Memory Management in Operating Systems, Assignments of Operating Systems

Address Spaces Memory API Address Translation Free Space Management Paging Segmentation Translation Lookaside Buffer (TLB) Swapping Demand Paging Thrashing Page Replacement Algorithms

Typology: Assignments

2024/2025

Uploaded on 09/28/2025

pvkkr
pvkkr 🇮🇳

1 document

1 / 31

Toggle sidebar

This page cannot be seen from the preview

Don't miss anything!

bg1
pf3
pf4
pf5
pf8
pf9
pfa
pfd
pfe
pff
pf12
pf13
pf14
pf15
pf16
pf17
pf18
pf19
pf1a
pf1b
pf1c
pf1d
pf1e
pf1f

Partial preview of the text

Download Memory Management in Operating Systems and more Assignments Operating Systems in PDF only on Docsity!

HA-3.° 24000 £000 € P- Vertada k\shor >) EXPlLan Yow address space Lorottey em Mane. System Secority g amon rece. | we COMEEE SD opaderers space ond | address tramsation wlan Uitewe zramelos so Add ver space Loletion & Secor ity, y Worl \o nun. addvess Space} e hn oddress Space & tle ¥ avege of Mmervly - Add vesex, -theF & Process S34 brog ran CONN vee : e Every prodrees in os Ld Glen Orr pr\ucbe address> wace x Logicot L Virctear Joddressey Whal- te precey Seetp Whlde Phystcad eddrersesarg aotug locoten Mm REMY Why @ddvess. Space Laatation (s tmeortat 1, prevents processes From tnterbacng om LOM otner . ‘e wt > Rkampe: A process rans over B preess Memo a& Enhonse 3ecwrity, ; epreverys Mokleroue pro grams trom veo" ld oa ee eee ee ee NEN o Pew’ OY er Pe ye AU FEN ony Pree. a-hewes 10 erece® stably, ecvasmhes by CVI IN Me Press don't omer Address Trarslat®n concert # log \od oddvess CLA), Addveas generate, G egane CPUTEPBG ‘Prog aH exdertion ~ Laphysccedl’ 8S CapCHAT! ‘ncsdk Adres sip “ pawnnaneay a2 RS rosaees oe « Wwarlation; *he ose veucbu Map logicat's Ply step ‘addresses Example of addvers *TOMSA tion: FMOQiNe , - of PYOCAS Seon mesiov “prot ote +14 ve ThE OS Maps 14 to Physleol meno Sarnwg of Lovo, Co qtcat Odden, pus neod address ie) ; (060 0 (o 50 — LL oo 4 4ag CRAM: oHere, Gore seqiers (000 CPrups cor Stars) (rant * Memrony - PUDrectio ? thal ProerA ma te CUED LAs SOOD> Oddeecw H bond 2 femeratey a Protection 0 av, Peguies = 500 = carat at POE TREE A y Pad kone = SS ee tian » Know Were ™ 1, programs clone Phil Brod meme, arn | BD Nay ga SEN ASS manne ewes On AIGA pay w4 t BOL? ane CEL THM CONPINATON Ensure cach er ane StOY in Ue oun Memory Area Proves. {mother PYOlCeESRy and tLe. of eX ey por ; @ pemomrrate Ko Fone tionality op we meroTf API Qn& oPpy Herory nmangmedr concepts to ceprarnte duferenk types of- MEMOTY Qesatated corth Y. pel Memory APT avd Types of Memayy - Wohak lp GA Memory API? cb tremory API PYOVIdes FOrcHop thot Programmer Lée to Tequesh -Manroge, ans free@ Memory dosing progtom -execwtlon ft Connects He Geplicotion win tbe Operating System a. Memory mong), Bgy Who ig prea: Merton AO\ inciaders fonctios (awe emauoc (2e) * courte (AdtheePe cn, se)? &wttabize AO “O° autocad e weroty Qquocake memart. eyeallotcpir, %izEID Terre BEM gn ; ebree Gr} => reese a other longuages a. eollectionm Werte onderlyire OS wre Cyova ryt) bw jocate d GS THohages net Ove twig