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Today Exceptional Control Flow: Exceptional Control Flow Exceptions Exceptions and Processes Processes Process Control CSci 2021: Machine Architecture and Organization December 3rd, 2018 Your instructor: Stephen McCamant Based

  1. Today Exceptional Control Flow:  Exceptional Control Flow  Exceptions Exceptions and Processes  Processes  Process Control CSci 2021: Machine Architecture and Organization December 3rd, 2018 Your instructor: Stephen McCamant Based on slides originally by: Randy Bryant, Dave O’Hallaron 1 2 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Control Flow Altering the Control Flow  Processors do only one thing:  Up to now: two mechanisms for changing control flow:  From startup to shutdown, a CPU simply reads and executes  Jumps and branches (interprets) a sequence of instructions, one at a time  Call and return  This sequence is the CPU’s control flow (or flow of control ) React to changes in program state Physical control flow  Insufficient for a useful system: <startup> Difficult to react to changes in system state inst 1  Data arrives from a disk or a network adapter inst 2  Instruction divides by zero Time inst 3  User hits Ctrl-C at the keyboard …  System timer expires inst n <shutdown>  System needs mechanisms for “exceptional control flow” 3 4 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Exceptional Control Flow Today  Exists at all levels of a computer system  Exceptional Control Flow  Low level mechanisms  Exceptions  1. Exceptions  Processes  Change in control flow in response to a system event  Process Control (i.e., change in system state)  Implemented using combination of hardware and OS software  Higher level mechanisms  2. Process context switch  Implemented by OS software and hardware timer  3. Signals  Implemented by OS software  4. Nonlocal jumps : setjmp() and longjmp()  Implemented by C runtime library 5 6 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 1

  2. Exception Tables Exceptions  An exception is a transfer of control to the OS kernel in response Exception to some event (i.e., change in processor state) numbers  Kernel is the memory-resident part of the OS Each type of event has a Code for   Examples of events: Divide by 0, arithmetic overflow, page fault, I/O exception handler 0 unique exception number k request completes, typing Ctrl-C Exception Code for Table exception handler 1 k = index into exception table  User code Kernel code 0 (a.k.a. interrupt vector) 1 Code for 2 exception handler 2 ... Exception Event I_current ... Handler k is called each time n-1  I_next Exception processing exception k occurs by exception handler Code for • Return to I_current exception handler n-1 • Return to I_next • Abort 7 8 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Asynchronous Exceptions (Interrupts) Synchronous Exceptions  Caused by events that occur as a result of executing an  Caused by events external to the processor instruction:  Indicated by setting the processor’s interrupt pin  Traps  Handler returns to “next” instruction  Intentional  Examples: system calls , breakpoint traps, special instructions  Returns control to “next” instruction  Examples:  Faults  Timer interrupt  Unintentional but possibly recoverable  Every few ms, an external timer chip triggers an interrupt  Examples: page faults (recoverable), protection faults  Used by the kernel to take back control from user programs (unrecoverable), floating point exceptions  I/O interrupt from external device  Either re- executes faulting (“current”) instruction or aborts  Aborts  Hitting Ctrl-C at the keyboard  Unintentional and unrecoverable  Arrival of a packet from a network  Examples: illegal instruction, parity error, machine check  Arrival of data from a disk  Aborts current program 9 10 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition System Call Example: Opening File System Calls User calls: open(filename, options)  Calls __ open function, which invokes system call instruction syscall  Each x86-64 system call has a unique ID number   Examples: 00000000000e5d70 <__open>: ... Number Name Description e5d79: b8 02 00 00 00 mov $0x2,%eax # open is syscall #2 e5d7e: 0f 05 syscall # Return value in %rax read 0 Read file e5d80: 48 3d 01 f0 ff ff cmp $0xfffffffffffff001,%rax write 1 Write file ... e5dfa: c3 retq open 2 Open file close 3 Close file stat 4 Get info about file User code Kernel code %rax contains syscall number  fork 57 Create process Other arguments in %rdi ,  execve %rsi , %rdx , %r10 , %r8 , %r9 59 Execute a program Exception syscall _exit Return value in %rax 60 Terminate process cmp  Open file kill 62 Send signal to process Negative value is an error Returns  corresponding to negative errno 11 12 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 2

  3. Fault Example: Invalid Memory Reference Fault Example: Page Fault int a[1000]; int a[1000]; main () User writes to memory location  main () { That portion (page) of user’s memory {  a[500] = 13; a[5000] = 13; is currently on disk } } 80483b7: c7 05 10 9d 04 08 0d movl $0xd,0x8049d10 80483b7: c7 05 60 e3 04 08 0d movl $0xd,0x804e360 User code Kernel code User code Kernel code Exception: page fault movl Exception: page fault Detect invalid address movl Copy page from Signal process disk to memory Return and reexecute movl Sends SIGSEGV signal to user process  User process exits with “segmentation fault”  13 14 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Processes Today  Definition: A process is an instance of a running  Exceptional Control Flow program.  Exceptions  One of the most profound ideas in computer science  Not the same as “program” or “processor”  Processes  Process Control  Process provides each program with two key Memory abstractions: Stack  Logical control flow Heap  Each program seems to have exclusive use of the CPU Data  Provided by kernel mechanism called context switching Code  Private address space CPU  Each program seems to have exclusive use of main memory. Registers  Provided by kernel mechanism called virtual memory 15 16 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Multiprocessing: The Illusion Multiprocessing Example Memory Memory Memory Stack Stack Stack Heap Heap Heap … Data Data Data Code Code Code CPU CPU CPU Registers Registers Registers  Computer runs many processes simultaneously  Applications for one or more users  Web browsers, email clients, editors, …  Running program “top” on Mac  Background tasks  System has 123 processes, 5 of which are active  Monitoring network & I/O devices  Identified by Process ID (PID) 17 18 Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 3

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