University of New Mexico1Exceptional Control Flow: Exceptions and ProcessesECE 335: Integrated Software Systems Instructor:Edward NavaUniversity of New Mexico2Today Exceptional Control Flow ProcessesUniversity of New Mexico3Control 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 flowTimeUniversity of New Mexico4Altering 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”University of New Mexico5Exceptional 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)University of New Mexico6Exceptions 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_nextUniversity of New Mexico7012...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 numbersUniversity of New Mexico8Asynchronous 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 PCUniversity of New Mexico9Synchronous 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 programUniversity of New Mexico10Trap 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 $0x80804d084: 5b pop %ebx. . .User Process OSexceptionopen filereturnsintpopUniversity of New Mexico11Fault 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 second tryint a[1000];main (){a[500] = 13;}80483b7: c7 05 10 9d 04 08 0d movl $0xd,0x8049d10User Process OSexception: page faultCreate page and load into memoryreturnsmovlUniversity of New Mexico12Fault Example: Invalid Memory Reference 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,0x804e360User Process OSexception: page faultdetect invalid addressmovlsignal processUniversity of New Mexico13Exception Table IA32 (Excerpt)ExceptionNumberDescriptionException Class0Divide errorFault13General protection faultFault14Page faultFault18Machine checkAbort32-127OS-definedInterrupt or trap128 (0x80)System callTrap129-255OS-definedInterrupt or trapCheck Table 6-1:http://download.intel.com/design/processor/manuals/253665.pdfUniversity of New Mexico14Today Exceptional Control Flow ProcessesUniversity of New Mexico15Processes Definition: A process is an instance of a running program. One of the most profound ideas in computer science Not the same as “program” or “processor” Process provides each program with two key abstractions: Logical control flow Each program seems to have exclusive use of the CPU Private virtual address space Each program seems to have exclusive use of main memory How are these Illusions maintained? Process executions interleaved (multitasking) or run on separate cores Address spaces managed by virtual memory system we’ll talk about this in a couple of weeksUniversity of New Mexico16Concurrent Processes Two processes run concurrently (are concurrent) if their flows overlap in time Otherwise, they are sequential Examples (running on single core): Concurrent: A & B, A & C Sequential: B & CProcess A Process B Process CTimeUniversity of New Mexico17User View of Concurrent Processes Control flows for concurrent processes are physically disjoint in time However, we can think of concurrent processes are running in parallel with each otherTimeProcess A Process B Process CUniversity of New Mexico18Context Switching Processes are managed by a shared chunk of OS code called the kernel Important: the kernel is not a separate process, but rather runs as part of some user process Control flow passes from one process to another via a context switchProcess A Process Buser codekernel codeuser codekernel codeuser codecontext switchcontext switchTimeUniversity of New Mexico19fork: Creating New Processes int fork(void)