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CMU CS 15213 - 14-exceptions

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Slide 1Perflab: Current StatusLast TimeLast Time: Symbol ResolutionLast Time: RelocationLast Time: Loading ExecutableTodayControl FlowAltering the Control FlowExceptional Control FlowExceptionsInterrupt VectorsAsynchronous Exceptions (Interrupts)Synchronous ExceptionsTrap Example: Opening FileFault Example: Page FaultFault Example: Invalid Memory ReferenceException Table IA32 (Excerpt)TodayProcessesConcurrent ProcessesUser View of Concurrent ProcessesContext Switchingfork: Creating New ProcessesUnderstanding forkFork Example #1Fork Example #2Fork Example #3Fork Example #4Fork Example #4exit: Ending a processZombiesZombie ExampleNonterminating Child Examplewait: Synchronizing with Childrenwait: Synchronizing with Childrenwait() Examplewaitpid(): Waiting for a Specific Processexecve: Loading and Running Programsexecve: Exampleexecl and exec Familyexec: Loading and Running ProgramsSummarySummary (cont.)Carnegie MellonIntroduction to Computer Systems15-213/18-243, spring 200914th Lecture, Mar. 3rdInstructors: Gregory Kesden and Markus PüschelCarnegie MellonPerflab: Current Statushttps://autolab.cs.cmu.edu/15213-s09/autolab.plCarnegie MellonLast TimeOptimization for the memory hierarchyLinkingSymbol resolution: Associate each symbol reference with exactly one definitionRelocation: Merge all .o files into one executableObject filesRelocatable object files (.o)ExecutablesShared object file (.so)ELF formatELF headerSegment header table(required for executables).text section.rodata section.bss section.symtab section.rel.txt section.rel.data section.debug sectionSection header table.data sectionCarnegie MellonLast Time: Symbol ResolutionSymbolsGlobalExternalLocalSymbolsStrongWeakextern int buf[]; static int *bufp0 = &buf[0];static int *bufp1;void swap(){ int temp; bufp1 = &buf[1]; temp = *bufp0; *bufp0 = *bufp1; *bufp1 = temp;}swap.cExternal LocalGlobalLinker knowsnothing of tempCarnegie MellonLast Time: Relocationmain()main.oint *bufp0=&buf[0]swap()swap.oint buf[2]={1,2}Headersmain()swap()System codeint *bufp0=&buf[0]int buf[2]={1,2}System dataMore system codeint *bufp1System dataRelocatable Object Files Executable Object File.text.text.data.text.data.text.data.bss .symtab.debug.dataUninitialized data.bssSystem codeCarnegie MellonLast Time: Loading ExecutableELF headerProgram header table(required for executables).text section.data section.bss section.symtab.debugSection header table(required for relocatables)0Executable Object FileKernel virtual memoryMemory-mapped region forshared librariesRun-time heap(created by malloc)User stack(created at runtime)Unused0%esp (stack pointer)Memoryinvisible touser codebrk0xc00000000x080480000x40000000Read/write segment(.data, .bss)Read-only segment(.init, .text, .rodata)Loaded from the executable file.rodata section.line.init section.strtabCarnegie MellonTodayExceptional Control FlowProcessesCarnegie MellonControl Flow<startup>inst1inst2inst3…instn<shutdown>Processors 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 CPU’s control flow (or flow of control)Physical control flowTimeCarnegie MellonAltering the Control FlowUp to now: two mechanisms for changing control flow:Jumps and branchesCall and returnBoth react to changes in program stateInsufficient for a useful system: Difficult to react to changes in system state data arrives from a disk or a network adapterinstruction divides by zerouser hits Ctrl-C at the keyboardSystem timer expiresSystem needs mechanisms for “exceptional control flow”Carnegie MellonExceptional Control FlowExists at all levels of a computer systemLow level mechanismsExceptions change in control flow in response to a system event (i.e., change in system state)Combination of hardware and OS softwareHigher level mechanismsProcess context switchSignalsNonlocal jumps: setjmp()/longjmp()Implemented by either:OS software (context switch and signals)C language runtime library (nonlocal jumps)Carnegie MellonExceptionsAn exception is a transfer of control to the OS in response to some event (i.e., change in processor state)Examples: div by 0, arithmetic overflow, page fault, I/O request completes, Ctrl-CUser Process OSexceptionexception processingby exception handler• return to I_current•return to I_next•abortevent I_currentI_nextCarnegie Mellon012...n-1Interrupt VectorsEach type of event has a unique exception number kk = index into exception table (a.k.a. interrupt vector)Handler k is called each time exception k occursExceptionTablecode for exception handler 0code for exception handler 1code forexception handler 2code for exception handler n-1...Exception numbersCarnegie MellonAsynchronous Exceptions (Interrupts)Caused by events external to the processorIndicated by setting the processor’s interrupt pinHandler returns to “next” instructionExamples:I/O interruptshitting Ctrl-C at the keyboardarrival of a packet from a networkarrival of data from a diskHard reset interrupthitting the reset buttonSoft reset interrupthitting Ctrl-Alt-Delete on a PCCarnegie MellonSynchronous ExceptionsCaused by events that occur as a result of executing 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 programCarnegie MellonTrap Example: Opening 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 . . .User Process OSexceptionopen filereturnsintpopCarnegie MellonFault Example: Page Fault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


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CMU CS 15213 - 14-exceptions

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