Computer Hardware

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.

A hardware interrupt causes the processor to save its state of execution and begin execution of an interrupt handler. Software interrupts are usually implemented as instructions in the instruction set, which cause a context switch to an interrupt handler similar to a hardware interrupt.

Interrupts are a commonly used technique for computer multitasking, especially in real-time computing. Such a system is said to be interrupt-driven.

An act of interrupting is referred to as an interrupt request (IRQ).

Types of Interrupts

1. Level-triggered

A level-triggered interrupt is an interrupt for which an unserviced interrupt is indicated by a particular state, high level or low level, of the interrupt request line. A device wishing to signal an interrupt drives the line to its active level, and then holds it at that level until serviced. It ceases asserting the line when the CPU commands it to or otherwise handles the condition that caused it to signal the interrupt.

2. Edge-triggered

An edge-triggered interrupt is an interrupt signalled by a level transition on the interrupt line, either a falling edge (high to low) or a rising edge (low to high). A device wishing to signal an interrupt drives a pulse onto the line and then releases the line to its inactive state. If the pulse is too short to be detected by polled I/O then special hardware may be required to detect the edge.

Edge-triggered interrupts do not suffer the problems that level-triggered interrupts have with sharing. Service of a low-priority device can be postponed arbitrarily, and interrupts will continue to be received from the high-priority devices that are being serviced. If there is a device that the CPU does not know how to service, it may cause a spurious interrupt, or even periodic spurious interrupts, but it does not interfere with the interrupt signalling of the other devices. However, it is fairly easy for an edge triggered interrupt to be missed - for example if interrupts have to be masked for a period - and unless there is some type of hardware latch that records the event it is impossible to recover. Such problems caused many "lockups" in early computer hardware because the processor did not know it was expected to do something. More modern hardware often has one or more interrupt status registers that latch the interrupt requests; well written edge-driven interrupt software often checks such registers to ensure events are not missed.

A common use of a hybrid interrupt is for the NMI (non-maskable interrupt) input. Because NMIs generally signal major -- or even catastrophic -- system events, a good implementation of this signal tries to ensure that the interrupt is valid by verifying that it remains active for a period of time. This 2-step approach helps to eliminate false interrupts from affecting the system.

4. Message-signaled

Main article: Message Signaled Interrupts

A message-signalled interrupt does not use a physical interrupt line. Instead, a device signals its request for service by sending a short message over some communications medium, typically a computer bus. The message might be of a type reserved for interrupts, or it might be of some pre-existing type such as a memory write.

Message-signalled interrupts behave very much like edge-triggered interrupts, in that the interrupt is a momentary signal rather than a continuous condition. Interrupt-handling software treats the two in much the same manner. Typically, multiple pending message-signalled interrupts with the same message (the same virtual interrupt line) are allowed to merge, just as closely spaced edge-triggered interrupts can merge.

500 MB can mean two thing: 500 MEGA BYTES of main (system) memmory or 500 Mega bytes of storage (disk drive). One Megabyte is roughly the equivalent of one full novel. its not exactly 500megabytes, this is due to each value increasing by itself, eg 1,2,4,8,16,32,64,128,256,512.....etc. Answer A byte is a unit of data storage. A megabyte is 1,000,000 bytes. And 500 megabytes is (of course) 500,000,000 bytes. Data storage comes in many forms (disk drives, memory (properly called RAM), flash memory like USB sticks, etc.) but the term 500 MB means the same thing either way. And yes, data storage that is equal to 500 MB is perfectly possible, the person who gave the second answer seems not to realize that multiples of two do not govern everything. RAM is usually retailed in such units (64 MB, 128 MB, 256 MB, etc.), but other methods of storage have no such constraints (like CDs that hold 700 MB, or floppy disks that hold 1.44 MB). AnswerWhoever gave the 3rd answer is forgetting that there is such thing as File System (FS) on mass Storage devices (such as CD's, DVD's, Harddrives, Floppy Disks etc) which take up space. EG if you get a 1GB Pendrive, it will have 0.99GB Space because the file system will take up the 0.01GB. So if you are looking at 500MegaBytes of RAM, there is a different type of file system, which is NOT stored on the RAM sticks, which means you get the full 500MB. :-)

Answer Is this kid serious asking this question? Yes their is such thing as 500 mb. Some people should not be allowed to own a computer...

Communications and Network Riser (CNR) is a interface on a computer motherboard for interfacing a communication device or peripheral. There have been several competing configurations but none of them seem to have really taken the lead.

This article puts it a little better perspective:

= The Strange Case of the AMR/CNR/ACR Slot =

article by: Demir Cutts

=== === Nearly everything inside a PC is designed to be user-installable. The Audio Modem Riser (AMR), Communications and Networking Riser (CNR), and Advanced Communication Riser (ACR) slots are exceptions. Although their presence on many recent motherboards intrigues some upgraders, these slots were never intended as general-purpose expansion slots. All of them were designed to be used by OEM system builders, not by backyard mechanics. Here's what you need to know about AMR, CNR, and ACR slots ; AMR slot : Intel developed the AMR slot to provide an easy, standardized way to integrate modem and audio functions into finished systems at minimal cost, but OEM system builders ignored it in droves. Why? Mainly because the AMR slot took the place of a standard PCI slot, and most motherboard designers and system builders rightly preferred having an extra PCI slot to having an AMR slot of dubious utility. The AMR slot also had limited functionality and no support for Plug and Play. The result was that, although some motherboards included an AMR slot, very few AMR-compatible cards were ever developed and those that were achieved only limited distribution. We've seen exactly one AMR card. ; CNR slot : Intel's answer to the problems of AMR was to redesign the AMR slot. The CNR slot, can coexist with a standard PCI slot, allowing either a CNR card or a standard PCI card to use the slot position interchangeably. CNR also adds Plug and Play support and other features of interest to system designers. AMR and CNR are incompatible, at both the physical and electrical level. Although we have seen a few CNR cards, mostly modems and sound adapters, CNR cards are not much easier to find than AMR cards. ; ACR slot : AMR and CNR are both Intel technologies. AMD, VIA and the rest of the everyone-who-is-not-Intel camp came up with an alternative called the ACR slot, which is found on some Intel-free motherboards. The ACR slot is physically a standard PCI slot connector, which you can recognize because it's turned 90 degrees to the other PCI connectors on the motherboard. In theory, the ACR slot offers several advantages over the AMR/CNR slot, including its use of standard connectors and its additional flexibility because of the greater number of available pins. In practice, we've never seen or even heard of a card designed to fit that slot, so it is effectively a wasted connector. Intel warns that the AMR and CNR interfaces are not rigidly defined, so it is quite possible that any given AMR or CNR card simply will not work in a particular AMR or CNR slot. If your motherboard has an AMR, CNR, or ACR slot, we suggest you pretend it's not there.