Transferring Data Between the CPU and I/O Device Types of Interrupts Processing Interrupts Interrupts Hardware and Priority Implementing Interrupts Inside the CPU Topics Covered
What is Interrupts ? Interrupts is a mechanism for alleviating the delay caused by this uncertainty and for maximizing system performance.
Transferring Data Between the CPU and I/O Device Polling: One method used in small system to alleviate the problem of I/O devices with variable delays. In polling, the CPU sends a request to transfer data to an I/O device. The I/O device processes the request and sets a device- ready signal when it is ready to transfer data. The CPU reads in this signal via another I/O address and checks the value. If the signal is set, it performs the data transfer. If not, it loops back, continually reading the value of the device ready signal.
Transferring Data Between the CPU and I/O Device (continue) Wait States: When wait states are used, the processor requests data from an I/O device, which then asserts a wait signal that is sent to the CPU via the control bus. As long as the signal is asserted, the CPU stays in a wait states, still outputting the address of the I/O device and the value of the control signal needed to access the device, but not doing anything else. The I/O device continues to assert this wait signal until it is ready to send or receive data. Once it is ready, the I/O device de-asserts it’s wait signal and the CPU completes the data transfer.
Transferring Data Between the CPU and I/O Device (continue) wait states simplifies the job of the programmer. Unlike polling, we need no additional code to accommodate the variability of the timing of the I/O device; like polling, the CPU does not perform any useful work while waiting for the I/O device to become ready to transfer data. To make use of this wasted CPU time, interrupts are developed.
Transferring Data Between the CPU and I/O Device (continue) Interrupts Unlike polling or wait states, they do not waste time waiting for the I/O device to become ready. --When interrupts are used with I/O devices, the CPU may output a request to the I/O device and, instead of polling the device or entering a wait state, the CPU then continues executing instructions, performing useful work. --When the device is ready to transfer data, it sends an interrupts request to the CPU; this is done via a dedicated signal on the control bus. --The CPU then acknowledges the interrupt, typically by asserting an interrupt acknowledge signal, and completes the data transfer.
External Interrupts External interrupts: are used by the CPU to interact with input/output devices. External interrupts improve system performance by allowing the CPU to execute instructions, instead of just waiting for the I/O device, while still performing the required data transfers.
Internal Interrupts Internal interrupts: occur entirely within the CPU; no input/output devices play any role in these interrupts. Internal interrupts could be used to allocate CPU time to different tasks in a multitasking operating system. This interrupts can also be used to handle exceptions that occur during the execution of valid instructions.
Software Interrupts Software interrupts: are generated by specific interrupt instructions in the CPU’s instruction set.
Processing Interrupts Who services the interrupt? An interrupt triggers a sequence of events to occur within the computer system. These events acknowledge the interrupt and perform the actions necessary to service the interrupt. These events only occur if the interrupt is enabled. Interrupt is also serviced by software which is written by the user, is called the handler, essentially a subroutine.
Sequence of Events Do nothing(until the current instruction has been executed) Get the address of the handler routine.(vector interrupts only) Invoke the handler routine
Sequence of Events (1) Do nothing (until the current instruction has been executed) If an execute routine is interrupted part way through, we would have to save the contents of many of the internal registers of the CPU, as well as the state information within the control unit. In contrast, it is not necessary to save this information if the execute routine has been completed.
Sequence of Events (2) Get the Address of the Handler Routine (Vectored interrupts only) Vectored interrupts supply the CPU with information, the interrupt vector, which is used to generate the address of the handler routine for that interrupt. Vectored interrupts are useful for CPUs that receive interrupt requests from several devices via the same control line.
Sequence of Events (3) Invoke the handler Routine When the CPU accesses the handler routine, it pushes the current value of the program counter into the stack and loads the address of the handler routine into the program counter. The handler routine then performs its tasks. When it is finished, it returns to the correct location by popping the value of the program counter off the stack.
