SYSTEM OPERATING WINDOWS2011GAS.docx

8/18/2019 SYSTEM OPERATING WINDOWS2011GAS.docx

1/10

SYSTEM OPERATING WINDOWS2011GAS

1.7.5 Virtual Machin!

The initial releases of OS/360 were strictly batch systems.

Nevertheless, many

360 users wanted to be able to work interactively at a terminal,

so various rou!s,

both inside and outside "#$, decided to write timesharin

systems for it. The official

"#$ timesharin system, TSS/360, was delivered late, and when it

finally arrived

it was so bi and slow that few sites converted to it. "t was

eventually abandoned

after its develo!ment had consumed some %&0 million '(raham,

)*+0. #ut

a rou! at "#$-s Scientific enter in ambride, $assachusetts,

!roduced a radically

different system that "#$ eventually acce!ted as a !roduct.

linear descendant

of it, called "#VM , is now widely used on "#$-s current

mainframes, the

Series, which are heavily used in lare cor!orate data centers,

for e1am!le, as

e2commerce servers that handle hundreds or thousands of

transactions !er second

and use databases whose sies run to millions of iabytes.

VM#$70

This system, oriinally called /$S and later renamed 4$/3+0

'Seawriht


2/10

and $ac5innon, )*+*, was based on an astute observation a

timesharin system

!rovides ') multi!rorammin and '7 an e1tended machine with a

more convenient

interface than the bare hardware. The essence of 4$/3+0 is to

com!letely

se!arate these two functions.

The heart of the system, known as the %irtual &achin &'nit'r,

runs on the

bare hardware and does the multi!rorammin, !rovidin not one,

but several virtual

machines to the ne1t layer u!, as shown in 8i. )279. :owever,

unlike all

other o!eratin systems, these virtual machines are not e1tended

machines, with

files and other nice features. "nstead, they are exact co!ies of

the bare hardware, includin

kernel/user mode, "/O, interru!ts, and everythin else the real

machine has.

"/O instructions here

Tra! here

Tra! here

System calls here

4irtual 3+0s

$S $S $S

4$/3+0

3+0 #are hardware

(i)ur 1*2+. The structure of 4$/3+0 with $S.

#ecause each virtual machine is identical to the true hardware,

each one can


3/10


4/10

new !rocess and the T?# flushed, to et rid of traces of the

!reviously e1ecutin

!rocess. The new !rocess- !ae table has to be made current,

usually by co!yin it

or a !ointer to it to some hardware reister's. O!tionally,

some or all of the !rocess-

!aes can be brouht into memory to reduce the number of !ae

faults initially

'e.., it is certain that the !ae !ointed to by the !roram

counter will be

needed.

=hen a !ae fault occurs, the o!eratin system has to read out

hardware reisters

to determine which virtual address caused the fault. 8rom this

information, it

must com!ute which !ae is needed and locate that !ae on disk.

"t must then find

an available !ae frame in which to !ut the new !ae, evictin

some old !ae if

need be. Then it must read the needed !ae into the !ae frame.

8inally, it must

back u! the !roram counter to have it !oint to the faultin

instruction and let that

instruction e1ecute aain.

=hen a !rocess e1its, the o!eratin system must release its !ae

table, its

!aes, and the disk s!ace that the !aes occu!y when they are on

disk. "f some of


5/10

the !aes are shared with other !rocesses, the !aes in memory

and on disk can be

released only when the last !rocess usin them has terminated.

$.,.2 Pa) (ault -anlin)

=e are finally in a !osition to describe in detail what ha!!ens

on a !ae fault.

The se reisters.

7. n assembly2code routine is started to save the eneral

reisters and

other volatile information, to kee! the o!eratin system from

destroyin

it. This routine calls the o!eratin system as a !rocedure.

3. The o!eratin system discovers that a !ae fault has

occurred, and

tries to discover which virtual !ae is needed. Often one of the

hardware

reisters contains this information. "f not, the o!eratin

system

or bitma!s are used to kee! track of free storae and how many

sectors there are in

a loical disk block are of no interest, althouh they are of

reat im!ortance to the


6/10

desiners of the file system. 8or this reason, we have

structured the cha!ter as several

sections. The first two are concerned with the user interface to

files and directories,

res!ectively. Then comes a detailed discussion of how the file

system is im!lemented

and manaed. 8inally, we iv e some e1am!les of real file

systems.

/.1 (IES

"n the followin !aes we will look at files from the user-s

!oint of view, that

is, how they are used and what !ro!erties they hav e.

/.1.1 (il Na&in)

file is an abstraction mechanism. "t !rovides a way to store

information on

the disk and read it back later. This must be done in such a way

as to shield the

user from the details of how and where the information is

stored, and how the disks

actually work.

robably the most im!ortant characteristic of any abstraction

mechanism is the

way the ob@ects bein manaed are named, so we will start our

e1amination of file

systems with the sub@ect of file namin. =hen a !rocess creates

a file, it ives the

file a name. =hen the !rocess terminates, the file continues to

e1ist and can be accessed


7/10


8/10

e1tensions to 8T 2)6, leadin to (AT*$2, but these two are


9/10

The first e1tension allows file names u! to 6E characters. The

second e1tension

enables the use of the >nicode character set for file names.

This e1tension is im!ortant

for software intended for use in countries that do not use the

?atin al!habet,

such as Da!an, "srael, and (reece. Since >nicode characters are

7 bytes, the

ma1imum file name in Doliet occu!ies )79 bytes.

?ike Cock Cide, the limitation on directory nestin is removed

by Doliet. ;irectories

can be nested as dee!ly as needed. 8inally, directory names can

have e1tensions.

"t is not clear why this e1tension was included, since =indows

directories

virtually never use e1tensions, but maybe some day they will.

/., RESEAR- ON (IE SYSTEMS

8ile systems have always attracted more research than other

!arts of the o!eratin

system and that is still the case. Fntire conferences such as

8ST, $SST,

and NS, are devoted larely to file and storae systems. =hile

standard file systems

are fairly well understood, there is still


10/10

Oh, 70)7B and Ghan et al., 70)3a, erasin data securely '=ei

et al., 70)), file

com!ression ':arnik et al., 70)3, flash file systems 'No, 70)7B

ark and Shen,

70)7B and Narayanan, 700*, !erformance '?eventhal, 70)3B and

Schindler et al.,

70)), C"; '$oon and Ceddy, 70)3, reliability and recovery

from errors 'hidambaram

et al., 70)3B $a et. al, 70)3B $c5usick, 70)7B and 4an

$oolenbroek et

al., 70)7, user2level file systems 'Ca@arhia and (ehani,

70)0, verifyin consistency

'8ryer et al., 70)7, and versionin file systems '$ashtiadeh

et al., 70)3.

Dust measurin what is actually oin in a file system is also a

research to!ic ':arter

et al., 70)7.

Security is a !erennial to!ic '#otelho et al., 70)3B ?i et al.,

70)3cB and ?orch

et al., 70)3. "n contrast, a hot new to!ic is cloud file

systems '$aurek et al.,

Documents

SYSTEM OPERATING WINDOWS2011GAS.docx