15 213 The course that gives CMU its Zip Exceptional Control Flow Part I Oct 17 2002 Topics n n n class16 ppt Exceptions Process context switches Creating and destroying processes Control Flow Computers do Only One Thing n From startup to shutdown a CPU simply reads and executes interprets a sequence of instructions one at a time n 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 02 Altering the Control Flow Up to Now two mechanisms for changing control flow n Jumps and branches n Call and return using the stack discipline n Both react to changes in program state Insufficient for a useful system n Difficult for the CPU to react to changes in system state l data arrives from a disk or a network adapter l Instruction divides by zero l User hits ctl c at the keyboard l System timer expires System needs mechanisms for exceptional control flow 3 15 213 F 02 Exceptional Control Flow n Mechanisms for exceptional control flow exists at all levels of a computer system Low level Mechanism n exceptions l change in control flow in response to a system event i e change in system state n Combination of hardware and OS software Higher Level Mechanisms n Process context switch n Signals n Nonlocal jumps setjmp longjmp n Implemented by either l OS software context switch and signals l C language runtime library nonlocal jumps 4 15 213 F 02 System context for exceptions Keyboard Keyboard Processor Processor Interrupt Interrupt controller controller Mouse Mouse Keyboard Keyboard controller controller Modem Modem Serial Serialport port controller controller Printer Printer Parallel Parallelport port controller controller Local IO Local IOBus Bus 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 5 CDROM 15 213 F 02 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 current next OS exception exception processing by exception handler exception return optional 6 15 213 F 02 Interrupt Vectors Exception numbers interrupt vector 0 1 2 n 1 n code codefor for exception handler exception handler00 Each type of event has a unique exception number k n code codefor for exception handler exception handler11 Index into jump table a k a interrupt vector n Jump table entry k points to a function exception handler n Handler k is called each time exception k occurs code codefor for exception handler exception handler22 code codefor for exception handler exception handlern 1 n 1 7 15 213 F 02 Asynchronous Exceptions Interrupts Caused by events external to the processor n Indicated by setting the processor s interrupt pin n handler returns to next instruction Examples n I O interrupts l hitting ctl c at the keyboard l arrival of a packet from a network l arrival of a data sector from a disk n Hard reset interrupt l hitting the reset button n Soft reset interrupt l hitting ctl alt delete on a PC 8 15 213 F 02 Synchronous Exceptions Caused by events that occur as a result of executing an instruction n Traps l Intentional l Examples system calls breakpoint traps special instructions l Returns control to next instruction n Faults l Unintentional but possibly recoverable l Examples page faults recoverable protection faults unrecoverable l Either re executes faulting current instruction or aborts n Aborts l unintentional and unrecoverable l Examples parity error machine check l Aborts current program 9 15 213 F 02 Trap Example Opening a File n User calls open filename options 0804d070 libc open 804d082 cd 80 804d084 5b int pop 0x80 ebx l Function open executes system call instruction int n OS must find or create file get it ready for reading or writing n Returns integer file descriptor User Process int pop OS exception Open file return 10 15 213 F 02 Fault Example 1 Memory Reference n User writes to memory location n That portion page of user s memory is currently on disk 80483b7 c7 05 10 9d 04 08 0d n Page handler must load page into physical memory n Returns to faulting instruction n Successful on second try User Process event movl 0xd 0x8049d10 OS page fault return 11 movl int a 1000 main a 500 13 Create page and load into memory 15 213 F 02 Fault Example 2 int a 1000 main a 5000 13 Memory Reference n User writes to memory location n Address is not valid 80483b7 c7 05 60 e3 04 08 0d 0xd 0x804e360 n Page handler detects invalid address Sends SIGSEG signal to user process n User process exits with segmentation fault n User Process event movl movl OS page fault Detect invalid address Signal process 12 15 213 F 02 Processes Def A process is an instance of a running program n One of the most profound ideas in computer science n Not the same as program or processor Process provides each program with two key abstractions n Logical control flow l Each program seems to have exclusive use of the CPU n Private address space l Each program seems to have exclusive use of main memory How are these Illusions maintained 13 n Process executions interleaved multitasking n Address spaces managed by virtual memory system 15 213 F 02 Logical Control Flows Each process has its own logical control flow Process A Process B Process C Time 14 15 213 F 02 Concurrent Processes Two processes run concurrently are concurrent if their flows overlap in time Otherwise they are sequential Examples n Concurrent A B A C n Sequential B C Process A Process B Process C Time 15 15 213 F 02 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 16 15 213 F 02 Context Switching Processes are managed by a shared chunk of OS code called the kernel n 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 code Process B code user code Time kernel code context switch user code kernel code context switch user code 17 15 213 F 02 Private Address Spaces Each process has its own private address space 0xffffffff kernel virtual memory code data heap stack 0xc0000000 0x40000000 user stack created at runtime read write segment data bss 18 0 esp stack pointer
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