Real-Time Kernel (Part 1) Operating Systems Design and Implementation

Basic Real-Time Concepts Soft versus Hard real-time soft--as fast as possible but missing deadline is tolerable hard--correct and on time

Foreground/Background Systems Often referred to as super-loops Infinite loop calling modules ISR Handle Asynchronous Events ISR ISR Interruption occurred Background (task level) Foreground (interrupt level)

Interrupt Service Routines Handle critical operations can take longer than they should make data available for background routines processing of such information is referred to task-level response

Example: EFI System (Electronic Fuel Injection) What are the components? Throttle Body Cold start solenoid Air-flow meter Injectors O2 sensor Water temperature sensor ECU Distributor sensors High pressure fuel pump manifold sensor

EFI System How does it work? fundamentally, it manages three necessities to start and maintain operation of a gasoline engine fuel spark air

EFI System What happen to the EFI system when you start a car? What happen to the EFI system when you drive a car?

Dissecting EFI System Tasks ISRs

EFI System Critical section (atomic or indivisible) Mutual exclusion? any possible critical regions in our tasks? Mutual exclusion? Reentrant code? functions can be used by multiple tasks without causing data corruption Priority inversion problem?

Priority Inversion Assume task 3 has lower priority than task 1. Task 1 is doing I/O so Task 3 gets to run Task 3 is in the middle of accessing a shared resource (obtain semaphore) Task 1 finishes so it preempts Task 3 Task 1 wants to access the same resource but can’t since Task 3 has the semaphore

Priority Inversion Priority Inheritance In this scenario, the priority of Task 1 is reduced to that of Task 3. What is a good solution? Priority Inheritance

Priority Inversion Priority Inheritance Task 1 is doing I/O so Task 3 gets to run Task 3 is in the middle of accessing a shared resource (obtain semaphore) Task 1 finishes so it preempts Task 3 Task 1 wants to access the same resource but can’t since Task 3 has the semaphore; thus the kernel raises the priority of Task 3 to the same as Task 1 Task 3 gets to finish and releases the resource. The priority is reset to the original value Task 1 is selected if it still has the highest priority

Assigning Task Priority Rate monotonic scheduling tasks with the highest rate of execution are given the highest priority Assume all tasks are periodic Tasks do not synchronize with another, share resources, and exchange data Preemptive scheduling is used

Assigning Task Priority Number of Tasks n(21/n - 1) 1 2 0.82 3 0.77 4 0.75 5 0.74 … - 0.69

Providing Mutual Exclusion Disabling interrupt Test and Set operation hardware support (TSL operation) Disabling scheduler Semaphores how is semaphore implemented?

Disabling Interrupt X86 CLI (disable interrupt) STI (enable interrupt)

Busy Waiting

Busy Waiting

Semaphore Is a type that has a counter and a delay queue require OS support as processes in the delay queue are blocked implementation often requires other primitive support (disabling interrupt, etc.)

Semaphore assumes the existence of binary semaphore operations upb and downb implemented with a test-and-set instruction and busy waiting

Intertask Communiciation Message mailbox Message queues often use to process interrupt

Interrupts A hardware mechanism used to notify the CPU that asynchronous events have occurred Upon completion, the programs return to: background for a foreground/background system the interrupted task for non-premptive kernel the higest priority task ready to run for premptive kernel

Example: Interrupt in NIOS IE bit to enabling interrupt PRI bits for priority MISC bits for interrupt control

Source of Exceptions (NIOS) External Hardware interrupt Sources External logic for producing the 6-bits interrupt number & asserting the IRQ input pin is automatically generated by the SOPC builder and is included in the Peripheral Bus Module (PBM). Internal Exception Sources 2 sources of internal exceptions Register window-overflow, Register window-underflow Direct Software Exceptions Software can request that control be transferred to an exception handler by issuing a TRAP instruction.

External Hardware Interrupts Active-high interrupt signal: irq Level sensitive Sampled synchronously at the rising edge of Nios clock Should stay asserted until the interrupt is acknowledged by software 6-bit Input Interrupt Number: irq_number[5:0] Identifies the highest priority interrupt currently requested Highest priority = 0 (irq #0 to #15 are reserved) Lowest priority = 63

External Hardware Interrupts Nios will process the indicated exception if IE= 1 – i.e. external interrupts & internal exceptions are enabled, AND The interrupt number is smaller (lower or equal) than the IPRI field value

Interrupt Service Routine Handler nr_installuserisr( int trapNumber, void *ISRProcedure, int context) trapNumber is the exception number to be associated with a service routine ISRProcedure is a routine which has a prototype of typedef void (*Nios_isrhandlerproc) (int context); context is a value that will be passed to the routine specified by isrProcedure

ISR Handler This routine installs an interrupt service routine for a specific exception number If nr_installuserisr() is used to set up the exception handler, then the exception handler can be an ordinary C routine

ISR Process Interrupt occurs Current state is saved (Context) Memory Main Program Save Context Interrupt occurs Current state is saved (Context) ISR address is retrieved from the vector table based on the interrupt number Jump to ISR routine Runs to completion Context is restored Program resumes Restore Context ISR Vector Table

ISR Implementation Specify your # IRQ Declare your IRQ subroutines Update the ISR vector table ROM instruction RAM stack @irq_subroutine 0 … @clock_adj_ISR @RealTime_ISR @irq_subroutine 63 Vector Table 0xFFFF 0xFF0F 0xFF0E 0xFFC0 Write your IRQ Subroutine Write your IRQ Subroutine