Interrupts

Table of Contents

• Interrupt stack

Interrupt stack

TNeoKernel provides a separate stack for interrupt handlers. This approach could save a lot of RAM: interrupt can happen at any moment of time, and if there's no separate interrupt stack, then each task should have enough stack space for the worse case of interrupt nesting.

Assume application's ISRs take max 64 words (64 * 4 = 256 bytes on PIC32) and application has 4 tasks (plus one idle task). Then, each of 5 tasks must have 64 words for interrupts: 64 * 5 * 4 = 1280 bytes of RAM just for 64 words for ISR.

With separate stack for interrupts, these 64 words should be allocated just once. Interrupt stack array should be given to tn_sys_start(). For additional information, refer to the section Starting the kernel.

In order to make particular ISR use separate interrupt stack, this ISR should be defined by kernel-provided macro, which is platform-dependent: see PIC32 details.

In spite of the fact that the kernel provides separate stack for interrupt, this isn't a mandatory: you're able to define your ISR in a standard way, making it use stask of interrupted task and work a bit faster. There is always a tradeoff. There are no additional constraints on ISR defined without kernel-provided macro: in either ISR, you can call the same set of kernel services.

When you make a decision on whether particular ISR should use separate stack, consider the following:

• When ISR is defined in a standard way, and no function is called from that ISR, only necessary registers are saved on stack. If you have such an ISR (that doesn't call any function), and this ISR should work very fast, consider using standard way instead of kernel-provided macro.
• When ISR is defined in a standard way, but it calls any function and doesn't use shadow register set, compiler saves (almost) full context on the task's stack, because it doesn't know which registers are used inside the function. In this case, it usually makes more sense to use kernel-provided macro (see below).
• Kernel-provided interrupt macros switch stack pointer between interrupt stack and task stack automatically, it takes additional time: e.g. on PIC32 it's about 20 cycles.
• Kernel-provided interrupt macro that doesn't use shadow register set always saves (almost) full context on the interrupt stack, independently of whether any function is called from an ISR.
• Kernel-provided interrupt macro that uses shadow register set saves a little amount of registers on the interrupt stack.