Chapter 8 Operating System Support

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Willia m St a llings

Com put e r Orga niza t ion a nd Arc hit e c t ure

7t h _{Edit ion}

Cha pt e r 8 Cha pt e r 8

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Obje c t ive s a nd Func t ions

• Convenience

—Making t he com put er easier t o use • Efficiency

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Ope ra t ing Syst e m Se rvic e s

• Program creat ion

• Program execut ion

• Access t o I / O devices

• Cont rolled access t o files

• Syst em access

• Error det ect ion and response

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T ype s of Ope ra t ing Syst e m

• I nt eract ive

• Bat ch

• Single program ( Uni- program m ing)

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Ea rly Syst e m s

• Lat e 1940s t o m id 1950s

• No Operat ing Syst em

• Program s int eract direct ly wit h hardware

• Two m ain problem s:

—Scheduling

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Sim ple Ba t c h Syst e m s

• Resident Monit or program

• Users subm it j obs t o operat or

• Operat or bat ches j obs

• Monit or cont rols sequence of event s t o process bat ch

process bat ch

• When one j ob is finished, cont rol ret urns t o Monit or which reads next j ob

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J ob Cont rol La ngua ge

• I nst ruct ions t o Monit or

• Usually denot ed by $

• e.g.

—$JOB

—$FTN

—... Som e Fort ran inst ruct ions

—$LOAD

—$RUN

—... Som e dat a

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De sira ble H a rdw a re Fe a t ure s

• Mem ory prot ect ion

—To prot ect t he Monit or • Tim er

—To prevent a j ob m onopolizing t he syst em • Privileged inst ruct ions

• Privileged inst ruct ions

—Only execut ed by Monit or

—e.g. I / O • I nt errupt s

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M ult i-progra m m e d Ba t c h Syst e m s

• I / O devices very slow

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T im e Sha ring Syst e m s

• Allow users t o int eract direct ly wit h t he com put er

—i.e. I nt eract ive

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Sc he duling

• Key t o m ult i- program m ing

• Long t erm

• Medium t erm

• Short t erm

• I / O

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Long T e rm Sc he duling

• Det erm ines which program s are subm it t ed for processing

• i.e. cont rols t he degree of m ult i-program m ing

• Once subm it t ed, a j ob becom es a process for t he short t erm scheduler

for t he short t erm scheduler

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M e dium T e rm Sc he duling

• Part of t he swapping funct ion ( lat er…)

• Usually based on t he need t o m anage m ult i- program m ing

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Short T e rm Sc he dule r

• Dispat cher

• Fine grained decisions of which j ob t o execut e next

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Proc e ss Cont rol Bloc k

• I dent ifier

• St at e

• Priorit y

• Program count er

• Mem ory point ers

• Cont ext dat a

• I / O st at us

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M e m ory M a na ge m e nt

• Uni- program

—Mem ory split int o t wo

—One for Operat ing Syst em ( m onit or)

—One for current ly execut ing program • Mult i- program

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Sw a pping

• Problem : I / O is so slow com pared wit h CPU t hat even in m ult i- program m ing

syst em , CPU can be idle m ost of t he t im e

• Solut ions:

—I ncrease m ain m em ory

– Expensive

– Leads t o larger program s

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Wha t is Sw a pping?

• Long t erm queue of processes st ored on disk

• Processes “ swapped” in as space becom es available

• As a process com plet es it is m oved out of m ain m em ory

m ain m em ory

• I f none of t he processes in m em ory are ready ( i.e. all I / O blocked)

—Swap out a blocked process t o int erm ediat e queue

—Swap in a ready process or a new process

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Pa rt it ioning

• Split t ing m em ory int o sect ions t o allocat e t o processes ( including Operat ing Syst em )

• Fixed- sized part it ions

—May not be equal size

—Process is fit t ed int o sm allest hole t hat will t ake it ( best fit )

t ake it ( best fit )

—Som e wast ed m em ory

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Fix e d

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V a ria ble Size d Pa rt it ions (1 )

• Allocat e exact ly t he required m em ory t o a process

• This leads t o a hole at t he end of m em ory, t oo sm all t o use

—Only one sm all hole - less wast e

• When all processes are blocked, swap out

• When all processes are blocked, swap out a process and bring in anot her

• New process m ay be sm aller t han swapped out process

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V a ria ble Size d Pa rt it ions (2 )

• Event ually have lot s of holes ( fragm ent at ion)

• Solut ions:

—Coalesce - Join adj acent holes int o one large hole

—Com pact ion - From t im e t o t im e go t hrough

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Re loc a t ion

• No guarant ee t hat process will load int o t he sam e place in m em ory

• I nst ruct ions cont ain addresses

—Locat ions of dat a

—Addresses for inst ruct ions ( branching)

