Multitasking and scheduling

Colin Walls , in Embedded RTOS Pattern, 2021

Context save

An ISR e'er needs to save the "context" and so that the interrupted code is unaffected by the computations of the ISR. In a system implemented without an RTOS, this is only a affair of preserving any registers used by the ISR—generally on the stack—and restoring them earlier render. Some processors have provision for a dedicated ISR stack—others simply use the same 1 as the application code.

When an RTOS is in utilise, the approach may be exactly the same. In the same way, the stack used by the ISR may be "borrowed" from the currently running chore or it may exist another stack dedicated to interrupts. Some kernels implement this adequacy even if the CPU itself does non facilitate an interrupt stack. The situation becomes more complex if the ISR makes an API telephone call which affects the scheduler. This may event in the interrupt returning to a different task from the i that was running when the interrupt occurred.

Read full chapter

URL:

https://world wide web.sciencedirect.com/science/article/pii/B9780128228517000023

Interrupts in Nucleus SE

Colin Walls , in Embedded RTOS Design, 2021

Low- and high-level ISRs

Low-level ISR

A low-level interrupt service routine (LISR) executes as a normal ISR, which includes using the electric current stack. Nucleus RTOS saves context before calling an LISR and restores context later the LISR returns. Therefore LISRs may exist written in C and may call other C routines. However, there are only a few Nucleus RTOS services available to an LISR. If the interrupt processing requires additional Nucleus RTOS services, a loftier-level interrupt service routine (HISR) must exist activated. Nucleus RTOS supports nesting of multiple LISRs.

High-level ISR

HISRs are created and deleted dynamically. Each HISR has its own stack infinite and its own control cake. The retention for each is supplied by the awarding. Of grade, the HISR must be created before it is activated past an LISR.

Since an HISR has its ain stack and control block, information technology can be temporarily blocked if it tries to access a Nucleus RTOS data construction that is already being accessed.

There are three priority levels available to HISRs. If a college priority HISR is activated during processing of a lower priority HISR, the lower priority HISR is preempted in much the same style equally a chore gets preempted. HISRs of the same priority are executed in the order in which they were originally activated. All activated HISRs are processed earlier normal task scheduling is resumed.

Read full chapter

URL:

https://world wide web.sciencedirect.com/science/article/pii/B9780128228517000163

Real-time operating systems

Peng Zhang , in Advanced Industrial Control Technology, 2010

16.4.v Interrupt service routines

An interrupt service routine (ISR) is a software routine that hardware invokes in response to an interrupt. ISR examines an interrupt and determines how to handle information technology executes the treatment, and and then returns a logical interrupt value. If no further treatment is required the ISR notifies the kernel with a return value. An ISR must perform very quickly to avoid slowing downward the operation of the device and the operation of all lower-priority ISRs.

Although an ISR might motility data from a CPU register or a hardware port into a retention buffer, in general it relies on a dedicated interrupt thread (or job), called the interrupt service thread (IST), to practise nearly of the required processing. If boosted processing is required, the ISR returns a logical interrupt value to the kernel. It so maps a physical interrupt number to a logical interrupt value. For instance, the keyboard might be associated with hardware interrupt four on one device and hardware interrupt xv on some other device. The ISR translates the hardware-specific value to the standard value corresponding to the specific device.

When an ISR notifies the kernel of a specific logical interrupt value, the kernel examines an internal table to map the logical interrupt value to an event handle. The kernel so wakes the IST by signaling the issue. An event is a standard synchronization object that serves as an alarm clock to wake upwards a thread when something interesting happens.

The interrupt service thread is a thread that does about of the interrupt processing. The operating system wakes the IST when it has an interrupt to process, otherwise, it remains idle. For the operating system to wake the IST, the IST must associate an event object with an interrupt identifier. Afterwards an interrupt is candy, the IST should wait for the next interrupt signal. This phone call is usually inside a loop.

When a hardware interrupt occurs, the kernel signals the result on behalf of the ISR, and then the IST performs necessary I/O operations in the device to collect the data and process them. When the interrupt processing is completed, the IST informs the kernel to re-enable the hardware interrupt.

