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AnneBracy
CS3410
ComputerScience
CornellUniversity
The slides were originally created by Deniz ALTINBUKEN.
P&HChapter 4.9,pages445–452,appendix A.7
Towhatextentdoestheclickergradecomponent
affectyourclassattendance?
A) Theclickersdonotaffectmyclassattendance.
B) Iattendthisclassslightlymoreoftenbecauseof
theclickers.
C) Iftherewerenoclickers,Iwouldbeherewayless
often.
D) Myclickerisansweringthisquestion,becausemy
friendisholdingmyclicker.Iamstillinbed.
E) Noneofthesedescribesme.
2
• Managesallofthesoftwareandhardware
onthecomputer
• Manyprocessesrunningatthesametime,
requiringresources
• CPU,Memory,Storage,etc.
• TheOperatingSystemmultiplexes these
resourcesamongstdifferentprocesses,
andisolates andprotects processesfrom
oneanother!
3
• OperatingSystem(OS)isatrustedmediator:
• Safecontroltransferbetweenprocesses
• Isolation(memory,registers)ofprocesses
untrusted
trusted
P1
P2
P3
P4
VM
filesystem net
driver driver
OS
MMU CPU disk network
card
software
hardware
4
Youarewhatyouexecute.
Brain
Personalities:
hailstone_recursive
MicrosoftWord
Minecraft
Linuxß yes,thisisjustsoftwarelike
everyotherprogram
thatrunsontheCPU
Aretheyallequal?
5
• Onlytrusted processesshouldaccess&
changeimportantthings
• EditingTLB,PageTables,OScode,OS$sp,
OS$fp…
• Ifanuntrusted processcouldchangethe
OS’$sp/$fp/$gp/etc.,OSwouldcrash!
6
CPUModeBitin ProcessStatusRegister
• Manybitsaboutthecurrentprocess
• Modebitisjustoneofthem
• Modebit:
• 0=usermode=untrusted:
“Privileged”instructionsandregistersare
disabledbyCPU
• 1=kernelmode=trusted
Allinstructionsandregistersareenabled
7
1. Bootsequence
• loadfirstsectorofdisk(containingOScode)to
predeterminedaddressinmemory
• Modeß 1;PCß predeterminedaddress
2.OStakesover
• initializesdevices,MMU,timers,etc.
• loadsprogramsfromdisk,setsuppagetables,etc.
• Modeß 0;PCß programentrypoint
– UserprogramsregularlyyieldcontrolbacktoOS
8
Ifanuntrustedprocessdoesnothaveprivilegesto
usesystemresources,howcanit
•
•
•
•
Usethescreentoprint?
Sendmessageonthenetwork?
Allocatepages?
Scheduleprocesses?
Solution:SystemCalls
9
putc(): Printcharactertoscreen
• Needtomultiplexscreenbetweencompeting
processes
send(): Sendapacketonthenetwork
• Needtomanipulatetheinternalsofadevice
sbrk(): Allocateapage
• Needstoupdatepagetables&MMU
sleep(): putcurrentprog tosleep,wakeother
• Needtoupdatepagetablebaseregister
10
Systemcall:Notjustafunctioncall
• Don’tletprocessjumpjustanywhereinOScode
• OScan’ttrustprocess’registers(sp,fp,gp,etc.)
SYSCALLinstruction: safecontroltransfertoOS
MIPSsystemcallconvention:
• Exceptionhandlersavestempregs,savesra,…
• $v0=systemcallnumber,whichspecifiesthe
operationtheapplicationisrequesting
11
CompilersdonotemitSYSCALLinstructions
• Compilerdoesn’tknowOSinterface
LibrariesimplementstandardAPIfromsystemAPI
libc (standardClibrary):
•
•
•
•
•
•
•
gets()à getc()
getc()à syscall
sbrk()à syscall
printf()à write()
write()à syscall
malloc()à sbrk()
…
12
char *gets(char *buf) {
while (...) {
buf[i] = getc();
}
}
int getc() {
asm("addiu $v0, $0, 4");
asm("syscall");
}
13
0xfffffffc
systemreserved
0x80000000
0x7ffffffc
stack
dynamicdata(heap)
0x10000000
0x00400000
0x00000000
staticdata
??
gets
code(text)getc
systemreserved
14
Initsownaddressspace?
– Syscallhastoswitchtoadifferentaddressspace
– Hardtosupportsyscallargumentspassedaspointers
...So,NOPE
Inthesameaddressspaceastheuserprocess?
