Control Flow 15 213 Processors do only one thing The course that gives CMU its Zip Exceptional Control Flow Processes October 2 2008 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 Topics Exceptions Processes and context switches Creating and destroying processes Time 15 213 F 08 2 lecture 11 ppt Altering the Control Flow Exceptional Control Flow Up to now two mechanisms for changing control flow Jumps and branches Mechanisms for exceptional control flow exists at all levels of a computer system Low level Mechanism Call and return Both react to changes in program state exceptions z change in control flow in response to a system event i e Insufficient for a useful system startup inst1 inst2 inst3 instn shutdown change in system state Difficult for the CPU to react to changes in system state z data arrives from a disk or a network adapter combination of hardware and OS software Higher Level Mechanisms z instruction divides by zero z user hits Ctrl C at the keyboard z System timer expires System needs mechanisms for exceptional control flow flow Process context switch Signals Nonlocal jumps setjmp longjmp implemented by either z OS software context switch and signals z C language runtime library nonlocal jumps 3 15 213 F 08 4 Page 1 15 213 F 08 Interrupt Vectors Exceptions An exception is a transfer of control to the OS in response to some event i e change in processor state User Process Exception numbers OS interrupt vector current next event exception 0 1 2 exception processing by exception handler exception return optional code codefor for exception exceptionhandler handler00 code codefor for exception exceptionhandler handler11 code codefor for exception exceptionhandler handler22 n 1 Each type of event has a unique exception number k Index into jump table a k a interrupt vector Entry k points to a function exception handler Handler k is called each time exception k occurs code codefor for exception exceptionhandler handlern 1 n 1 15 213 F 08 5 Asynchronous Exceptions Interrupts Synchronous Exceptions Caused by events external to the processor Indicated by setting the processor s interrupt pin handler returns to next instruction 15 213 F 08 6 Caused by events that occur as a result of executing an instruction z Intentional Examples z Examples system calls breakpoint traps special instructions I O interrupts z Returns control to next instruction z hitting Ctrl C at the keyboard Faults z arrival of a packet from a network z Unintentional but possibly recoverable z arrival of data from a disk z Examples page faults recoverable protection faults Hard reset interrupt unrecoverable floating point exceptions z Either re executes faulting current instruction or aborts z hitting the reset button Traps Soft reset interrupt Aborts z unintentional and unrecoverable z hitting Ctrl Alt Delete on a PC z Examples parity error machine check z Aborts current program 7 15 213 F 08 8 Page 2 15 213 F 08 Fault Example 1 Trap Example Memory Reference Opening a File User calls open filename options 0804d070 libc open 804d082 cd 80 804d084 5b int pop 0x80 ebx 80483b7 z Function open executes system call instruction int OS must find or create file get it ready for reading or writing Returns integer file descriptor User Process c7 05 10 9d 04 08 0d event return User writes to memory location Address is not valid c7 05 60 e3 04 08 0d movl User process exits with segmentation fault User Process Definition A process is an instance of a running program Logical control flow Private address space z Each program seems to have exclusive use of the CPU z Each program seems to have exclusive use of main memory How are these Illusions maintained OS event movl One of the most profound ideas in computer science Not the same as program or processor Process provides each program with two key abstractions 0xd 0x804e360 Page handler detects invalid address Sends SIGSEGV signal to user process 15 213 F 08 10 int a 1000 main a 5000 13 Invalid Memory Reference 80483b7 Create page and load into memory Processes Fault Example 2 page fault movl return 15 213 F 08 0xd 0x8049d10 OS Open file 9 movl Page handler must load page into physical memory Returns to faulting instruction Successful on second try User Process OS exception int pop User writes to memory location That portion page of user s memory is currently on disk int a 1000 main a 500 13 page fault Process executions interleaved multitasking Address spaces managed by virtual memory system z we ll talk about this in a couple of weeks Detect invalid address Signal process 11 15 213 F 08 12 Page 3 15 213 F 08 Concurrent Processes Logical Control Flows Two processes run concurrently are concurrent if their flows overlap in time Each process has its own logical control flow Otherwise they are sequential Examples Concurrent A B A C Sequential B C Process A Process B Process C Time 15 213 F 08 13 User View of Concurrent Processes Context Switching Processes are managed by a shared chunk of OS code called the kernel 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 15 213 F 08 14 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 C Process A code Time Process B code user code Time kernel code context switch user code kernel code context switch user code 15 15 213 F 08 16 Page 4 15 213 F 08 fork Creating New Processes Fork Example 1 Key Points int fork void creates a new process child process that is identical to the calling process parent process Start with same state but each has private copy z Distinguish parent from child by return value from fork z Including shared output file descriptor z Relative ordering of their print statements undefined void fork1 int x 1 pid t pid fork if pid 0 printf Child has x d n x else printf Parent has x d n x printf Bye from process d with x d n getpid x Fork is interesting and often confusing because it is called once but returns twice 15 213 F 08 17 Fork Example 3 Key Points Key Points Both parent and child can continue forking void fork2 printf L0 n fork printf L1 n fork printf Bye n 15 213 F 08 18 Fork Example 2 19 Parent and child both run same code returns 0
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