Control Flow 15 213 Computers do Only One Thing The course that gives CMU its Zip 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 Exceptional Control Flow Part I March 4 2004 Physical control flow Topics Exceptions Process context switches Creating and destroying processes Time startup inst1 inst2 inst3 instn shutdown 15 213 S 04 2 class16 ppt Altering the Control Flow Exceptional Control Flow Mechanisms for exceptional control flow exists at all levels of a computer system Up to Now two mechanisms for changing control flow Jumps and branches Call and return using the stack discipline Both react to changes in program state Low level Mechanism exceptions change in control flow in response to a system event i e change in system state Insufficient for a useful system Difficult for the CPU to react to changes in system state Combination of hardware and OS software data arrives from a disk or a network adapter Instruction divides by zero User hits ctl c at the keyboard System timer expires Higher Level Mechanisms Process context switch Signals Nonlocal jumps setjmp longjmp Implemented by either System needs mechanisms for exceptional control flow OS software context switch and signals C language runtime library nonlocal jumps 3 15 213 S 04 4 Page 1 15 213 S 04 Exceptions System context for exceptions USB Ports Keyboard Keyboard Processor Processor Interrupt Interrupt controller controller Mouse Mouse Modem Modem Serial Serialport port controllers controllers Timer Timer An exception is a transfer of control to the OS in response to some event i e change in processor state Printer Printer Parallel Parallelport port controller controller User Process OS Super I O Chip Memory Memory IDE IDEdisk disk controller controller SCSI SCSI controller controller Video Video adapter adapter Network Network adapter adapter Display Display Network Network SCSI SCSIbus bus disk disk current next event Local IO Local IOBus Bus exception exception processing by exception handler exception return optional CDROM 15 213 S 04 5 15 213 S 04 6 Asynchronous Exceptions Interrupts Interrupt Vectors Caused by events external to the processor Exception numbers code codefor for exception exceptionhandler handler00 interrupt vector 0 1 2 n 1 code codefor for exception exceptionhandler handler11 code codefor for exception exceptionhandler handler22 Indicated by setting the processor s interrupt pin handler returns to next instruction 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 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 code codefor for exception exceptionhandler handlern 1 n 1 7 hitting ctl alt delete on a PC 15 213 S 04 8 Page 2 15 213 S 04 Synchronous Exceptions Precise vs Imprecise Faults Caused by events that occur as a result of executing an instruction Precise Faults the exception handler knows exactly which instruction caused the fault All prior instructions have completed and no subsequent instructions had any effect Traps Intentional Examples system calls breakpoint traps special instructions Returns control to next instruction Imprecise Faults the CPU was working on multiple instructions concurrently and an ambiguity may exists as to which instruction cause the Fault For example multiple FP instructions were in the pipe and one caused an exception Alpha Microprocessors 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 15 213 S 04 9 15 213 S 04 10 Fault Example 1 Trap Example Memory Reference Opening a File User writes to memory location That portion page of user s memory is currently on disk User calls open filename options 0804d070 libc open 804d082 cd 80 804d084 5b int pop 0x80 ebx 80483b7 Function open executes system call instruction int int pop User Process OS event exception 0xd 0x8049d10 movl OS page fault Open file return return 11 movl Page handler must load page into physical memory Returns to faulting instruction Successful on second try 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 int a 1000 main a 500 13 15 213 S 04 12 Page 3 Create page and load into memory 15 213 S 04 Fault Example 2 Fault Example 3 int a 1000 main a 500 13 Memory Reference with TLB miss Memory Reference User writes to memory location That portion page of user s memory is currently in physical memory but the processor has forgotten how to translate the this virtual address to the physical address TLB must be reloaded with current translation Returns to faulting instruction Successful on second try User Process OS or Hardware event User writes to memory location Address is not valid 80483b7 return movl 0xd 0x804e360 User process exits with segmentation fault User Process event Look up address translation and store it in a TLB entry OS page fault movl Detect invalid address Signal process 15 213 S 04 13 c7 05 60 e3 04 08 0d Page handler detects invalid address Sends SIGSEG signal to user process TLB miss movl int a 1000 main a 5000 13 15 213 S 04 14 Processes Logical Control Flows Definition A process is an instance of a running program One of the most profound ideas in computer science Each process has its own logical control flow Not the same as program or processor Process provides each program with two key abstractions Process A Process B Process C Logical control flow Time 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 Process executions interleaved multitasking Address spaces managed by virtual memory system 15 15 213 S 04 16 Page 4 15 213 S 04 Concurrent Processes User View of Concurrent Processes Two processes run concurrently are concurrent if their flows overlap in time Control flows for concurrent processes are physically disjoint in time
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