My understanding on the both are slightly unclear. Many people on the internet say they are both the same. There are a few questions similar to my one, however none of them give a good real life example at a software level.
Would it be possible for someone to give me a clear example of both which will help me understand the differences between one another?
For example, is a division by zero a software interrupt? Or an exception?
Interrupt and exceptions have the same method of dispatch (usually through the system interrupt vector). However, interrupts and exceptions are triggered differently.
An exception occurs through the execution of the instruction stream. Thus, exceptions occur at predictable points in an application.
Interrupts occur as the result of events external to the execution stream.
Division by zero is occurs as the result of the instruction stream making it an exception.
Some operating systems are interrupt-based (e.g., Windoze and VMS).They allow the application to be interrupted in user (or other modes) for various reasons.
For example. in both those operating systems you can queue I/O operation and then have the application be interrupted when the I/O completes (a software interrupt triggered by the operating system rather than the hardware).
Related
Interrupts are hardware based and happen asynchronous (data incoming to a socket, some I/O is ready to read from or write to, A user pressed the keyboard.
Exceptions are are also hardware based but they are synchronous and caused by the CPU when executing instructions. for example a page in the virtual memory address space that has no actual chunk of RAM mapped to it will cause a page fault. Exceptions are the general name for fault, trap and abort.
Interrupts and Exceptions are generated by hardware and handled by handlers in the kernel space. They can be seen as mean of communication between the Hardware and the kernel.
Signals Signals can be viewed as a mean of communication between the running processes and the kernel. In some cases an Interrupt/Exception will use signals as part of the handling by the kernel.
Interrupts
In computing, an interrupt is an asynchronous signal indicating the
need for attention or a synchronous event in software indicating the
need for a change in execution.
(definition obtained from stackoverflow's tag description)
So, it's not unnecessarily asynchronous. It's asynchronous only if it is emitted by the hardware. Think of a virtual device or an emulator for examples of synchronous interrupts, when you are programming a camera and instead of the real device you have an emulator, which you can program to simulate interrupts.
Exceptions
From Microsoft docs:
Most of the standard exceptions recognized by the operating system are
hardware-defined exceptions. Windows recognizes a few low-level
software exceptions, but these are usually best handled by the
operating system.
Windows maps the hardware errors of different processors to the
exception codes in this section. In some cases, a processor may
generate only a subset of these exceptions. Windows preprocesses
information about the exception and issues the appropriate exception
code.
Exceptions are not necessarily hardware-generated and not necessarily synchronous.
If they are synchronous, then a software emitted it (like a camera emulator). Asynchronous exceptions can be raised just about anywhere.
In more advanced programming languages one can use exception handlers and different kinds of exceptions have their own exception subclass. The program can emit an exception with a command, usually the throw keyword which is paired with an exception instance. See: https://www.geeksforgeeks.org/throw-throws-java/
One can implement custom exception classes according to business logic, see https://www.baeldung.com/java-new-custom-exception.
So, the realm of exceptions is much wider than you first thought.
Signals
A signal is a notification to a process that an event occurred. Signals are sometimes described as software interrupts. Signals are analogous to hardware interrupts in that they interrupt the normal flow of execution of a program; in most cases, it is not possible to predict exactly when a signal will arrive. They are defined in the C standards and extended in POSIX, but many other programming languages/systems provide access to them as well.
You are more-or-less correct about signals.
I'm working with the RISC-V specification and have a problem with pending interrupts/exceptions. I'm reading version 1.10 of volume II, published in May 7, 2017.
In section 3.1.14, describing the registers mip and mie it is said that:
Multiple simultaneous interrupts and traps at the same privilege level are handled in the following decreasing priority order: extern interrupts, software interrupts, timer interrupts, then finally any synchronous traps.