An interrupt notification is a signal from an IST that notifies the operating arrangement that an event must be candy. For devices that connect to a platform through intermediate hardware, the device driver for that hardware should pass the interrupt notification to the acme-level device driver. By and large, the intermediate hardware's device driver has some facility that allows some other device driver to register a call-dorsum function, which the intermediate device driver calls when an interrupt occurs.

Equally an example, PC cards connect to hardware platforms through a PC card slot, which is an intermediate piece of hardware with its own device driver. When a PC card device sends an interrupt, information technology is actually the PC bill of fare slot hardware that signals the physical interrupt on the system motorcoach. The device driver for the PC card slot has an ISR and IST that handle the physical interrupt. They use a function to pass the interrupt to the device driver for the PC card device. Devices with like connection methods carry similarly.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9781437778076100166

Interrupts

Xiaocong Fan , in Real-Time Embedded Systems, 2015

4.5 Design Patterns for ISRs

At present, we talk over a few design patterns for writing ISRs. These patterns are applicable to external interrupts, software interrupts, and internal interrupts.

iv.5.one Full general ISR Design Pattern

A simple ISR design pattern is given by ISR-Pattern-min().

ISR-Pattern-min()

one  Save the processor context (registers) onto interrupt stack;   //some processor/compiler does this automatically.

2  Articulate the interrupt source;

3  Service section (may access hardware ports for hardware interrupts);

4  Switching contexts: restore context from interrupt stack;   //some processor/compiler does this automatically.

The first step of ISR-Pattern-min() is to push button the current context onto the interrupt stack then that the context can be restored upon interrupt render. To protect context switching, this step has to be performed with the interrupts disabled.

The second stride is to clear the interrupt source. This is necessary specially for level-sensitive devices. At this time, interrupts can as well be enabled, if interrupt nesting is desirable.

The tertiary stride is the principal body of the ISR: service the interrupt source. For a hardware interrupt, this is the place to access the ports associated with the hardware to read inputs from external devices or write outputs to external devices. For example, in the ISR for a serial device, this is the place to transfer data from/to the universal asynchronous receiver/transmitter.

After the interrupt service has finished, at the fourth step the original context is restored and the processor is ready to execute the adjacent didactics prior to the interruption. During context switching, interrupts have to exist disabled.

ISR-Pattern-min() can exist used to write ISRs for those interrupt sources that have to be fully serviced rapidly (eastward.g., clock).

4.5.2 ISR with a Server Job

Not all interrupt sources accept to be fully serviced at the third step of ISR-Pattern-min(). In such a case, information technology is critical to optimize the third step to keep information technology equally brusk as possible.

This gives usa another pattern called ISR-Blueprint-SERVER(), where it is causeless that corresponding to the interrupt source there is a dedicated "server task," or device driver, which may be part of the operating system's kernel and offer services to user programs. The "server task" is ordinarily waiting (blocked) for service requests, and becomes active whenever (a) it receives a indicate (say, a semaphore) from an ISR—this is the time it needs to finish the bulk of the work deferred past the ISR, or (b) it receives a service call from a user program—this is the time it needs to relay relevant information to the user program.

Isr-Pattern-Server()

one  Save the processor context (registers) onto interrupt stack;   //some processor/compiler does this automatically.

two  Clear the interrupt source;

3  Service section (minimal processing only);

4  Indicate the corresponding server task (commuter task) for further processing;

5  Switching contexts: restore context from interrupt stack;   //some processor/compiler does this automatically.

The separation of concerns as implemented in Isr-Pattern-Server() has the following benefits:

•

Information technology allows the system to reply to a device faster, leading to higher hardware concurrency. The ISR may contain merely the part that has to exist done immediately in guild to keep the device working correctly. For case, in the ISR of a network device, it is necessary to copy the incoming packet off hardware. The processing of the parcel, nevertheless, can be deferred.