cacy “wd 2 prem procesSes From ome tA MNeTrOTK a HEMP allocate Merteye AYMOMICOZ Er Fortine tyeeF oy Herory. in aprogzam gener & program VY loaded into Meng, 7 iy dudes wiro. drttergg memory 7EGIrr le stack, euped: ¥OF Fonction” Cal Sees ad es aased © Manogea -aLtommarrcally by Ave COMP tly, e Group and Shrinid dyramica ligudn ? Bounction alls loa vowia res 2. Heap: used For dynamic programmer erplicrity DeqQvessd memoy wy mallott? Gre yreats bse kreeg menosy ariocevion ap lite TJ Mey? nod breed > leads to memory leaks 2. paka segment + Stor ep grovokt Lem ayied) ANA Statre VO © dwided \nto; . votes ) = inwralized Oala C verane wih asigne > Unwarrobizad SS segmect) aw PROBES Segeise oe awd © TranslaalOon eccors © PFOF*FAMNP dom vera & PFOYFAMD Can be rE lacaled IA RAM EMILY © Secortty & \solarion vrw erecesse? Se. Bddvess transatio Lang Baa & RegiaeysS Nardware Metvos tor protect, (mt othe simpte 94 O sransation 2 Gare Teguter— > — NHoved tre Storing Phuyricol aga veon He MEAS in memory © Limit Reg sey 7 Stores He L124 Crame y of ta proceeP! p loqicar AddVe*S Save, Translation roe f PA = G@a+x LA’ ! i LA varlaue ¢ UnStrdesiens mapee A Rnged ro. Menory adorersea tip praese 1b calles address bindirg, Co) Compile-time Binding ely We prow exachy mere He PYOyram WI vedade in meme ak Cam pie Hme one Bereratea proqram musk lwous be loaderah tro some \OCatilon - o&% wo He mba y % velocattoa NOt Passive en: —~ caompiter tires Storing oddvess gp fen instroction Load x (nuerted to 0Ot Hero x TE memory lotion w pod GUNNa ne tre Program Canis be\ooded (©) Load. time binding 2 CFACE Phy NAR \ecotion ty NOX trowap (ormphe Hme. - ° Compiler “generates Ye \oe@table todo. -C logico® addresses fo? Finer landing lp done Py ive loam Coen The Preq cam w leaded Into Memory ‘6 YW -Some Flex iar tty X can Mose dovg ae Section oe Com plies Benerates loan Lovet 200 * loader proves Program ak Pryaderey ray & Ema = 5 20a {c) Eaecotion. time Gnaug : Bwayy te deayed untt| tte progray \ ACAVAN GG To yone>”g), Requyred hardvcasa supe ¢ pia? | en) a, Sc | een re car. Se ee nT te! | Frame numver =found Ry 100 Icing up te pege tarle entry Using tte Page Num ber GS Inder. pA = (rome nomver % page size) ose. Complle time binding -» 4\xed al compile tre VOad time binding» deaded at loadtime Execotion time eirdire3 done. duynamicaty by Mrtu C suppores Paging, Qurriuad Menory), In POQWA , A24-bI-F Uivar addiregy sphip into ) {2-bU page NUMbert j2- bie O}+Se- and ~ page table maps tie Pege toa Physteak framoa, ® Nemonsivrate tre function op a Trarslaty, [dor aride Bother TLS) in gadress travalaton &\\usiate ite dy nificance, wwn_aneof Rl@cke O lagran | we) Translation |ookaside Bolten CTLB) Whol & TLB7 | We TR wa specie cache, ‘inide Cro “ store reeentig Oded Ulrtvued page 4 Pryieol trame mapeingys HY orea to - speedup wre address j Translatioy OF OCess ber rovnyng tote | ' Page rtae’ In morsry iy “low , _ i pune too of TLE. inadadress Sansa ite, 1. CP? DEANS OO Wy tun Aad res a. vrrruak pye numer ys Let He, 4. 