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CMU CS 15213 - Exceptional Control Flow

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Exceptional Control FlowPart IMarch 4, 2008TopicsTopicsExceptionsProcess context switchesCreating and destroying processesclass14.ppt15-213“The course that gives CMU its Zip!”215-213, S’08Control Flow<startup>inst1inst2inst3…instn<shutdown>Computers do only one thing:Computers do only one thing:From startup to shutdown, a CPU simply reads and executes (interprets) a sequence of instructions, one at a time.This sequence is the system’s physical control flow (or flow of control).Physical control flowTime315-213, S’08Altering the Control FlowUp to now: two mechanisms for changing control Up to now: two mechanisms for changing control flow:flow:1. Jumps and branches2. Call and return using the stack discipline.Both react to choices the program makes voluntarilyInsufficient for a useful systemInsufficient for a useful systemDifficult for the CPU to react to changes in system state. Data arrives from a disk or a network adapter.Instruction divides by zeroUser hits Control-C at the keyboardInterval timer expires (time for this processor to run another program)System needs mechanisms for “exceptional control System needs mechanisms for “exceptional control flow”flow”415-213, S’08Exceptional Control FlowMechanisms for exceptional control flow exists at all levels of a computer system.Low level MechanismLow level MechanismExceptions change in control flow in response to a system event (i.e., change in system state)Combination of hardware and OS softwareHigher Level MechanismsHigher Level MechanismsProcess context switchSignalsNon-local jumps (setjmp()/longjmp())Implemented by either:OS software (context switch and signals).C language runtime library: non-local jumps.515-213, S’08System context for exceptionsLocal/IO BusMemoryNetworkadapterIDE diskcontrollerVideoadapterDisplayNetworkProcessorInterruptcontrollerSCSIcontrollerSCSI busSerial port controllersParallel portcontrollerTimerKeyboard Mouse PrinterModemdiskdisk CDROMUSB PortsSuper I/O Chip615-213, S’08ExceptionsAn An exceptionexception is a transfer of control to the OS in is a transfer of control to the OS in response to some response to some eventevent (i.e., change in processor (i.e., change in processor state)state)User Process OSexceptionexception processingby exception handlerexception return (optional)event currentnext715-213, S’08Interrupt VectorsEach type of event has a unique exception number kIndex into jump table (a.k.a., interrupt vector)Jump table entry k points to a function (exception handler).Handler k is called each time exception k occurs. interruptvector012...n-1code for exception handler 0code for exception handler 1code forexception handler 2code for exception handler n-1...Exception numbers815-213, S’08Asynchronous Exceptions (Interrupts)Caused by events external to the processorCaused by events external to the processorIndicated by setting the processor’s interrupt pinhandler returns to “next” instruction.Examples:Examples:I/O interruptshitting ctl-c at the keyboardarrival of a packet from a networkarrival of a data sector from a diskHard reset interrupthitting the reset buttonSoft reset interrupthitting ctl-alt-delete on a PC915-213, S’08Synchronous ExceptionsCaused by events that occur as a result of executing Caused by events that occur as a result of executing an instruction:an instruction:TrapsIntentionalExamples: system calls, breakpoint traps, special instructionsReturns control to “next” instructionFaultsUnintentional but possibly recoverable Examples: page faults (recoverable), protection faults (unrecoverable), floating point exceptions.Either re-executes faulting (“current”) instruction or aborts.Abortsunintentional and unrecoverableExamples: parity error, machine check.Aborts current program1015-213, S’08Trap ExampleUser Process OSexceptionOpen filereturnintpopOpening a FileOpening a FileUser calls open(filename, options)Function open executes system call instruction intOS must find or create file, get it ready for reading or writingReturns integer file descriptor0804d070 <__libc_open>: . . . 804d082: cd 80 int $0x80 804d084: 5b pop %ebx . . .1115-213, S’08Fault Example #1User Process OSpage faultCreate page and load into memoryreturnevent movlMemory ReferenceMemory ReferenceUser writes to memory locationThat portion (page) of user’s memory is currently on diskPage handler must load page into physical memoryReturns to faulting instructionSuccessful on second tryint a[1000];main (){ a[500] = 13;} 80483b7: c7 05 10 9d 04 08 0d movl $0xd,0x8049d101215-213, S’08Fault Example #2User Process OSpage faultDetect invalid addressevent movlInvalid Memory ReferenceInvalid Memory ReferenceUser writes to memory locationAddress is not validPage handler detects invalid addressSends SIGSEGV signal to user processUser process exits with “segmentation fault”int a[1000];main (){ a[5000] = 13;} 80483b7: c7 05 60 e3 04 08 0d movl $0xd,0x804e360Signal process1315-213, S’08ProcessesDefinition: A Definition: A processprocess is an instance of a running is an instance of a running program.program.One of the most profound ideas in computer science.Not the same as “program” or “processor”Process provides each program with two key Process provides each program with two key abstractions:abstractions:Logical control flowEach program seems to have exclusive use of the CPU.Private address spaceEach program seems to have exclusive use of main memory.How are these Illusions maintained?How are these Illusions maintained?Process executions interleaved (multitasking)Address spaces managed by virtual memory system1415-213, S’08Logical Control FlowsTimeProcess A Process B Process CEach process has its own logical control flow1515-213, S’08Concurrent ProcessesTwo processes Two processes run concurrentlyrun concurrently ( (are concurrent)are concurrent) if if their flows overlap in time.their flows overlap in time.Otherwise, they are Otherwise, they are sequential.sequential. Examples:Examples:Concurrent: A & B, A & CSequential: B & CTimeProcess A Process B Process C1615-213, S’08User View of Concurrent ProcessesControl flows for concurrent processes are physically Control flows for concurrent processes are physically


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CMU CS 15213 - Exceptional Control Flow

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