•

It tin amend the performance of the whole arrangement. The execution of an ISR blocks the highest-priority job from running. Lengthy processing tin can defer the execution (thus interruption the deadlines) of other ISRs and user tasks.

•

Only nonblocking functions tin can be used in an ISR. Complex operations can at present be moved out of the interrupt context.

•

Information technology offers flexibility for service direction. Some job tin be more than important than servicing interrupts. In such a case, the task can be designed with a higher priority than the server tasks for interrupts. This is non possible in ISR-Pattern-min().

4.5.3 ISR Chaining

Interrupt chainingallows multiple ISRs to share a unmarried interrupt vector. For example, the BIOS may install an ISR for timer interrupts (or real-time clock interrupts, or keyboard interrupts, etc.). An operating system may determine to install its own timer ISR to offer add-on services (say, for existent-time scheduling). Later, a user program may too wish to install a timer ISR to go a time-out result every 100 ms. When an operating arrangement or a user installs a new ISR to a vector that is currently in utilise, it is normal to copy the original ISR to a new location and phone call the original ISR immediately earlier the end of the new ISR.

This chaining process is illustrated in Figure iv.7. Here, the ISR installed past a user is at the beginning of the chain: its location is referenced past the vector x. Unless the user-installed ISR could get all the jobs done and done well, information technology typically will invoke the previous ISR to manus over the remainder of the task. This is how interrupt chaining gets its proper name. It is worth noting that only the last ISR on the concatenation should study EOI.

Figure 4.7. Interrupt chaining.

Next time y'all employ the system call InterruptAttach() to install an ISR, yous will know that this ISR is actually being placed in the front of a chain with other ISRs sharing the same vector number. Figure iv.8 gives an instance, where the 8254 programmable interval timer (PIT) bit runs at roughly 1.193182 MHz. The PIT chip has a sixteen-bit register used as a frequency divider, which can take values from 0 to 65,535 (0 represents 65,536).

Figure 4.8. Chaining the timer interrupt.

During the boot procedure, the BIOS typically sets the register to 0xFFFF, which gives an output frequency of xviii.2065 Hz (or one output every 54.9255 ms). This output is connected to the IRQ0 line of the PIC. Consequently, the Picture generates interrupts every 54.9255 ms, and the corresponding ISR (with the vector number 0x08 past default) is triggered to execute every 54.9255 ms. In improver, the ISR installed by the BIOS at 0x08 also invokes the ISR at 0x1C, which handles timer ticks—say, updates the fourth dimension of day. By the organisation phone call InterruptAttach(), a user timer ISR is installed prior to the default timer ISR. This user timer ISR can be used to generate customized periodic fourth dimension-out events demanded by other tasks.

Chaining can too be used by a debug monitor to gather system footprints.

iv.5.four ISR Cascading

Interrupt cascadingallows multiple interrupt sources to share i interrupt vector. This pattern is typically used by an operating system to group multiple relevant services together under i entry point. For example, DOS uses 0x21 for its services; all software interrupts on ARM processors share a unmarried vector, 0x0008.

Figure four.9 illustrates the idea of interrupt cascading, where the ISR direct referenced by the interrupt vector is called a level 1 ISR, and those ISRs referenced past the level 1 ISR are called level 2 ISRs, and so on. The branching typically happens at the outset of an ISR (switching points): an appropriate next-level ISR is invoked according to the specified function number (software interrupts) or past checking the status registers of hardware devices (hardware interrupts). It is also worth noting that for each IRQ there is just ane ISR to be executed to completion, and later on that the control is immediately returned to the interrupted program.

Figure 4.9. Interrupt cascading.

4.v.5 Data Sharing with ISRs

Problems may occur when there are shared data (or resource) in the system that demand to be accessed simultaneously past two or more processes. It is even more critical when a user process shares data with an ISR.

We utilize a classic case [71] to explain this effect. Suppose the fourth dimension-of-mean solar day information is stored separately in 4 shared variables: ms, sec, min, and hour. Every l ms these four variables are updated by a real-time clock ISR. In add-on, in that location is a user task that refreshes its time-of-day display on a GUI twice a 2d.

