libkernel.c


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// This is the privileged code, that gets placed somewhere in
// unprivileged process' address space. It's compiled as relocatable,
// so it can be loaded at different address for different processes.

void call_unprivileged(void (*address) (void)) {
  // switch to PL0
  asm("cps #0b10000\n\r"
      "isb" ::: "memory");

  // jump to that unprivileged code
  address();
}