Carnegie Mellon Introduction to Computer Systems 15 213 18 243 spring 2009 14th Lecture Mar 3rd Instructors Gregory Kesden and Markus P schel Carnegie Mellon Perflab Current Status https autolab cs cmu edu 15213 s09 autolab pl Carnegie Mellon Last Time Optimization for the memory hierarchy Linking Symbol resolution Associate each symbol reference with exactly one definition Relocation Merge all o files into one executable Object files Relocatable object files o Executables Shared object file so ELF format ELF header Segment header table required for executables text section rodata section data section bss section symtab section rel txt section rel data section debug section Section header table Carnegie Mellon Last Time Symbol Resolution Symbols External extern int buf Global External Local static int bufp0 buf 0 static int bufp1 void swap int temp Symbols Strong Weak Local Linker knows nothing of temp Global bufp1 buf 1 temp bufp0 bufp0 bufp1 bufp1 temp swap c Carnegie Mellon Last Time Relocation Relocatable Object Files System code text System data data Executable Object File Headers System code main main o swap main text int buf 2 1 2 data More system code text System data int buf 2 1 2 int bufp0 buf 0 Uninitialized data symtab debug swap o swap int bufp0 buf 0 data int bufp1 bss text data bss Carnegie Mellon Last Time Loading Executable Executable Object File ELF header 0 Kernel virtual memory 0xc0000000 Program header table required for executables User stack created at runtime init section text section rodata section Memory invisible to user code esp stack pointer Memory mapped region for shared libraries 0x40000000 data section bss section brk Run time heap created by malloc symtab debug Read write segment data bss line strtab Section header table required for relocatables Read only segment init text rodata 0x08048000 0 Unused Loaded from the executable file Carnegie Mellon Today Exceptional Control Flow Processes Carnegie Mellon Control Flow Processors 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 CPU s control flow or flow of control Physical control flow Time startup inst1 inst2 inst3 instn shutdown Carnegie Mellon Altering the Control Flow Up to now two mechanisms for changing control flow Jumps and branches Call and return Both react to changes in program state Insufficient for a useful system Difficult to react to changes in system state data arrives from a disk or a network adapter instruction divides by zero user hits Ctrl C at the keyboard System timer expires System needs mechanisms for exceptional control flow Carnegie Mellon Exceptional Control Flow Exists at all levels of a computer system Low level mechanisms 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 Carnegie Mellon 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 I current I next OS exception exception processing by exception handler return to I current return to I next abort Examples div by 0 arithmetic overflow page fault I O request completes Ctrl C Carnegie Mellon Interrupt Vectors Exception numbers code for exception handler 0 Exception Table 0 1 2 n 1 code for exception handler 1 code for exception handler 2 code for exception handler n 1 Each type of event has a unique exception number k k index into exception table a k a interrupt vector Handler k is called each time exception k occurs Carnegie Mellon 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 Ctrl C at the keyboard arrival of a packet from a network arrival of data from a disk Hard reset interrupt hitting the reset button Soft reset interrupt hitting Ctrl Alt Delete on a PC Carnegie Mellon 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 Carnegie Mellon Trap Example Opening File User calls open filename options Function open executes system call instruction int 0804d070 libc open 804d082 cd 80 804d084 5b User Process int pop int pop 0x80 ebx OS exception open file returns OS must find or create file get it ready for reading or writing Returns integer file descriptor Carnegie Mellon Fault Example Page Fault User writes to memory location That portion page of user s memory is currently on disk 80483b7 c7 05 10 9d 04 08 0d User Process movl movl 0xd 0x8049d10 OS exception page fault returns int a 1000 main a 500 13 Create page and load into memory Page handler must load page into physical memory Returns to faulting instruction Successful on second try Carnegie Mellon Fault Example Invalid Memory Reference int a 1000 main a 5000 13 80483b7 c7 05 60 e3 04 08 0d User Process movl movl 0xd 0x804e360 OS exception page fault detect invalid address signal process Page handler detects invalid address Sends SIGSEGV signal to user process User process exits with segmentation fault Carnegie Mellon Exception Table IA32 Excerpt Exception Number Description Exception Class 0 Divide error Fault 13 General protection fault Fault 14 Page fault Fault 18 Machine check Abort 32 127 OS defined Interrupt or trap 128 0x80 System call Trap 129 255 OS defined Interrupt or trap Check pp 183 http download intel com design processor manuals 253665 pdf Carnegie Mellon Today Exceptional Control Flow Processes Carnegie Mellon 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 virtual address space
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