• Protectionbitspreventusercodefromwritingkernel
• Higherpartofvirtualmemory
• Lowerpartofphysicalmemory
...Yes,thisishowwedoit.
15
Allkerneltext&mostdata:
• Atsamevirtualaddressin
everyaddressspace
0xfffffffc
OSStack
0x80000000
OSHeap
OSData
OSText
0x7ffffffc
OSisomnipresent,availableto
helpuser-levelapplications
• Typicallyinhighmemory
stack
dynamicdata(heap)
0x10000000
staticdata
0x00400000
code(text)
0x00000000
systemreserved
16
VirtualMemory
0xfffffffc
0x80000000
0x7ffffffc
OSStack
OSHeap
OSData
OSText
stack
dynamicdata(heap)
0x10000000
staticdata
0x00400000
code(text)
0x00000000
systemreserved
VirtualMemory
OSStack
0x00...00
OSHeap
OSData
OSText
17
PhysicalMemory
0xfffffffc
0x80000000
0x7ffffffc
systemreserved
implementation of
getc() syscall
stack
dynamicdata(heap)
0x10000000
0x00400000
0x00000000
staticdata
gets
code(text)getc
systemreserved
18
WhichstatementisFALSE?
A) OSmanagestheCPU,Memory,Devices,and
Storage.
B) OSprovidesaconsistentAPItobeusedbyother
processes.
C) TheOSkernelisalwayspresentonDisk.
D) TheOSkernelisalwayspresentinMemory.
E) AnyprocesscanfetchandexecuteOScodein
usermode.
19
SYSCALL instructiondoesanatomicjumptoa
controlledlocation(i.e.MIPS0x80000180)
•
•
•
•
•
•
Switchesthesp tothekernelstack
Savestheold(user)SPvalue
Savestheold(user)PCvalue(=returnaddress)
Savestheoldprivilegemode
Setsthenewprivilegemodeto1
SetsthenewPCtothekernelsyscallhandler
21
Kernelsystemcallhandlercarriesoutthedesired
systemcall
•
•
•
•
•
•
•
Savescallee-saveregisters
Examinesthesyscallnumber
Checksargumentsforsanity
Performsoperation
Storesresultinv0
Restorescallee-saveregisters
Performsa“returnfromsyscall”(ERET)instruction,
whichrestorestheprivilegemode,SPandPC
22
Anythingthatisn’tauserprogramexecutingits
ownuser-levelinstructions.
SystemCalls:
• justonetypeofexceptionalcontrolflow
• ProcessrequestingaservicefromtheOS
• Intentional– it’sintheexecutable!
23
Trap
Intentional
Examples:
System call
(OS performs service)
Breakpoint traps
Privileged instructions
Fault
Unintentional but
Possibly recoverable
Examples:
Division by zero
Page fault
Abort
Unintentional
Not recoverable
Examples:
Parity error
24
Exceptionprogramcounter(EPC)
• 32-bitregister,holdsaddr ofaffectedinstruction
• Syscallcase:AddressofSYSCALL
Causeregister
• Registertoholdthecauseoftheexception
• Syscallcase:8,Sys
SpecialinstructionstoloadTLB
• Onlydo-ablebykernel
25
Precise
Hardwareguarantees
•
•
•
•
•
Previousinstructionscomplete
Laterinstructionsareflushed
EPCandcauseregisterareset
JumptoprearrangedaddressinOS
Whenyoucomeback,restart instruction
• Disableexceptionswhilerespondingtoone
– OtherwisecanoverwriteEPCandcause
26
AKA Exceptions
Hardware interrupts
Asynchronous
= caused by events
external to CPU
Software exceptions
Synchronous
= caused by CPU
executing an instruction
Maskable
Can be turned off by CPU
Unmaskable
Cannot be ignored
Example: alert from network device
that a packet just arrived, clock
notifying CPU of clock tick
Example: alert from the
power supply that electricity
is about to go out
27
NoSYSCALL instruction.Hardware stepsin:
•
•
•
•
•
•
•
•
SavesPCofexceptioninstruction(EPC)
Savescauseoftheinterrupt/privilege(Causeregister)
Switchesthesp tothekernelstack
Savestheold(user)SPvalue
Savestheold(user)PCvalue
SYSCALL
Savestheoldprivilegemode
Setsthenewprivilegemodeto1
SetsthenewPCtothekernelsyscallhander
interrupt/exceptionhandler
28
interrupt/exceptionhandlerhandlesevent
Kernelsystemcallhandlercarriesoutsystemcall
•
•
•
•
•
•
•
all
Savescallee-saveregisters
Examinesthesyscallnumbercause
Checksargumentsforsanity
Performsoperation
Storesresultinv0 all
Restorescallee-saveregisters
PerformsaERETinstruction(restorestheprivilege
mode,SPandPC)
29
WhatothertaskrequiresbothHardwareand
Software?
