Question

we know CPU can reord instructions. and OS simulate multithreading by dispatch.

following cpu machine instructions:

ins0
ins1
ins2

thread0 partition these three ins into 2 parts. part0 execute ins0, part1 execute ins1 and ins2. before part0 start, part2 complete. and then thread1 is triggered.

my question is:should part0 be completed before thread1 start? or thread1 start while thread0 complete ins1 and ins2 without completing ins0?

Answer 1

The short answer is that, with very few exceptions, when a CPU switches from one thread to another, it picks an instruction boundary and completes every instruction prior to that boundary and doesn’t start any after that boundary. When it resumes, it starts from the first instruction after that boundary.

To allow the restarting of program or task following the handling of an exception or
an interrupt, all exceptions (except aborts) are guaranteed to report exceptions on
an instruction boundary. All interrupts are guaranteed to be taken on an instruction
boundary.

…
Interrupts rigorously support restarting of interrupted programs and tasks without
loss of continuity. The return instruction pointer saved for an interrupt points to the
next instruction to be executed at the instruction boundary where the processor took
the interrupt. If the instruction just executed has a repeat prefix, the interrupt is
taken at the end of the current iteration with the registers set to execute the next
iteration. — Intel x86 System Programming Guide, Volume 3A, Part 1, Section 6.6