CPU Operations in Unix/Linux

Every CPU has a Program Counter (PC), also known as the Instruction Pointer (IP), a flag or status register (SR), a Stack Pointer (SP) and several general registers, where PC points to the next instruction to be executed in memory, SR contains current status of the CPU, e.g. operating mode, interrupt mask and condition code, and SP points to the top of the current stack. The stack is a memory area used by the CPU for special operations, such as push, pop call and return, etc. The operations of a CPU can be modeled by an infinite loop.

while(power-on){

1. 1.  fetch instruction: load *PC as instruction, increment PC to point to the next instruction in memory;
   2.  decode instruction: interpret the instruction’s operation code and generate operands;
   3.  execute instruction: perform operation on operands, write results to memory if needed; execution may use the stack, implicitly change PC, etc.
   4.  check for pending interrupts; may handle interrupts;

}

In each of the above steps, an error condition, called an exception or trap, may occur due to invalid address, illegal instruction, privilege violation, etc. When the CPU encounters an exception, it follows a pre-installed pointer in memory to execute an exception handler in software. At the end each instruction execution, the CPU checks for pending interrupts. Interrupts are external signals from I/O devices or coprocessors to the CPU, requesting for CPU service. If there is a pending interrupt request but the CPU is not in the state of accepting interrupts, i.e. its status register has interrupts masked out, the CPU will ignore the interrupt request and continue to execute the next instruction. Otherwise, it will direct its execution to do interrupt processing. At the end of interrupt processing, it will resume the normal execution of instructions. Interrupts handling and exceptions processing are handled in the operating system kernel. For the most part they are inaccessible from user level programs but they are keys to understanding timer services and signals in operating systems, such as Linux.

Source: Wang K.C. (2018), Systems Programming in Unix/Linux, Springer; 1st ed. 2018 edition.