• Logical address - relat ive t o beginning of program

• Physical address - act ual locat ion in m em ory ( t his t im e)

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Pa ging

• Split m em ory int o equal sized, sm all chunks - page fram es

• Split program s ( processes) int o equal sized sm all chunks - pages

• Allocat e t he required num ber page fram es t o a process

t o a process

• Operat ing Syst em m aint ains list of free fram es

• A process does not require cont iguous page fram es

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V irt ua l M e m ory

• Dem and paging

—Do not require all pages of a process in m em ory

—Bring in pages as required • Page fault

—Required page is not in m em ory

—Operat ing Syst em m ust swap in required page

—May need t o swap out a page t o m ake space

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T hra shing

• Too m any processes in t oo lit t le m em ory

• Operat ing Syst em spends all it s t im e swapping

• Lit t le or no real work is done

• Disk light is on all t he t im e

• Solut ions

—Good page replacem ent algorit hm s

—Reduce num ber of processes running

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Bonus

• We do not need all of a process in m em ory for it t o run

• We can swap in pages as required

• So - we can now run processes t hat are bigger t han t ot al m em ory available!

• Main m em ory is called real m em ory

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T ra nsla t ion Look a side Buffe r

• Every virt ual m em ory reference causes t wo physical m em ory access

—Fet ch page t able ent ry

—Fet ch dat a

• Use special cache for page t able

—TLB

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Se gm e nt a t ion

• Paging is not ( usually) visible t o t he program m er

• Segm ent at ion is visible t o t he program m er

• Usually different segm ent s allocat ed t o program and dat a

program and dat a

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Adva nt a ge s of Se gm e nt a t ion

• Sim plifies handling of growing dat a st ruct ures

• Allows program s t o be alt ered and

recom piled independent ly, wit hout re-linking and re- loading

• Lends it self t o sharing am ong processes

• Lends it self t o prot ect ion

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Pe nt ium I I

• Hardware for segm ent at ion and paging

• Unsegm ent ed unpaged

— virt ual address = physical address

— Low com plexit y

— High perform ance

• Unsegm ent ed paged

— Mem ory viewed as paged linear address space

— Prot ect ion and m anagem ent via paging

— Berkeley UNI X

• Segm ent ed unpaged

— Collect ion of local address spaces

— Prot ect ion t o single byt e level

— Translat ion t able needed is on chip when segm ent is in m em ory

• Segm ent ed paged

— Segm ent at ion used t o define logical m em ory part it ions subj ect t o access cont rol

— Paging m anages allocat ion of m em ory wit hin part it ions

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Pe nt ium I I Se gm e nt a t ion

• Each virt ual address is 16- bit segm ent and 32- bit offset

• 2 bit s of segm ent are prot ect ion m echanism

• 14 bit s specify segm ent

• Unsegm ent ed virt ual m em ory 232 ₌ 4Gbyt es

• Segm ent ed 246_{= 64 t erabyt es}

—Can be larger – depends on which process is act ive

—Half ( 8K segm ent s of 4Gbyt es) is global

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Pe nt ium I I Prot e c t ion

• Prot ect ion bit s give 4 levels of privilege

—0 m ost prot ect ed, 3 least

—Use of levels soft ware dependent

—Usually level 3 for applicat ions, level 1 for O/ S and level 0 for kernel ( level 2 not used)

—Level 2 m ay be used for apps t hat have

—Level 2 m ay be used for apps t hat have int ernal securit y e.g. dat abase

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Pe nt ium I I Pa ging

• Segm ent at ion m ay be disabled

—I n which case linear address space is used • Two level page t able lookup

—First , page direct ory

– 1024 ent ries m ax

– Split s 4G linear m em ory int o 1024 page groups of

– Split s 4G linear m em ory int o 1024 page groups of 4Mbyt e

– Each page t able has 1024 ent ries corresponding t o 4Kbyt e pages

– Can use one page direct ory for all processes, one per process or m ixt ure

– Page direct ory for current process always in m em ory

—Use TLB holding 32 page t able ent ries

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Pow e rPC M e m ory M a na ge m e nt H a rdw a re

• 32 bit – paging wit h sim ple segm ent at ion

—64 bit paging wit h m ore powerful segm ent at ion

• Or, bot h do block address t ranslat ion

—Map 4 large blocks of inst ruct ions & 4 of m em ory t o bypass paging

—e.g. OS t ables or graphics fram e buffers • 32 bit effect ive address

—12 bit byt e select or

– = 4kbyt e pages

—16 bit page id

– 64k pages per segm ent

—4 bit s indicat e one of 16 segm ent regist ers

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Re quire d Re a ding

• St allings chapt er 8

• St allings, W. [ 2004] Operat ing Syst em s, Pearson