Exceptional Control Flow I

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Exceptional Control Flow I
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Outline

• Operating Systems
• Exceptions
• Suggested reading 1.7, 8.1, 8.2

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Operating Systems

• Contemporary Applications never access hardware
  resources directly
• They rely on the services provided by the
  operating system

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Operating System

• Is a program that manages the computer hardware
• Also provides a basis for application programs
• And acts as an intermediary between a user of a
  computer and the computer hardware

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Two primary purposes

• Protect hardware from misuse by applications
• Provide applications with simple and uniform
  mechanisms for manipulating hardware devices

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Operating Systems

• Fundamental abstractions
• Process
• Virtual memory
• files

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Process phenomenon

• When a program runs on a modern system,
• the operating system provides the illusion
• that the program is the only one running on the
  system

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Process phenomenon

• The program appears to have exclusive use of all
  the processor, main memory, and I/O devices
• The program appears to execute the instructions
  in the program, one after the other, without
  interruption
• The code and data of the program appear to the
  only objects in the systems memory

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Processes

• Def A process is an instance of a running
  program.
• Process is one of the most profound ideas in
  computer science.
• Each program in the system runs in the context of
  some process

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When a New Process is Created

• Each time a user runs a program
• by typing the name of an executable object file
  to the shell,
• the shell creates a new process
• and then runs the executable object file
• in the context of this new process

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When a New Process is Created

• Application programs can also
• create new processes
• and run
• either their own
• or other application
• in the context of new process

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Processes

• 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

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Processes

• How is this illusion maintained?
• Process executions interleaved (multitasking)
• Address spaces managed by virtual memory system

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Process

• Concurrency
• Multiple processes can run concurrently
• The instructions of one process are interleaved
  with the instructions of another process

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Logical control flows

• Each process has its own logical control flow
• does not affect the states of any other processes
• Preempted
• Time slice

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Concurrent processes

• Two processes run concurrently
• are concurrent, if their flows overlap in time.
• Otherwise, they are sequential.
• Examples
• Concurrent A B, AC
• Sequential B C

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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.
• Multitasking or time slicing

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Contexts of Processes

• The context consists of the state that the
  program needs to run correctly
• This state includes
• the programs code and data stored in memory
• its stack
• the contents of its general-purpose registers
• its program counter
• environment variables
• and the set of open file descriptors

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Private address spaces

• Each process has its own private address space

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Virtual Memory

• An abstraction
• Each process appears to have exclusive use of the
  main memory
• Virtual address space
• Program code and data (global variables)
• Heap
• Shared libraries (standard and math libraries)
• Stack (function calls)
• Kernel (operating system)
• Hardware supports to translate the virtual address

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Files

• A sequence of bytes
• Each I/O device is modeled as a file
• Unix I/O
• using a small set of system calls reading and
  writing files
• All input and output in the system is performed
  by Unix I/O

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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 systems physical control
  flow (or flow of control).

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Altering the Control Flow

• Weve 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.

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Altering the Control Flow

• 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

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Exceptions (review)

• Conditions under which pipeline cannot continue
  normal operation
• Causes
• Halt instruction (Current)
• Bad address for instruction or data (Previous)
• Invalid instruction (Previous)
• Pipeline control error (Previous)

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Exceptions (review)

• Desired Action
• Complete some instructions
• Either current or previous (depends on exception
  type)
• Discard others
• Call exception handler
• Like an unexpected procedure call

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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

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System context for exceptions
Keyboard
Mouse
Printer
Modem
Processor
Interrupt controller
Serial port controller
Parallel port controller
Keyboard controller
Local/IO Bus
Network adapter
Video adapter
Memory
IDE disk controller
SCSI controller
SCSI bus
disk
Network
Display
disk
CDROM
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Exceptional control flow

• Higher level mechanisms
• process context switch (OS level)
• signals
• nonlocal jumps (setjmp/longjmp)
• Implemented by either
• OS software (context switch and signals).
• C language runtime library nonlocal jumps.

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Exceptions

• An exception is a transfer of control to the OS
  in response to some event (i.e., change in
  processor state)

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Interrupt vectors

• 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.

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Asynchronous exceptions (interrupts)

• Caused by events (changes in state) external to
  the processor
• Indicated by setting the processors interrupt
  pin
• handler returns to next instruction.

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Asynchronous exceptions (interrupts)

• 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

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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

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Synchronous exceptions

• Caused by events (changes in state) that occur as
  a result of executing an instruction
• Faults
• unintentional but possibly recoverable
• either re-executes faulting (current)
  instruction or aborts.
• Examples page faults (recoverable), protection
  faults (unrecoverable).

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Synchronous exceptions

• Caused by events (changes in state) that occur as
  a result of executing an instruction
• Aborts
• unintentional and unrecoverable
• aborts current program
• Examples parity error, machine check.

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Trap Example

• Opening a File
• User calls open(filename, options)
• Function open executes system call instruction int

0804d070 lt__libc_opengt . . . 804d082 cd 80
int 0x80 804d084 5b
pop ebx . . .
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Trap Example

• Opening a File
• OS must find or create file, get it ready for
  reading or writing
• Returns integer file descriptor

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Fault Example 1

• Memory Reference
• User writes to memory location
• That portion (page) of users memory is currently
  on disk

int a1000 main () a500 13
80483b7 c7 05 10 9d 04 08 0d movl
0xd,0x8049d10
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Fault Example 1

• Memory Reference
• Page handler must load page into physical memory
• Returns to faulting instruction
• Successful on second try

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Fault Example 1
int a1000 main () a500 13
80483b7 c7 05 10 9d 04 08 0d movl
0xd,0x8049d10
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Fault Example 2

• Memory Reference
• User writes to memory location
• Address is not valid

int a1000 main () a5000 13
80483b7 c7 05 60 e3 04 08 0d movl
0xd,0x804e360
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Fault Example 2

• Memory Reference
• Page handler detects invalid address
• Sends SIGSEG signal to user process
• User process exits with segmentation fault

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Fault Example 2
int a1000 main () a5000 13
80483b7 c7 05 60 e3 04 08 0d movl
0xd,0x804e360