The registers aren't shared here, but the main memory is. The example has each thread write '1' into a [shared, main] memory location and then read the other th

The registers aren't shared here, but the main memory is. The example has each thread write '1' into a [shared, main] memory location and then read the other thread's memory location into a register. If both CPUs' registers are 0, that means that both reads occurred before both writes.