Interrupt Hardware and Priority Three hardware samples for interrupts: Hardware and Timing of a non-vectored interrupt for a single device. Hardware and Timing of a vectored interrupt for a single device. Hardware of multiple non-vectored interrupts
(a) (b) (a)Hardware of a non-vectored interrupt for a single device An external device sent an interrupt to the CPU by asserting its interrupt request (IRS) signal. When the CPU is ready to process the interrupt request, it assert the its interrupt acknowledge signal (IACK), thus informing the I/O device that is ready to proceed. (b)Timing of a non-vectored interrupt for a single device The device set the IRQ low, which cause the CPU set the IACK low. As the handler routine proceeds, it transfers data between the CPU and the interrupting device. CPU IRQ IACK Data Device valid IRQ IACK Data Interrupt Request Interrupt Acknowledge Data bus
(a)Hardware of a vectored interrupt for a single device A vector interrupt is more complex. After acknowledge the interrupt, the CPU must input an interrupt vector from the device and call an interrupt service routine(handler); the address of this routine is a function of the vector. (b)Timing of a vectored interrupt for a single device CPU IRQ IACK Data Device valid IRQ IACK Data (b) Interrupt Request Interrupt Acknowledge Interrupt Vector (a)
CPU IRQ 0 IACK 0 IRQ 1 IACK 1 IRQ n IACK n Data Device #0 Device #1 Device #n Hardware for multiple non-vectored interrupts In addition to enabling and disabling interrupts, we must also consider the priority of the interrupts. In general, the second interrupt is processed if its priority is higher than that of the interrupt currently being processed. If not, it remains pending until the current handler routine is complete.
Two methods used for prioritizing multiple interrupts Daisy Chaining Parallel Priority
Daisy Chaining: The interrupt request signals from the devices are wire-ORed together. When the CPU receives an active IRQ input, it cannot know which device generated the interrupt request. It sends out an acknowledge signal and leaves it to the devices to work that out among themselves. CPU IACKin IRQ Device #n D IACKout IRQ Device#n-1D IRQ Device#0 D IACKin IACKout IACKin IACKout IACK IRQ Data Interrupt Acknowledge Interrupt Request Vector
Device is blocked from from interrupting by a device with higher priority(device may not may not be issuing an interrupt request 00 Invalid state 10 Device interrupts CPU 01 Device has priority to interrupt but does not 11 StateIACKoutIACKin Possible values of IACKin and IACKout and their states The invalid state (IACKin = 0 and IACKout = 1) is shown to account for all possible value of IACKin and IACKout, but a device should never be in this state.
CPU Priority encoder IRQ Device #1 IRQ Device #n IRQ Data IACK IRQ Device #0 0 1 n Interrupt request Vector Interrupt Acknowledge Implementing priority interrupts in parallel The IRQ input to the CPU ids generated as in the daisy chain configuration, using a wired-OR of the IRQ signals from the devices. Note that buffers are needed to prevent the signals from the values input to priority encoder. Unlike daisy chaining, however, the IACK signal simply enables a priority device requesting an interrupt. This value is placed on the data bus as the interrupt vector and is read in by the CPU, which then proceeds as before.
The most difficult part of handling the interrupt is recognizing it and accessing the states to process the interrupt. This is done every execute cycle, and could be done in one or two ways. 1. Using separate FETCH1 and INT1 states 2. Modifying FETCH1 to support interrupts
1. Using separate FETCH1 and INT1 states The branches that go to state FETCH1 are broken into two branches. If interrupts are enabled (IE=1) and an interrupt is pending (IP=1), these states branch to the beginning of the interrupt handler routine, state INT1, rather than to FETCH1. If either the IE or IP is 0, no interrupt is processed and the CPU proceeds to FETCH1 to continue processing instructions. execute routines FETCH1 execute routines IE’VIP’ IE^IP FETCH1 INT1
2.Modifying FETCH1 to support interrupts The micro-operations associated with the state can be modified. State FETCH1 would consist of two sets of micro-operations. The CPU could branch to either FETCH2 or INT2. FETCH 1FETCH 2 Modified FETCH INT 2 IE’VIP’ IE^IP FETCH 2
How to Access the Interrupt Handler? 1. CPU pushes the return address on to the stack 2. Reads in the interrupt vector 3.Jumps to the address corresponding to this vector, 1111(vector) 0000.