At present, suppose the electric current situation is 06:59:59:950, and the real-time clock ISR is triggered when the user task has merely refreshed the infinitesimal display. This is shown in Figure 4.10.

Figure 4.10. Shared data upshot.

Patently, the ISR would interrupt the user task. Inside the ISR, ms is increased past 50 ms, which will in plow trigger the variables sec, min, and hour to exist updated to their new values. Upon the completion of the ISR, the new time recorded in the variables is 07:00:00:00.

The execution of the user chore is at present resumed, and 07:59:59 is displayed on the GUI. This is almost 1 h ahead of the true time, which is annoying to people with abrupt eyes (although this could be corrected within 0.five south).

Figure four.11 gives a solution, which requires the user program to mark the display department every bit a critical department. The critical department has to be protected past disabling interrupts earlier it is entered and enabling interrupts subsequently.

Figure iv.11. Interrupt disabling.

While interrupts are disabled, the critical department is protected because the current job has sectional use of the processor (no other chore or interrupt tin take control). Pending interrupts or higher-priority tasks, if whatever, are not able to take control until after interrupts have been enabled.

This solution does come with drawbacks. First, disabling interrupts affects the timely update of the fourth dimension-of-day information contained by the shared variables. Over time, the system time could drift considerably away from the real fourth dimension. Second, disabling interrupts will also defer the execution of other ISRs, causing other interrupt-driven tasks to miss their deadlines. When such a solution is used, it is brash to ensure that a disquisitional department encloses necessary code simply then that the fourth dimension for executing the disquisitional department can exist minimized.

Some other solution, as shown in Effigy 4.12, involves the utilise of double buffers. In item, two sets of variables are used, and the ISR is coded such that it modifies but the fix of variables that is not currently being used by the user task.

Figure 4.12. Double buffering.

This solution tin avoid the drawbacks of the interrupt-disabling solution. However, it does need more memory for shared data. The displayed fourth dimension tin can drift slightly owing to the utilize of two variable sets.

Read full chapter

URL:

https://world wide web.sciencedirect.com/scientific discipline/article/pii/B9780128015070000043

More C and the wider C environment

Tim Wilmshurst , in Designing Embedded Systems with Flick Microcontrollers (Second Edition), 2010

17.four Interrupts

Interrupts present a number of challenges in the C surround. When working with interrupts we are working very shut to the hardware, nevertheless a loftier-level language tends to distance united states from it. A number of distinct and important actions must be taken in guild to allow the 18 Series interrupts to work successfully. The interrupt must exist enabled and allocated to the desired priority. The ISR must be located in program memory at the right get-go accost (noting that there are 2 interrupt vectors in the 18 Serial structure) and context saving must be managed. Check Figure 12.7 and the accompanying description for a reminder of these points if needed.

17.4.ane The Interrupt Service Routine

Interrupt Service Routines in C are like to C functions, except of course they are called by occurrence of an interrupt and stop with a return from interrupt instruction. They can accept local variables (i.e. variables declared inside the ISR) and access global ones. Global variables which are accessed by an ISR should, however, exist designated volatile. This indicates that the variable value can be changed outside normal programme operation. As the ISR can be called anywhere, it is not allowed to transfer any parameters or return values.

17.4.ii Locating and identifying the Interrupt Service Routine

The C18 compiler uses several pragmas, first to locate the outset of the ISR at the reset vector and and then to distinguish the ISR from a regular function.

Similar the reset vector, the C18 compiler does non automatically outset the ISR at the high or depression interrupt vector in program retention. This requires the use of the #pragma code, already described. This is used to locate the outset of the ISR correctly.

The ISR is identified in the program through use of a pragma. Two are bachelor for ISR definition:

•

#pragma interrupt function_name (save = save_list). This pragma declares function_name to be a loftier-priority ISR. The Fast Register Stack (Department xiii.6.iii) is used to relieve the minimum context – the STATUS, WREG and BSR registers. The interrupt is ended with a fast render from interrupt.