4H CPU Creer tirst Check ive Tle th pete Hita the +¥arne Rornocy iy FONE Arrechy trom te Thee, FITLE Miss -+ne CPU Geer 46 tle page table in main Memory to sei +e frame NUMber, GNA then Vedaies tLe Ws. 4. Once tte ftrawe numer 4, ag . oh 2 combired qin Te P OB CHa 46 Lom ain, prignie@d address why TL® la wnportant > ¢ Wthoot TLR A eacn Memory acess Q memory lootay 1 one +04 PAM? Tang 2- Oretor actoae tavie data 6 Whos?! TLB ama) Avanslatiom axe toune immed iaiety Chu Yatio “YW OSvahy 40-4927 + thus, reducers EGectwe heceas hme Cean emaieyp, drosticany g's , ’ ar S POLE teo i Brock DIAgQ*TAM oO} Hddvess 7 Lady, pvee Monagmed neues loan TUB sgt : poner om PO Gram ye quest Memon, ss : : : : o+ ama — apes Memory bwees & @ th mem. , Later, When | weviwa address 7. auocot Perens 1, F rraage i Cav j freed + me o Age i & space iy nab a Page nomoey ot seh pasted : MOIN +tECKnLaques TLE lWorvp ? se ne ; a Sete, (TES “over | ¥ + Coales 1 Tre Miss + eens ¢ SMi.s. Bima, T13 HUD : ; A Unted WUSte S. pree Sts ei “when a pregram requests memory $69 Clears) ans X*2 OS hada emt : ; ye [ie Boe ne mee a eae i be * ONE [0068 blécr attocary p= OME Poe Lobe) ; ae = _—_—— = F pdwantages ~REedvien axsie op Memory es) Limitert lors + rey lerd ‘ *O many smait mre Draco C$F0G Mentatan ) + OnE (0OkKB block ¢ sittree) Tne TBacts Ire a fast shortcuf toxoddra troraletion avoiding «= Slow pagg tcveie lookups 2. Coalescing: in most Casep » Gnen adgocent free Blocys Rarsts que OS © Deveur & derrorsirate yee speue Meiges tem into ome \eryes tree blot, rae) “uy yedured Late, rrAanogmens *enniauer / \ncivarcg spliivg ? NEE HOG MEAL Ion (oalpacing , Hitmraes lnved Mets, & bree 3. Blkumapa: (WatS , with Sorrtavre Cranmpres the memorg Y dwided inig yuwed size 5 UMHS (eA) 4 ke). - * 6-747 bICth kas 4 =) alwcofed block 2 Smemeti’ Pragmentadienn eHappens ceoren tree memoy exbts Sut HUG Statler€d “in gmalr pleres ACYOSs memory ’ fren the totea Pree mm enogy ip best Fi > Finds Smomesd Free ney eb. FAP Cveduces ehrouer sro2k ) * Fi0 $14 > gioaten Hot aallone Hee ues . WOMt fiz y Por“ Perea In labigeaa sige’ ‘ bt leouirg g lef ove. tor laren Comporion : ee ~ type Cause footed Sire feamag, 7S pil tiwed bloc | jaside ailoalel protens Sire too bly block eqn bt - 3 Extesvad. | Free SPae. | Oui sige aoe aS Batti Alilocazed| 3 $™4)| Free L bloce blocty cant sO Satisky one a ie. \L lerges request gnrernol traqgmentart oie [Inside ploclys Coasted Space C¥tes nak pragmenrtttion = Gmsted Pace Ouro ide Yorn, oltocot ln Solwrlen= Cooipactiory, pasy ing Better allo Aigorithr sz, r mee. No intecrot asihialy «Be explain Vartoos menmoty aocat iy pav ? “% Svag aay deckmrqvuen for doth Contigvoy and Hon- Contr gous EMO rionag MWenb » Mearraghaing trew advantage @ (witatte, Qos J Meacory &llocation Tecunique : theve are. 2 wroad Cate gore: Y + Conti gove memory eWocalton 2 Non Contigova mMenrosy aloetica the Contlg oop Memo qitocabion . tden : ian Se * ae “protes GcuPle, ora sine Cominey oto ck ob merrory 1 oo ceempa a tane of wricn blocks a)rized Partition ng e memory swided to fired -dize par titLonp * ean -partlon nolds CeACIIY Oe PYOCes Pduwantage', samp 720234 +o Umelement XLimakten ; internar