POSIX IPC on Cortex-M architectures

Tyler Gilbert of CoActionOS explains how to implement the system components required send and receive POSIX-style signals-a form of IPC–between concurrently running tasks on the Cortex-M architecture and then specifically with the Cortex-M3 (CM3).

The ARM Cortex-M architecture is a 32-bit microcontroller core designed to replace many 8-bit and 16-bit devices. This is in contrast to the ARM Cortex-A architecture, an application processor that typically runs Linux, iOS, or Windows 8 (at least in development environments).

The M-series runs lesser-known, embedded operating systems such as FreeRTOS, ThreadX, and my favorite (of course) CoActionOS. These operating systems use the advanced features of the Cortex-M architecture to implement system functionality such as context switching and inter-process communication (IPC).

This article explains how to implement the system components required to send and receive POSIX-style signals – a form of IPC – between concurrently running tasks on the Cortex-M architecture, and deals specifically with the Cortex-M3 (CM3).

Processor interrupt vs. task interrupt
A processor interrupt, when talking in general about interrupts on microcontrollers, is usually triggered by peripheral circuitry such as a timer or input/output port, and causes the processor to suspend its current activities and execute an interrupt service routine (ISR). On the CM3, the Nested Vectored Interrupt Controller (NVIC) handles processor interrupts. By contrast, a task interrupt is when a function is placed on a dormant task’s stack and executed when the task resumes.

For purposes of this discussion, a ‘task’ is defined as an independent execution context (either a process or thread) that is preemptively managed by a scheduling algorithm. An example of a task interrupt in a POSIX-style operating system is when a task calls the sigqueue() or kill() function. The operating system inserts a function on the stack of the task receiving the signal. When the scheduler resumes execution of the receiving task, the signal handling function is executed before the task resumes.

On the CM3, a processor interrupt has two attributes that make task interrupts useful. First, the CM3’s NVIC always executes ISRs in handler mode rather than process mode. Second, ISR execution is prioritized over normal execution. Using a task interrupt allows application developers to execute the response to an event in process mode and at the same priority level as the task that is responding to the event.

CM3 system support hardware and context switching
Note: To implement task interrupts on the CM3, knowledge of advanced CM3 features as well as context switching is required.

Advanced features of the CM3 include two execution modes and two stack pointers. The execution modes are handler mode and process mode while the stacks are the main stack pointer (MSP) and the process stack pointer (PSP). ISRs always execute in handler mode: privileged and using the MSP. Process mode can be configured to use either the MSP or the PSP as well as to be privileged or unprivileged. Unprivileged code execution has limited access to critical system registers.

Additionally, the memory protection unit (MPU) can be configured to allow privileged software to have different permissions than unprivileged code. Code executing in process mode must use the SVCall interrupt to switch to handler mode; this allows application developers access to kernel features such as context switching.

The CM3 architecture provides two processor interrupts that facilitate context switching: the PendSV and SysTick interrupts. The PendSV interrupt can be triggered by software while the SysTick interrupt is triggered when the SysTick timer counts down to zero. One systems design approach is to use PendSV for FIFO algorithms and SysTick for round robin scheduling.

When an interrupt is handled, the NVIC hardware immediately pushes the following registers (referred to in the figures as the ‘hardware stack frame’): r0, r1, r2, r3, r4, r12, LR, PC, and xPSR. By convention, only these registers are required to be saved/restored when handling interrupts. However, for context switching, all registers must be saved/restored.

When the NVIC starts execution of the context switcher, the context switcher pushes the remainder of the registers (the ‘software stack frame’ in the figures): r4, r5, r6, r7, r8, r9, r10, and r11. The context switcher then decides which task to execute next, updates the process stack pointer (PSP), and pops the software stack frame for the new task. When the context switcher exits, the ISR returns, the NVIC pops the hardware stack frame, and execution of the next task resumes. The chronology of a context switch is illustrated in Figure 1 below.

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Figure 1: Chronology of a Context Switch

(More details on context switching on the CM3 can be found in this article: “Taking advantage of the Cortex M3’s pre-emptive context switches”.)

Implementing Task Interrupts
Implementing a task interrupt on the CM3 architecture involves manipulating the stack of the receiving task (Figure 2). This method assumes the sending task is different than the receiving task. If the sending and receiving tasks are the same, the sending task can simply make a direct call to the task interrupt handling function.

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Figure 2: Task Interrupt Stack Timeline

Figure 2 shows a timeline for inserting a task interrupt handler on the receiving task as well as how the stack is restored after the handler returns.

The NVIC and context switcher insert the hardware and software stack frames when switching from the receiving task to some other task.

The sending task decrements the receiving task’s stack with a new hardware stack frame and an uninitialized software stack frame.

When the receiving task resumes, the context switcher pops the new software stack frame, and the NVIC pops the hardware stack frame causing the task interrupt handling function to be executed.

When the task interrupt handling function exits, an interrupt is forced, and the stack is adjusted such that the original hardware as well as software stack frames are restored.

When the forced interrupt exits, the NVIC uses the original hardware stack frame to restore execution of the task to its pre-task-interrupt state.

The sending task adjusts the receiving task’s stack and initializes the new hardware stack frame; the new software stack frame is uninitialized by convention. The registers in the hardware stack frame are crucial to the execution of and return from the task interrupt:

r0-r3: These are the parameters (a,b,c and d respectively, see task_interrupt_handler() below) passed to the task interrupt-handling routine.
pc: This is the location of the task interrupt-handling routine.
lr: This is the location of the return routine which restores normal task execution.