A) VirtualtoPhysicalAddressTranslation
B) BranchingandJumping
C)Clearingthecontentsofaregister
D)PipelininginstructionsintheCPU
E)Whatareweeventalkingabout?
30
Virtualà physicaladdresstranslation!
Hardware
• hasaconceptofoperatinginphysicalorvirtualmode
• helpsmanagetheTLB
• raisespagefaults
• keepsPageTableBaseRegister(PTBR)andProcessID
Software/OS
• managesPageTablestorage
• handlesPageFaults
• updatesDirtyandReferencebitsinthePageTables
• keepsTLBvalidoncontextswitch:
• FlushTLBwhennewprocessruns(x86)
• Storeprocessid(MIPS)
32
I/ODevices:monitor,disk,keyboard,network,
mouse,etc.
Network
Keyboard
Disk
Display
33
Modernsystemsseparatehigh-performanceprocessor,
memory,displayinterconnectfromlower-performance
interconnect
Core0
Cache
Core1
Cache
HighPerformance
Interconnect
Memory
Controller
I/O
Controller
LowerPerformance
LegacyInterconnect
I/O
Controller
I/O
Controller
I/O
Controller
Disk
Keyboard
Network
Memory
Display
Aside:Memory-MappedI/O
0xFFFFFFFF
Virtual
Address
Space
0x00FFFFFF
I/O
Controller
Physical
Address
Space
I/O
Controller
Display
Disk
I/O
Controller
Keyboard
I/O
Controller
Network
0x00000000
0x00000000
Less-favoredalternative=ProgrammedI/O:
• SyscallinstructionsthatcommunicatewithI/O
• Communicateviaspecialdeviceregisters
ProgrammedI/O
•
•
•
•
Requiresspecialinstructions
Canrequirededicatedhardwareinterfacetodevices
Protectionenforcedviakernelmodeaccesstoinstructions
Virtualizationcanbedifficult
Memory-MappedI/O
•
•
•
•
Re-usesstandardload/storeinstructions
Re-usesstandardmemoryhardwareinterface
Protectionenforcedwithnormalmemoryprotectionscheme
Virtualizationenabledwithnormalmemoryvirtualization
scheme
Howdoesprogramlearndeviceisready/done?
1.Polling: PeriodicallycheckI/Ostatusregister
• Commoninsmall,cheap,orreal-timeembeddedsystems
+ Predictabletiming,inexpensive
– WastesCPUcycles
2.Interrupts: DevicesendsinterrupttoCPU
•
•
+
–
–
Causeregisteridentifiestheinterruptingdevice
Interrupthandlerexaminesdevice,decideswhattodo
Onlyinterruptwhendeviceready/done
ForcedtosaveCPUcontext(PC,SP,registers,etc.)
Unpredictable,eventarrivaldependsonotherdevices’activity
Whichoneisthewinner?Whichoneistheloser?
1.ProgrammedI/O:Deviceßà CPUßà RAM
for(i =1..n)
• CPUissuesreadrequest
• Deviceputsdataonbus
&CPUreadsintoregisters
• CPUwritesdatatomemory
CPU
RAM
DISK
2.DirectMemoryAccess(DMA):Deviceßà RAM
• CPUsetsupDMArequest
• for(i =1...n)
Deviceputsdataonbus
&RAMacceptsit
• DeviceinterruptsCPUafterdone
CPU
RAM
DISK
Whichoneisthewinner?Whichoneistheloser?
DiverseI/Odevicesrequirehierarchicalinterconnect
whichismorerecentlytransitioningtopoint-to-point
topologies.
Memory-mappedI/Oisaneleganttechniqueto
read/writedeviceregisterswithstandardload/stores.
Interrupt-basedI/Oavoidsthewastedworkin
polling-basedI/Oandisusuallymoreefficient.
Modernsystemscombinememory-mappedI/O,
interrupt-basedI/O,anddirect-memoryaccess
tocreatesophisticatedI/Odevicesubsystems.