fragmenta ren » NO of PTCCES32D Nynited t. Nook PO ena |B). Py namie partitionirg: * partitlons ase created process are e EMeM precede gore eractld tre amon ~ ok Memory WwW reqvuedrt,, Aygnamical y as a [ynnitcctlon : €tterma +ragmentation (yee spacey poy Scottereay \location Ssrro1e- c )A\ “ MEDD Cred wn Aynarnic @\wocad Ton: eevee 4\t= Alorate Wrst bree Chocy leap. treat CIFOR pe ray Carpe FIOGMEMTO eR of beggnryg ). » pect fit J Sllocote srraile st Free WW ric ted EMD (Minimizes [ereouey Ub slous Search) + WOsst FH A A\ocate laagest tree Clee C Leven lomrger Free dlock tor t+erure, bet not always, pep mat) a) hon Contigous Meeovy \oced On. A procens oern+ a5> need to ke Strvedina Single block. VW Can be split (ntd SmMalier PIE te@y SPread ACCYOSS — phy dicad MeMory oS mairaind MmAPPHY logical BD Phy Neod. addvert a) OAD: we dn j precesS WV EAC S into Roged Cine dave 400) Prey stcal MEMOTYA — qiced AMO dveme Any paye an 2 locrdea (nto ary trame, Aduantage 2 No ttternct pragmentara, _ Lawns atten! eS Camp # (arerral pregved Wee atlon (last aye may Nal r Mapped to Some lrame in RAM, Benekis of pera tay : } a ee : y "Remover cxremakl breqmemoanon Chine q Memory Ww Allocared. * Allow, men ovy ny dued- size cMonksy NON- Cons \g oud allocation o}- —) IMPTOVE, » L4'L120 Fon. * Simplutles Allocation Con Need fre Hams faa). Limitations of Pqing ; “int er nag 4 ¥Og menratien C \oat frame may Not ve tolly vaedD " overhead ef page tales Charge PYOCE3%q aur lange PAGE ta bien ‘Slower addvess translation (€ needs Page tary] lookup ¢ Sbuing OF1NA THB CtranSlatign Leorosde Botte, D) R remory Biccation strategies, CON UNGLOD MEeMoty ANVecortig Schemes Cnort Paging, were Memory yK CONS dered alisi of Free. HOC \csC Moles) ang AUocased blocks, tnLe aAPAly Wm - 2) Pirsd fiz - AWocate HE Hrs, Bock thar to leage eraegh 2 eg: Os yee ccks [ jwOkB, TOK G, 200k 8, Boakp, Gookead . Processes . 22k, > Pirst Fu putg In Sooke block “firsi Sutad i 24" bree Bloc 5- ; Eloo¢a, soe 206, 200, Proress = Dre boo S 7 O20FST Bik ete in bee 6 SlecKE [longest Sxa\la bie] a seay ae PIOS cons Bas Pg cay _ UA FU Cirsi Sortable \Pocartion $722 ment mae oT Se 4 = j tlonaF rir Bes. Fit Strate. | Lae | Uleak Scnabe Min iHe: S\oux+ hol. teste Seascr, w : / Space Creates mn - M unsab iene + a ao ley oy & Worst fit (axgeat % [Feeer medromn\ oy oa batchore | oer oY be ° | fetore. ea DWD | Chants , + ————— to) SLOSS AYMENcer Problems o4 Poging & OdkTESS tranWatlor Car catucurole tre YO ok PXGeP EBIZS Of- lve poze tabic for OL SYSTEM coAH % 24°F Virtua addregs — [LBEEB play or1cok Memory 2 ke Paamye Sve ,% 4 byte page rtavie WIM QE PAG Size Chere page 0-3 We Mapped to frames 17 3,2 2G Ae the eh 4 S\COR address Virtua Adare s sos Corvesponding to te, 2760 C dectrmad ) Nis tvar addwcas space Go‘ 6+! S6k-6o) SLE Sb 48e-S2 44K- 4 § erwad wpe - - Duilze tre pagerable Provide inte quay } Here typ calverWe e determine TL empreot oaddverns OPAL FOOD vartwal abdversep, oa) (2) tleo CAD 2A2AKD fe; 2 Cc) 830 von: ae Y et ae VMN -Cb) For H ston, Correaspendirg © cach A DB oO A OE Ss A ity ¢ =X cnot Present) 4° % 8: x 