Up until that point I thought that exceptions, e.g. a misaligned instruction fetch exception on a JAL/JALR instruction, would be handled immediately by a trap because
a) there is no way to continue executing your stream of instructions and
b) there is no description of how an exception could be pending, i.e. there are no concepts described by the specification that could manage state for exceptions (for example registers like mip but for exceptions).
However, the paragraph cited above indicates something different.
My questions are:
Are there pending exceptions in RISC-V?
If yes, how is it possible that the exception still can be handled after an interrupt was handled and isn't forgotten?
In my option there are pending exceptions in RISCV-V, exactly by the reason you stated. It is a matter of semantics, if two events occur simultaneously, and one is deferred, it must be pending. One must cater for the possibility of an asynchronous event (interrupt) occurring simultaneously with a trap, and (by section 3.1.14) the asynchronous event has priority. Depending on the implementation one does not neccesarely need to save any state in this case, after the interrupt is handled, the instruction that triggers a trap is re-fetched, and duly leads to an exception. In my view section 3.1.14 describes the serialization of asynchronous events.
I was going through the book The Design and Implementation of the FreeBSD operating system and I came across this:
This ability to restart an instruction is called a precise exception.
The CPU may implement restarting by saving enough state when an
instruction begins that the state can be restored when a fault is
discovered. Alternatively, instructions could delay any modifications
or side effects until after any faults would be discovered so that the
instruction execution does not need to back up before restarting.
I could not understand what does
modification or side effects
refer to in the passage. Can anyone elaborate?
That description from the FreeBSD book is very OS centric. I even disagree with its definition, This ability to restart an instruction is called a precise exception. You don't restart an instruction after a power failure exception. So rather than try to figure out what McKusick meant, I'm gonna suggest going elsewhere for a better description of exceptions.
BTW, I prefer Smith's definition:
An interrupt is precise if the saved process state corresponds with
the sequential model of program execution where one instruction completes before the next begins.
https://lagunita.stanford.edu/c4x/Engineering/CS316/asset/smith.precise_exceptions.pdf
Almost all modern processors support precise exceptions. So the hard work is already done. What an OS must do is register a trap handler for the exceptions that the hardware takes. For example, there will be a page fault handler, floating point, .... To figure out what is necessary for these handlers you'd have to read the processor theory of operations.
Despite what seems like gritty systems detail, that is fairly high level. It doesn't say anything about what the hardware is doing and so the FreeBSD description is shorthanding a lot.
To really understand precise exceptions, you need to read about that in the context of pipelining, out of order, superscalar, ... in a computer architecture book. I'd recommend Computer Architecture A Quantitative Approach 6th ed. There's a section Dealing With Exceptions p C-38 that presents a taxonomy of the different types of exceptions. The FreeBSD description is only describing some exceptions. It then gets into how each exception type is handled by the pipeline.
Also, The Linux Programming Interface has 3 long chapters on the POSIX signal interface. I know it's not FreeBSD but it covers what an application will see when, for example, a floating point exception is taken and a SIGFPE signal is sent to the process.
My understanding : An interrupt (hardware interrupt) occurs asynchronously generally been caused by an external event directly interrupting the CPU. The CPU will then vector to the particular ISR to handle the interrupt.
Obviously an ISR cannot have a return value or have parameters passed because the event happen at anytime at any point of execution in our code.
With exceptions however, my understanding is that this is a software interrupt which is caused by a special instruction pd within the software.
I've heard that exceptions are handled in a similar fashion to handling an ISR. Can an exception handler in that case behave differently to an ISR , by taking arguments from the code and return a value, because we know where we were in our code when it was executed?
Thanks in advance
A hardware exception is not a software interrupt, you do not explicitly call it - it occurs on some hardware detectable error such as:
invalid address
invalid instruction
invalid alignment
divide-by-zero
You can of course write code to deliberately cause any of these and therefore use them as software interrupts, but then you may loose the utility of them as genuine error traps. Exceptions are in some cases used for this purpose - for example in a processor without an FPU on an architecture where and FPU is an option, the invalid instruction handler can be used to implement software emulation of an FPU so that the compiler does not need to generate different code for FPU and non-FPU variants. Similarly an invalid address exception can invoke a memory manager to implement a virtual memory swap-file (on devices with an MMU).
