15 213 The course that gives CMU its Zip Exceptional Control Flow I Oct 18 2001 Topics Exceptions Process context switches Creating and destroying processes class16 ppt Control flow 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 class16 ppt startup inst1 inst2 inst3 instn shutdown 2 CS 213 F 01 Altering the Control Flow We ve discussed two mechanisms for changing the 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 class16 ppt 3 CS 213 F 01 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 Implemented as a combination of both 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 class16 ppt 4 CS 213 F 01 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 disk SCSI SCSI controller controller SCSI SCSIbus bus disk class16 ppt 5 Video Video adapter adapter Network Network adapter adapter Display Display Network Network CDROM CS 213 F 01 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 class16 ppt 6 CS 213 F 01 Interrupt vectors Exception numbers interrupt vector 0 1 2 code codefor for exception handler exception handler00 code codefor for exception exceptionhandler handler11 code codefor for exception handler exception handler22 n 1 code codefor for exception exceptionhandler handlern 1 n 1 class16 ppt 7 1 Each type of event has a unique exception number k 2 Jump table interrupt vector entry k points to a function exception handler 3 Handler k is called each time exception k occurs CS 213 F 01 Asynchronous exceptions interrupts Caused by events changes in state 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 class16 ppt 8 CS 213 F 01 Synchronous exceptions Caused by events changes in state that occur as a result of executing an instruction Traps intentional returns control to next instruction Examples system calls breakpoint traps Faults unintentional but possibly recoverable either re executes faulting current instruction or aborts Examples page faults recoverable protection faults unrecoverable Aborts unintentional and unrecoverable aborts current program Examples parity error machine check class16 ppt 9 CS 213 F 01 Opening a File Trap Example 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 descriptor Userfile Process OS Returns integer int pop exception Open file return class16 ppt 10 CS 213 F 01 Fault Example 1 Memory Reference User writes to memory location That portion page of user s memory is currently on disk Page handler must 80483b7 c7load 05page 10 into 9d physical 04 08 memory 0d movl Returns to faulting instruction Successful on second try User Process event movl 0xd 0x8049d10 OS page fault return class16 ppt int a 1000 main a 500 13 11 Create page and load into memory CS 213 F 01 Fault Example 2 Memory Reference User writes to memory location Address is not valid 80483b7 c7 05 60 e3 04 08 0d Page handler detects invalid address Sends SIGSEG signal to user process User process exits with segmentation fault User Process event movl movl int a 1000 main a 5000 13 0xd 0x804e360 OS page fault Detect invalid address Signal process class16 ppt 12 CS 213 F 01 Processe s of a running program Def A process is an instance One of the most profound ideas in computer science Process provides each program with two key abstractions Logical control flow gives each program the illusion that it has exclusive use of the CPU Private address space gives each program the illusion that has exclusive use of main memory How is this illusion maintained Process executions interleaved multitasking Address spaces managed by virtual memory system class16 ppt 13 CS 213 F 01 Logical control flows Each process has its own logical control flow Process A Process B Process C Time class16 ppt 14 CS 213 F 01 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 class16 ppt 15 CS 213 F 01 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 class16 ppt 16 CS 213 F 01 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 code Process B code user code kernel code Time context switch user code kernel code context switch user code class16 ppt 17 CS 213 F 01 Private address spaces Each process has its own private address space 0xffffffff 0xc0000000 0x40000000 kernel virtual memory code data heap stack user stack created at runtime read write segment data bss class16
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