Preemptive Multitasking (ARM)

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This page documents how Embedded Xinu implements preemptive multitasking on ARM-architecture platforms that it has been ported to, such as the Raspberry Pi.

Thread context

The format and size of the thread context used for ARM ports of Embedded Xinu rests on two factors: the registers available on the ARM architecture, and the standard ARM calling convention[1]:

  • ARM processors have 16 "general-purpose" registers numbered r0-r15, as well as the Current Program Status Register (cpsr). However, despite being considered "general-purpose" registers, r13-r15 actually have special purposes; namely, r13 is used as the stack pointer (sp), r14 is used as the link register (lr), and r15 is used as the program counter (pc).
  • The standard ARM calling convention specifies which registers are callee-save (r4-r11, r13-r14) and which are caller-save (r0-r3, r11), as well as how arguments are passed to procedures (up to four arguments in r0-r3; additional arguments spill onto stack).

The thread context we chose is 15 words long and is the following:


The ARM version of ctxsw() is implemented in system/arch/arm/ctxsw.S and contains a detailed explanation that will not be replicated here.


The means of generating a timer interrupt for preemption is not standard to the ARM architecture; instead, software must make use of a board-specific or chip-specific device such as the BCM2835 System Timer.


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