4 = (oO. x Ch og (2 TL 20500 /agagg Jos" obtsery= ROSOG -2WO*IO = RO PMY 5 > frame 3. — 12,2363: = uw ewe Bx *orb 20 @® Desecrve row Segerertation new Wrorganta Pragramp into module? Awl Segmentation in. Memory Mancge ment ? pet: 5 Segmentation s O.memsoyy TTUPAGMER! ‘eounine where tre \SGlcak addrers oy aApsacery, u dwided wto vormave- Sized seqmeny , each DEPFESENIINS, §Alogicad CALF of Ave pres Hapis GUM 2 functions, ; Astdsy Staccpa 1 mass Eact, Seamer nos: Ve > segymerd nomper Fiber Mase) {| “> segment bese addres ( stattirg, \ocaten i) WY Phigsicag Memory ) => Segment lergar © emt td se qmet) (©Gi\co’ Bldvrers— y OllLeoss’y we gre ot Ve C Segenert. NU rVvbeRg te [a Werle | re or OO wa @ ene Pa.) reaps Sle Cheer Ceartlcms, WOM OATES to me whergtra Segrcenak\ore “ei eg yor how prod ar aaniwe. MMO Module, 4 pog cer S°AB(On of A Prog Tar, ° m rm set 4 prog COM. SPU hte See rest Perso, kK prin FOACTION, proceaeres, doto 3 ivetore, nace SyAM~O} zovles, libraries 2, = eteo2 ~ x. each mode Y Stored ines OULON Hoenene @ andcependerr prouwtn BR erwtec tig, eacn Seqgmerklor Hoon limit Preven, ane MoOasie From accessing Porrtsttg anomer- eg * Brack’ QFOusp enacperdemtty OF Glo wm, dakar seq mMeEnNrA 3. Ease CF SHOUD Comm M4 ped Moss Fre raya: fan be Placed IM Semvate seaqments 2% ADA Cotten: : 4- Seppor-t FO¥ Madoian PI OG varning. e€ NCOo? OO Speeckured ara = ymode\|ay Pry wan des ign swnee MoadkS Gan Compred ANA leaded ince rendentyy Se es Rynamee liaying Revo cadre, Only le segment ere address needs tO be CManged . esvherra ‘us moved MEMOVY —7 SIMPLE TelocottoNZ eh a kl) > ey ee i Examples, Soppe se AProgan I drvided roto Segment, © Segment 6: moan proysam CIO KB) ® Segmen 4. Assay orkS) © segmenr 2: [ounction \Wosary 6 8kG) eSseqment 3: stackc +S). Cam Yas tt own ware sinreah "Wn the Segment AUANS . wlen tre cpu. gem rates. @ logical adsérers KE 62,1000 7 Id mean, © Segment 2 C}unction Wosary Jottse. low. * the NNO add $ of-Set 7£ 10) 9 tle eae. Adress of 5 egment Q to prt tre Address PhegPcog > Seger tartar mince the 10g sarmmeA Logteal Weus ot memon,, AWowire PxOgraQnD to be peevided into legiag- motes tee Code, dara, Maco ® VWbraries. thy _ Umerovep modvolasrity) Protection » sranvg- DW ret ocatten. Vorng tre vworval addweas sere stro segment gegurer Vawe Gwen tre Poe place © Segment in Shy sical Memory, ij QxsemMe A Teperence "WS made to varrva address (00, 4290, 1168 15360 owsit]e Flow al Qe@ai taro Toque a seq MALMLATION contro\ > | berg segment” y (del VPI ae ace yee yore ove as Signe Boxe | Spas Code a2e pe Dd eae BAT aK gto’ aed — seq ret reg whew Vales’ piace ale Seg ment, WTO Pa oiceQ® memay rs a poarsiare Caen Gwen ~ Sta addres ona 4. owe ‘asvorh, exam -Teady Searme ntaken ContTol-FlL0U> ~—_ ¢psewdcco se) OPINg tle segment — SEQet ers e Code dsegmert OccoPlep OLB — AB C se 2B. > eAD OccuoPle VWittveg Addres 4¥R-TIrAl rae VystaaQ add wrsey @ SACK Gecople> Yrwadaddvess 14-16. racsue 2KC ID ploce 2] SEOMEMA In pryncs MEMOY Coarg-a) Consest cose toKnste? and add stzm. © cose © . eee = Breas B2AxX toate B2 TES MSY SVC» WB Mm ~KlO24 = IGtSE bytes