A central processing unit leads probably the most repetitive existence know to man (or woman). It does the same thing, nanosecond after nanosecond, never gets tired and never makes a mistake. Amazing. In this lesson, we investigate the cycle of its life. We are learning ...
So that we can ...
Remember that the operation of a von Neumann machine requires that both instructions and data are accessible from the same data bus. Therefore, the Fetch (Decode) Execute cycle can only work with these so-called 'Stored Program Concept' machines.
In a previous topic, we learnt about the operation of the processor, how it utilised its connection to the components on the motherboard in order to carry out its tasks. However, we didn't consider the sequence in which it carries out those tasks. Firstly, look at the following animation. You will need to enlarge it so you can read it better. Click to enlarge, 'cause it's a bit small. You'll notice the strange notation used to represent the separate stages in the Fetch (Decode) Execute cycle. This is called register transfer notation and is commonly used to denote data flowing between registers and memory in the processor. In essence ...
Yes, but I'd still make some notes! Yes and no. This is a grossly over-simplistic view of this processes - in reality, the processor is doing a lot more work than this keeping track of the operating system and the hardware for instance.
Virtually all computer systems provide a mechanism for handling interrupts from ...
Once the processor receives an interrupt from the 'device' (via a Programmable Interrupt Controller or PIC), the processor will respond by suspending its current activities, saving its current state and executing a function called an Interrupt Service Routine, or ISR, to deal with the event. After the interrupt is handled, control is passed back to the previous activity. High priority interrupts are dealt with before low priority ones, as you might expect. Interrupts in computer systems can either be vectored or polled. In a vectored interrupt, the device directs the processor towards an appropriate Interrupt Service Routine whereas in polled interrupt, a single interrupt service routine needs to 'poll' the devices to determine which one initiated the interrupt. Interrupts are a normal part of computation - they do not necessarily indicate errors Vectored interrupts require that each device be assigned a unique identifier called an IRQ (Interrupt ReQuest). The processor knows the address of the Interrupt Service Routine in advance; all it needs is the IRQ code from the device and it can look up the ISR memory address in its 'Interrupt-Vector Table' and execute the interrupt by setting the program counter to the memory address of the ISR. The original (i.e. old) PC based on 16-bit ISA (Industry Standard Architecture) had 16 hardware interrupt lines, maintained, originally, by two Programmable Interrupt Controllers (PIC) which are still available for legacy reasons even in today's PCs ...
... although very few of these legacy IRQs are used these days ... Screenshot of msdiag32.exe from my laptop More modern PCI architectures (using APIC, Advanced Programmable Interrupt Controller) increased the theoretical number of hardware IRQ to 24 helping to avoid IRQ sharing / conflicts and enable connectivity of more hardware devices. IRQs above 23 are maintained dynamically by the operating system.
How about these?
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