docs/lesson01/rpi-os.md reads:

.globl _start
_start:
    mrs    x0, mpidr_el1        
    and    x0, x0,#0xFF        // Check processor id
    cbz    x0, master        // Hang for all non-primary CPU
    b    proc_hang

The first thing this function does is check the processor ID. The Raspberry Pi 3 has four core processors, and after the device is powered on, each core begins to execute the same code. However, we don't want to work with four cores; we want to work only with the first one and put all of the other cores in an endless loop. This is exactly what the _start function is responsible for. **It gets the processor ID** from the [mpidr_el1](http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0500g/BABHBJCI.html) system register. **If the current process ID is 0**, then execution is transferred to the master function:

The correct term should be "processor ID" because mpidr_el1 identifies the processor core (not a software process). At this point in the boot process, there's no OS and hence no concept of process IDs.

I’d be happy to submit a pull request with a fix!