Carnegie Mellon Exceptional Control Flow Exceptions and Processes 15 213 Introduction to Computer Systems 12th Lecture Oct 5 2010 Instructors Randy Bryant and Dave O Hallaron 1 Carnegie Mellon Today Exceptional Control Flow Processes 2 Carnegie Mellon Control Flow 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 flow Time startup inst1 inst2 inst3 instn shutdown 3 Carnegie Mellon Altering the Control Flow Up to now two mechanisms for changing control flow Jumps and branches Call and return Both 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 4 Carnegie Mellon Exceptional 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 5 Carnegie Mellon Exceptions An exception is a transfer of control to the OS in response to some event i e change in processor state User Process event I current I next OS exception exception processing by exception handler return to I current return to I next abort Examples div by 0 arithmetic overflow page fault I O request completes Ctrl C 6 Carnegie Mellon Interrupt Vectors Exception numbers code for exception handler 0 Exception Table 0 1 2 n 1 code for exception handler 1 code for exception handler 2 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 occurs code for exception handler n 1 7 Carnegie Mellon Asynchronous 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 PC 8 Carnegie Mellon Synchronous 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 program 9 Carnegie Mellon Trap Example Opening File User calls open filename options Function open executes system call instruction int 0804d070 libc open 804d082 cd 80 804d084 5b User Process int pop int pop 0x80 ebx OS exception open file returns OS must find or create file get it ready for reading or writing Returns integer file descriptor 10 Carnegie Mellon Fault Example Page Fault User writes to memory location That portion page of user s memory is currently on disk 80483b7 c7 05 10 9d 04 08 0d User Process movl movl 0xd 0x8049d10 OS exception page fault returns int a 1000 main a 500 13 Create page and load into memory Page handler must load page into physical memory Returns to faulting instruction Successful on second try 11 Carnegie Mellon Fault Example Invalid Memory Reference int a 1000 main a 5000 13 80483b7 c7 05 60 e3 04 08 0d User Process movl movl 0xd 0x804e360 OS exception page fault detect invalid address signal process Page handler detects invalid address Sends SIGSEGV signal to user process User process exits with segmentation fault 12 Carnegie Mellon Exception Table IA32 Excerpt Exception Number Description Exception Class 0 Divide error Fault 13 General protection fault Fault 14 Page fault Fault 18 Machine check Abort 32 127 OS defined Interrupt or trap 128 0x80 System call Trap 129 255 OS defined Interrupt or trap Check Table 6 1 http download intel com design processor manuals 253665 pdf 13 Carnegie Mellon Today Exceptional Control Flow Processes 14 Carnegie Mellon Processes 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 weeks 15 Carnegie Mellon Concurrent 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 C Process A Process B Process C Time 16 Carnegie Mellon User 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 other Process A Process B Process C Time 17 Carnegie Mellon Context 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 switch Process A Process B user code kernel code Time context switch user code kernel code context switch user code 18 Carnegie Mellon fork Creating New Processes int fork void creates a new process child process that is identical to the calling process parent process returns 0 to the child process returns child s pid to the parent process pid t pid fork if pid 0 printf hello from child n else printf hello from parent n Fork is interesting and often confusing because it is called once but returns twice 19 Carnegie Mellon Understanding fork Process n Child Process m pid t pid fork if pid 0 printf hello from child n else printf hello from parent n pid t pid fork if pid 0 printf
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