What is the difference between Trap and Interrupt

Question

What is the difference between Trap and Interrupt   If the terminology is different for different systems  then what do they mean on x86

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Generally speaking  terms like exceptions  faults  aborts  Traps  and Interrupts all mean the same thing and are called  Interrupts    Coming to the difference between Trap and Interrupt      Trap  Is a programmer initiated and expected transfer of control to a special handler routine   For ex  80x86 INT instruction is a good example    Where as      Interrupt Hardware   Is a program control interruption based on an external hardware event external to the CPU  For ex  Pressing a key on the keyboard or a time out on a timer   chip

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A trap is called by code like programs and used e  g  to call OS routines  i  e  normally synchronous   An interrupt is called by events  many times hardware  like the network card having received data  or the CPU timer   and - as the name suggests - interrupts the normal control flow  as the CPU has to switch to the driver routine to handle the event

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I think Traps are caused by the execution of current instruction and thus they are called as synchronous events  where as interrupts are caused by an independent instruction that is running in the processor which are related to external events and thus are known as asynchronous ones

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A trap is an exception in a user process  It s caused by division by zero or invalid memory access  It s also the usual way to invoke a kernel routine  a system call  because those run with a higher priority than user code  Handling is synchronous  so the user code is suspended and continues afterwards   In a sense they are  active  - most of the time  the code expects the trap to happen and relies on this fact   An interrupt is something generated by the hardware  devices like the hard disk  graphics card  I O ports  etc   These are asynchronous  i e  they don t happen at predictable places in the user code  or  passive  since the interrupt handler has to wait for them to happen eventually   You can also see a trap as a kind of CPU-internal interrupt since the handler for trap handler looks like an interrupt handler  registers and stack pointers are saved  there is a context switch  execution can resume in some cases where it left off

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A trap is a software interrupt If you write a program in which you declare a variable having divide by zero value then it is treated as a trap Whenever you run this program it will throw same error at the same time System call is a special version of trap in which a program asks os for its required service  In case of interrupt a general word for hardware interrupts like an i o error the cpu is interrupted at random time and off course it is not the fault of our programmers It is the hardware that brings them up

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A Trap can be identified as a transfer of control  which is initiated by the programmer  The term Trap is used interchangeably with the term Exception  which is an automatically occurring software interrupt   But some may argue that a trap is simply a special subroutine call  So they fall in to the category of software-invoked interrupts  For example  in 80  86 machines  a programmer can use the int instruction to initiate a trap  Because a trap is always unconditional the control will always be transferred to the subroutine associated with the trap  The exact instruction  which invokes the routine for handling the trap is easily identified because an explicit instruction is used to specify a trap

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An interrupt is a hardware-generated change-of-flow within the system  An interrupt handler is summoned to deal with the cause of the interrupt  control is then returned to the interrupted context and instruction  A trap is a software-generated interrupt  An interrupt can be used to signal the completion of an I O to obviate the need for device polling  A trap can be used to call operating system routines or to catch arithmetic errors

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Interrupts are hardware interrupts  while traps are software-invoked interrupts  Occurrences of hardware interrupts usually disable other hardware interrupts  but this is not true for traps  If you need to disallow hardware interrupts until a trap is served  you need to explicitly clear the interrupt flag  And usually the interrupt flag on the computer affects  hardware  interrupts as opposed to traps  This means that clearing this flag will not prevent traps  Unlike traps  interrupts should preserve the previous state of the CPU

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A trap is a special kind of interrupt which is commonly referred to as a software interrupt  An interrupt is a more general term which covers both hardware interrupts  interrupts from hardware devices  and software interrupts  interrupts from software  such as traps

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Traps and interrupts are closely related  Traps are a type of exception  and exceptions are similar to interrupts  Intel x86 defines two overlapping categories  vectored events  interrupts vs exceptions   and exception classes  faults vs traps vs aborts   All of the quotes in this post are from the April 2016 version of the Intel Software Developer Manual  For the  definitive and complex  x86 perspective  I recommend reading the SDM s chapter on Interrupt and Exception handling  Vectored Events Vectored Events  interrupts and exceptions  cause the processor to jump into an interrupt handler after saving much of the processor s state  enough such that execution can continue from that point later   Exceptions and interrupts have an ID  called a vector  that determines which interrupt handler the processor jumps to  Interrupt handlers are described within the Interrupt Descriptor Table  Interrupts  Interrupts occur at random times during the execution of a program  in response to signals from hardware  System hardware uses interrupts to handle events external to the processor  such as requests to service peripheral devices  Software can also generate interrupts by executing the INT n instruction   Exceptions  Exceptions occur when the processor detects an error condition while executing an instruction  such as division by zero  The processor detects a variety of error conditions including protection violations  page faults  and internal machine faults   Exception Classifications  Exceptions are classified as faults  traps  or aborts depending on the way they are reported and whether the instruction that caused the exception can be restarted without loss of program or task continuity   Summary  traps increment the instruction pointer  faults do not  and aborts  explode   Trap  A trap is an exception that is reported immediately following the execution of the trapping instruction  Traps allow execution of a program or task to be continued without loss of program continuity  The return address for the trap handler points to the instruction to be executed after the trapping instruction   Fault  A fault is an exception that can generally be corrected and that  once corrected  allows the program to be restarted with no loss of continuity  When a fault is reported  the processor restores the machine state to the state prior to the beginning of execution of the faulting instruction  The return address  saved contents of the CS and EIP registers  for the fault handler points to the faulting instruction  rather than to the instruction following the faulting instruction   Example  A page fault is often recoverable  A piece of an application s address space may have been swapped out to disk from ram  The application will trigger a page fault when it tries to access memory that was swapped out  The kernel can pull that memory from disk to ram  and hand control back to the application  The application will continue where it left off  at the faulting instruction that was accessing swapped out memory   but this time the memory access should succeed without faulting  An illegal-instruction fault handler that emulates floating-point or other missing instructions would have to manually increment the return address to get the trap-like behaviour it needs  after seeing if the faulting instruction was one it could handle   x86  UD is a  quot fault quot   not a  quot trap quot     The handler would need a pointer to the faulting instruction to figure out which instruction it was   Abort  An abort is an exception that does not always report the precise location of the instruction causing the exception and does not allow a restart of the program or task that caused the exception  Aborts are used to report severe errors  such as hardware errors and inconsistent or illegal values in system tables   Edge Cases Software invoked interrupts  triggered by the INT instruction  behave in a trap-like manner  The instruction completes before the processor saves its state and jumps to the interrupt handler

[x86] What is the difference between Trap and Interrupt?

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