ery coRtarye = 20,465 --. B2 768 + DoAS-( = Baws-- St sls— pe ¢ B2kR~ Sto olimyt« Mae ag tha segment. —— A. check por Valid ty . reqrnen “\F ovsep Dilwni then wale See Mor Foolh ¢ wvad access) S Catecolate Plug near address: > reer olset sphyrical addres 4omem, 6 Send He phy orcad adaress “ a Lor acessing tte seQuired Instrocton /date. : TLA c Ex piain He wole oF ee dering prowen erecotton Ans} Roveof TLG conter Seartcuing, Por'ng- proces Execution ; |. Whok ‘© TLG? e tle TLAL speclaR, SMa, bign-speed cacte wtrne ceo “ Et Storer recert Marepires From Urtia.d POR? ROMID Ob wien Alo rag tbe) . Pome KOI, \oox oes, & War hopeem dozing o cootexf Switch 2A comer SOMCH OcCorA Gen to ceucress FFOM CrecatlRS ONE proceas 40 Ono tes] €ach process ros 1+ Eton Rage tabe Utstoad to — Pry col Pca pre TEA raQsy ny comtaiws enrvye, F¥Ory mot pIelOY eroce ty Ve TLE enkrieg \eadunp ko “Trane yee rN0TA = Accor», pole ot TELS In Contert sSatichnwg, fe Conteak’ SWiiching, 442 OS mop Lets avel- tLe ThA, erie tle Convery addve*$ Seace toy tie new Process CALs" Can BE: handies ~ in. A Mrarn, tary, aE PN Ctriva dae On Sutth) FUME Basticn ing Processes b, Me oS clea, +e tLe a 7a the New proces yelaqas dem ONE (lA Rege tame 7) Sbmple “Pot increases Gve Freavernr Tus misses FlGenrya cn le okups) : RA depte %% Togaed TB 4 ddvess Space \cerinlo: 2 7 CAME tLe energy LA AOQQeEd wi ar) . dentiteyY Fo: pe process VEINS Content Stattevirg He OS soap Changep. +¥e 4S(o Yea ster. Old enever remain in TLOQ ant ese ignores For 4X6 new proceap GANG 1293 dont Meen ) Redoc® Qreshead, UMP roten Pesipasmance: if b py ADV OONMEN Ce ¢ etc), Yoy ts dws IMportany ¢ | eensvuvep MEMOR) FOC CMON? CNG PYOCeENy | | | Candy access anoitte wv MEMO *Mcantous Come ct addveas tranlehio done, MaAaatastins — eimproves PEXFormance Vy 2edvcing He Cost OFF2€Q0EM Content — SuttCrey ' PLTIPg® ® BDNtez db Sasrtch, Ala oS Mark Clie, Hoenra we 46105 INnThQ -entrled So tt.) eacyn. etece» OS Only it cen wha Urwes to Petcal Mapeirgy ~hy Gronrentees em Seeonty & rE eo t now Meniway Ucers (2 A 3susxem L>5G> A evel Pazpire, a TLS. We PRametes cse P TLS loorup twme = lone OCCR> 3 tire ~mertery 2 laon> ° TLG hit ratw> 90% ° BSt CORR APArectlon TeQurea gs memory Aes (C1 Woreectlar fetch + | debracg e pag Htovid yare . 1 A 10,000 © Pap faut seovire Atume. Reps Cartkesole tre epecstwe Cteustion TAQ C EIT) tomidering mentony acexe) Ard page Fovlty, be TO occe?? one ALG Ory wr wath, (a B> 9 Aerie, pen ervey A VN Page ete ony rear a POe ren, ALCOKES Te read f 4 He t REMC rs Atcenn +e AOAXNOLD doy Lirat toch, C356 ent Nas pin» +2 4 Merron, Alem PD tO “d6, tras ete ere) © PK HOF vob) ans per memary, Occes, eTIM® Conuerte in C te \Yrne_ feoc Coons in cog =6-Co6} Jy 1) Cort pes Memory access ( \etosuney TL (00%ve ) + TLG lCotwtime = lon. . menory access time \e TEA wiee = loons TLE \oct pe menary accer a 10% 10 = lens ‘ Te \COtoer Cease AUME OfCease + Lackebon. ; . RS) x monn - ~accey Te TLS mass 2 cosy 5 'XBo= FP qt3] =/ng Wire. ; cacr leoyrectte Tea res Dmemeny cvey C Gere Adora } _SOmemary . oot As; Rue OE eres oo Beacon» @ MAO Cork Pex