a Cache?

What is it used for?

Answer 14

Stores the most heavily used data and instructions in fast memory close to core
Goal is to increase performance
Fast but expensive

Question 15

What is a cache hit?

Answer 15

The data needed by the CPU is in the cache

Question 16

What is a cache miss?

Answer 16

The CPU needs to fetch the data from main memory

Question 17

Describe a typical cache hierarchy

Answer 17

L1 (~32KiB) Fastest, private per core
L2 (~0.5MiB) Slower than L1, may be shared by multiple cores
L3 (xMB) Faster than main mem, usually shared by all cores

Question 18

What are some issues with the concept of a cache?

Answer 18

• when to put a new item into the cache
• which cache line to put the new item in
• which item to remove from the cache when cache is full
• where to put a newly evicted item in the larger memory
• multiple cache synchronization
• how long is the data in cache valid (expiration)

Question 19

What is memoization?

Answer 19

Similar to caching

Optimization technique used to speed up programs by storing results of expensive computations

Question 20

What are the two parts of a typical I/O device?

Answer 20

Device Controller

Device

Question 21

What is the Device Controller?

Answer 21

A chip(s) that physically controls the device
Presents a simpler interface to OS

Question 22

What is a device (For hardware IO)

Answer 22

Connects to computer through controller

Follows some agreed upon standard of communication

Question 23

What is the Device Driver?

Answer 23

Software that talks to the device controller

Can be implemented as kernel modules and loaded on demand

Question 24

What is a BUS

Answer 24

A communication system for transferring data between different computer systems
Modern systems have multiple buses

Question 25

What is Booting? How does it work?

Answer 25

1. when the computer is booted, the BIOS is started (Basic Input Output System) is a program stored on motherboard 2. check the RAM, keyboard, other devices by scanning the ISA and PCI buses 3. record interrupt levels and I/O addresses of devices, or configure new ones 4. determine the boot device (ie. try list of devices stored in CMOS) 5. read & run primary boot loader program from first sector of boot device 6. read & run secondary boot loader from potentially another device 7. read in the OS from the active partition and start it 8. OS queries the BIOS to get the configuration information and initialize all device drivers in the kernel 9. OS creates a device table, and necessary background processes, then waits for I/O events

Question 26

What does BIOS stand for?

Answer 26

Basic Input Output System

Question 27

What does MBR stand for?

Answer 27

Master Boot Record - is the information in the first sector of any hard disk or diskette that identifies how and where an operating system is located so that it can be boot (loaded) into the computer's main storage or random access memory.

Question 28

What does GRUB stand for?

Answer 28

GRand Unified Bootloader

Question 29

What does the OS do after it has been read in?

Answer 29

OS queries the BIOS to get the configuration information and initialize all device drivers in the kernel OS creates a device table, and necessary background processes, then waits for I/O events

Question 30

What is the kernel?

Answer 30

'Heart' of the OS Running at all times Provides services to applications via system calls Handles all interrupts

Question 31

Where is the kernel located

Answer 31

Located and started by a bootstrap program (boot loader)

Question 32

Are application programs part of the OS?

Answer 32

NO

Question 33

Are system programs part of the OS? The Kernel?

Answer 33

YES NO

Question 34

What are the two privilege levels of a CPU?

Answer 34

Kernel mode | User mode

Question 35

Which register controls the CPU mode?

Answer 35

Status register

Question 36

What is allowed in Kernel Mode?

Answer 36

* all instructions are allowed * all I/O operations are allowed * all memory can be accessed * note: most of the kernel runs in kernel mode

Question 37

What is allowed in User Mode?

Answer 37

• only subset of operations are allowed • eg. accessing the status register is disallowed (of course), I/O instructions not allowed, access to some parts of memory not allowed, … • illegal instructions result in traps (exceptions)

Question 38

Which applications run in kernel mode?

Answer 38

All of them - System and Application programs

Question 39

Why would a program want to talk to the kernel?

Answer 39

To do I/O

Question 40

What is a system call?

Answer 40

A function that a program can call to interact with the operating system

Question 41

What is a trap?

Answer 41

An exception in a user process. caused by div/0 or invalid memory access or to invoke a system call

Question 42

When a trap is called what mode is the cpu in?

Answer 42

Kernel mode

Question 43

Invoking a system call will invoke a trap | True or False?

Answer 43

True

Question 44

Applications run in user mode. | True or False?

Answer 44

True

Question 45

Device Drivers run in kernel mode. | True or False?

Answer 45

Both. Usually in kernel mode, but could be user mode as well.