•

#pragma interruptlow function_name (save = save_list). This pragma declares function_name to be a low-priority ISR. The software stack is used to save the minimum context. This slows response to the interrupt. The interrupt is ended with a normal return from interrupt.

Further context saving, across the minimum, can be accomplished in either interrupt type by specifying annals(s) to be saved in the optional save section of the pragma.

Read full affiliate

URL:

https://www.sciencedirect.com/science/article/pii/B9781856177504100216

Cortex-M Architecture

Trevor Martin , in The Designer's Guide to the Cortex-1000 Processor Family unit (Second Edition), 2016

Interrupt Handling—Exit

Once the ISR has finished its task, it volition strength a return from the interrupt to the point in the groundwork code from where information technology left off. Nevertheless, the Pollex-2 instruction set does not have a render or return from interrupt instruction. The ISR volition use the same return method every bit a noninterrupt routine namely a co-operative on the R14 the link register. During normal operation, the link register will comprise the correct return accost. However, when we entered the interrupt, the current contents of R14 was pushed onto the stack and in its place the CPU entered a special code. When the CPU tries to branch on this code instead of doing a normal branch, it is forced to restore the stack frame and resume normal processing (Table 3.6).

Table iii.6. At the first of an exception or interrupt R14 (Link Register) is pushed onto the stack. The CPU and so places a control word in R14. At the end of the interrupt, the code will co-operative on R14. The control word is non a valid return address and will cause the CPU to recall a stack frame and return to the correct operating style

Interrupt Return Value Meaning
0xFFFFFFF9 Return to Thread mode and use the Master Stack Arrow
0xFFFFFFFD Return to Thread mode and use the Procedure Stack Pointer
0xFFFFFFF1 Return to Handler style

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780081006290000037

Cortex-M Architecture

Trevor Martin , in The Designer'southward Guide to the Cortex-g Processor Family, 2013

Interrupt Treatment—Exit

In one case the ISR has finished its task, it volition force a return from the interrupt to the point in the background code from where it left off. Nonetheless, the Pollex-two instruction set does non have a return or return from interrupt instruction. The ISR volition use the same return method as a noninterrupt routine, namely a branch on R14, the link annals. During normal operation, the link register will contain the correct return address. However, when we entered the interrupt, the current contents of R14 were pushed onto the stack and in their place the CPU entered a special code. When the CPU tries to branch on this code instead of doing a normal co-operative, information technology is forced to restore the stack frame and resume normal processing.

Table 3.6. At the Outset of an Exception or Interrupt R14 (Link Annals) Is Pushed Onto the Stack

Interrupt Return Value Pregnant
0xFFFFFFF9 Return to thread way and use the MSP
0xFFFFFFFD Return to thread mode and utilize the PSP
0xFFFFFFF1 Return to handler fashion

The CPU then places a command word in R14. At the stop of the interrupt, the lawmaking volition branch on R14. The command word is not a valid render address and will crusade the CPU to recollect a stack frame and render to the correct operating mode.

Read total affiliate

URL:

https://www.sciencedirect.com/science/article/pii/B9780080982960000037

Interrupts

Kevin One thousand. Lynch , ... Matthew L. Elwin , in Embedded Computing in C with the PIC32 Microcontroller, 2016

6.three Steps to Configure and Use an Interrupt

The bootloader (and NU32_Startup()) enables the CPU to receive interrupts, setting multi-vector mode by setting INTCONbits.MVEC to 1. Subsequently being in the correct mode, there are seven steps to configure and employ an interrupt. Nosotros recommend your program execute steps 2-vii in the order given below. The details of the syntax are left to the examples in Section vi.4.

1.

Write an ISR with a priority level 1-7 using the syntax

void __ISR(vector_number, IPLnXXX) interrupt_name(void) { ... }

where vector_number is the interrupt vector number, n=1 to 7 is the priority, XXX is either SOFT or SRS, and interrupt_name can be anything just should describe the ISR. SOFT uses software context save and restore, and SRS uses the shadow register prepare. (The bootloader on the NU32 allows only priority level 6 to use the SRS, so if y'all use the SRS, you should utilize the syntax IPL6SRS.) No subpriority is specified in the ISR function. The ISR should articulate the appropriate interrupt flag IFSxbit.

ii.