A software interrupt is explicitly called by an SWI instruction. It's benefit over a straightforward function call is that the application does not need to know the location of the handler - that is determined by the vector table, and is often used to make operating system or BIOS calls in simple operating systems can dynamically load code, but that do not support dynamic-linking (MS-DOS for example worked in this way).
What hardware interrupts, software interrupts and exceptions all have in common is that they execute in different processor context that normal code - typically switching to an independent stack and automatically pushing registers (or using an alternate register bank). They all operate via a vector table, and you cannot pass or return parameters via formal function parameter passing and return. With SWI and forced-exceptions, it is possible to load values into specific registers or memory locations known to the handler.
The above are general principles - the precise details will vary between different architectures. Consult technical reference for the specific device used.
The term "exception" can mean completely different things.
There are "software exceptions" in the form of exception handling, as a high-level language feature in languages like C++. An "exception handler" in this context would be something like a try { } catch block.
And there are "hardware exceptions" which is a term used by some CPU cores like PowerPC. These are a form of critical interrupts corresponding to an error state. An "exception handler" in this context would be similar to an interrupt vector table, although when a hardware exception occurs, there is usually nothing the software can do about it.
Hardware exceptions take no arguments and return no data, just like interrupts. Architectures like PowerPC separate hardware exceptions from hardware interrupts, where the former are various error states, and the later are interrupts from the application-specific hardware.
It isn't all that meaningful for a hardware exception to communicate with the software, as they would be generated from critical failures like wrong OP code executed, CPU clock gone bad, runaway code etc etc. That is, the execution environment has been compromised so there is nothing meaningful that the software executing in that environment can possibly do.
When I was searching for a distinction between Exceptions and Interrupts,
I found this question Interrupts and exceptions on SO...
Some answers there were not suitable (at least for assembly level):
"Exception are software-version of an interrupt" But there exist software interrupts!!
"Interrupts are asynchronous but exceptions are synchronous" Is that right?
"Interrupts occur regularly"
"Interrupts are hardware implemented trap, exceptions are software implemented" Same as above!
I need to find if some of these answers were right , also I would be grateful if anyone could provide a better answer...
Thanks!
Interrupts are typically a method of signaling a change in hardware state. Peripherals will be tied by electrical signal to an interrupt controller which prioritizes and assigns address vectors to each possible signal. the interrupt controller forwards a detected interrupt condition to the CPU which may or may not 'interrupt' its present execution state to process the signaled state change (depending on whether interrupts are enabled and/or whether this particular input is non-maskable). Interrupt conditions may, on some architectures, be initiated by software (such as on the x86 there is an int mnemonic) in addition to hardware input.
Exceptions span a greater range of implementation. In some CPU architectures such as 68K, an exception can be similar to an interrupt but is generated by some CPU state that needs to be handled. For example there are conditions such as divide by zero, illegal instruction, I/O bus timeout, etc. that generate exceptions. By handling those exceptions one can do things such as emulate instructions and virtually extend the instruction set.
Exceptions may also be a software-only concept such as in the C++ language where certain error conditions can be trapped and handled.
So in general, the statements you are trying to find the validity of may be true or false depending on the exact platform you are applying them to.
An exception as it is used most often is a form of control flow in a programming language to deal with events outside the normal logic flow of the program to avoid that the business logic of a program drowns in the error handling logic. The 'handling' of the exception is context specific. It is more like a kind of GoTo for a number of use-cases where it was useful.
An interrupt is a hardware assisted 'trap' to trigger certain actions when certain events occur, as a timer tick or program "calling" INT21. There is a handler registered which does something predefined.
Both may or may not be synchronous or asynchronous.