15 213 The course that gives CMU its Zip Exceptional Control Flow Part I October 14 2003 Topics class15 ppt Exceptions Process context switches Creating and destroying processes Control Flow Computers 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 system s physical control flow or flow of control Physical control flow Time 2 startup inst1 inst2 inst3 instn shutdown 15 213 F 03 Altering the Control Flow 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 Insufficient for a useful system Difficult for the CPU to react to changes in system state data arrives from a disk or a network adapter Instruction divides by zero User hits ctl c at the keyboard System timer expires System needs mechanisms for exceptional control flow 3 15 213 F 03 Exceptional Control Flow Mechanisms for exceptional control flow exists at all levels of a computer system Low level Mechanism 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 4 15 213 F 03 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 Printer Printer Parallel Parallelport port controller controller Super I O Chip Local IO Local IOBus Bus Memory Memory IDE IDEdisk disk controller controller disk SCSI SCSI controller controller SCSI SCSIbus bus disk 5 Video Video adapter adapter Network Network adapter adapter Display Display Network Network CDROM 15 213 F 03 Exception s An exception is a transfer of control to the OS in response to some event i e change in processor state User Process event current next OS exception exception processing by exception handler exception return optional 6 15 213 F 03 Interrupt Vectors Exception numbers interrupt vector 0 1 2 n 1 code codefor for exception exceptionhandler handler00 Each type of event has a unique exception number k code codefor for exception handler exception handler11 Index into jump table a k a interrupt vector code codefor for exception exceptionhandler handler22 Jump table entry k points to a function exception handler Handler k is called each time exception k occurs code codefor for exception handler exception handlern 1 n 1 7 15 213 F 03 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 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 hitting ctl alt delete on a PC 8 15 213 F 03 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 15 213 F 03 Precise vs Imprecise Faults Precise Faults the exception handler knows exactly which instruction caused the fault All prior instructions have completed and no subsequent instructions had any effect 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 10 15 213 F 03 Trap Example Opening a File User calls open filename options 0804d070 libc open 804d082 cd 80 804d084 5b int pop 0x80 ebx 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 int pop 11 OS exception Open file return 15 213 F 03 Fault Example 1 Memory Reference 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 try 80483b7 c7 05 10 9d 04 08 0d User Process event movl 0xd 0x8049d10 OS page fault return 12 movl int a 1000 main a 500 13 Create page and load into memory 15 213 F 03 Fault Example 2 Memory Reference with TLB miss 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 event movl OS or Hardware TLB miss return 13 int a 1000 main a 500 13 Look up address translation and store it in a TLB entry 15 213 F 03 Fault Example 3 int a 1000 main a 5000 13 Memory Reference User writes to memory location Address is not valid Page handler detects invalid address 80483b7 c7 05 60 e3 04 08 0d Sends SIGSEG signal to user process User process exits with segmentation fault User Process event 14 movl movl 0xd 0x804e360 OS page fault Detect invalid address Signal process 15 213 F 03 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 address space Each program seems to have exclusive use of main memory How are these Illusions maintained 15 Process executions interleaved multitasking Address spaces managed by virtual memory system 15 213 F 03 Logical Control Flows Each process has its own logical control flow Process A Process B Process C Time 16 15 213 F 03 Concurrent Processes Two processes run concurrently are concurrent if their flows overlap in time Otherwise they are sequential Examples Concurrent A B A C Sequential B C Process A Process B Process C Time 17 15 213 F 03 User View
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