Disable interrupts at the CPU to forbid spurious generation of interrupts while you are configuring. Although interrupts are disabled by default on reset, NU32_Startup() enables them. To disable all interrupts y'all tin utilize the special compiler instruction __builtin_disable_interrupts().

three.

Configure the device (east.g., peripheral) to generate interrupts on the appropriate event. This procedure involves configuring the SFRs of the particular peripheral.

four.

Configure the interrupt priority and subpriority in IPCy.The IPCy priority should match the priority of the ISR defined in Pace i.

5.

Clear the interrupt flag condition bit to 0 in IFSx.

6.

Set the interrupt enable fleck to 1 in IECx.

7.

Re-enable interrupts at the CPU. You tin can use the compiler instruction __builtin_enable_interrupts().

Read full chapter

URL:

https://world wide web.sciencedirect.com/science/article/pii/B9780124201651000068

Operating Systems

Jean J. Labrosse , in Software Applied science for Embedded Systems (Second Edition), 2019

x.2 NonKernel-Enlightened Interrupt Service Routine (ISR)

The above sequence assumes that the ISR signals or sends a message to a job. Notwithstanding, in many cases, the ISR may not demand to notify a task and can simply perform all its work within the ISR (assuming information technology can be done speedily). In this case, the ISR will appear every bit shown in Listing 5.

Listing 5

List 5. Nonkernel-aware interrupt service routine.
(ane)

As mentioned higher up, an ISR is typically written in assembly language. MyNonKernelAwareISR() corresponds to the proper noun of the handler that will handle the interrupting device.

(ii)

Here, yous save sufficient registers required to handle the ISR.

(3)

The user probably needs to clear the interrupting device to prevent it from generating the same interrupt once the ISR returns.

(4)

You should non reenable interrupts at this point since some other interrupt could be kernel aware thus forcing a context switch to a higher priority task. This means that the above ISR would complete, but at a much after time.

(5)

At present you can take intendance of the interrupting device in assembly linguistic communication or call a C function, if necessary.

(half-dozen)

In one case finished, simply restore the saved CPU registers.

(7)

The ISR completes past performing a render from interrupt to resume the interrupted chore.

Read full chapter

URL:

https://world wide web.sciencedirect.com/science/article/pii/B9780128094488000060

EXCEPTION AND INTERRUPT HANDLING

ANDREW N. SLOSS , ... CHRIS WRIGHT , in ARM Organization Developer's Guide, 2004

9.3.8 VIC PL190 BASED INTERRUPT SERVICE ROUTINE

To take advantage of the vector interrupt controller, the IRQ vector entry has to be modified.

This instruction loads an ISR accost from the retentivity mapped location 0xffffff030 into the pc which bypasses any software interrupt handler since the interrupt source can be obtained straight from the hardware. It also reduces interrupt latency since there is only a single jump to a specific ISR.

Here is an example of VIC service routine:

This routine saves the context and due south psr_irq before clearing the interrupt source. One time this is consummate, the IRQ exceptions tin exist reenabled by clearing the i fleck, and the processor manner is ready to system mode. The service routine can then process the interrupt in system fashion. Once consummate, the IRQ exceptions are disabled by setting the i bit, and the processor mode is switched back to IRQ style.

The spsr_irq is restored from the IRQ stack, preparing the routine to return to the interrupted job.

The service routine and so writes to the VICVectorAddr register in the controller. Writing to this address indicates to the priority hardware that the interrupt has been serviced.

Note that since the VIC is basically a hardware interrupt handler, the assortment of ISR addresses must be preprogrammed into the VIC before it is activated.

Read full chapter

URL:

https://world wide web.sciencedirect.com/scientific discipline/commodity/pii/B9781558608740500101