Hi,
I am wondering if Intel's processor provides instructions in their instruction set to turn on and off the multithreading or hyperthreading capability? Basically, I wanna know if an Operating System can control these feature via instructions somehow?
Thank you so much Mareike
Most operating systems have a facility for changing a process' CPU affinity, thereby restricting it to a single physical or virtual core. But multithreading is a program architecture, not a CPU facility.
Multithreading and multitasking are not special instructions or modes in the CPU. They're just fancy ways people who write operating systems use interrupts. There is a hardware timer, basically a counter being incremented by a clocking signal, that triggers an interrupt when it overflows. The exact interrupt is platform specific. In the olden days this timer is actually a separate chip/circuit on the motherboard that is simply attached to one of the CPU's interrupt pin. Modern CPUs have this timer built in. So, to turn off multithreading and multitasking the OS can simply disable the interrupt signal.
Alternatively, since it's the OS's job to actually schedule processes/threads, the OS can simply decide to ignore all threads and not run them.
Hyperthreading is a different thing. It sort of allows the OS to see a second virtual CPU that it can execute code on. Never had to deal with the thing directly so I'm not sure how to turn it off (or even if it is possible).
I think that what you are trying to ask is, "Is there a way to prevent the OS from utilizing hyperthreading and/or multiple cores?"
The answer is, definitely. This isn't governed by a single instruction, and indeed it's not like you can just write a device driver that would automagically disable all of that hardware. Most of this depends on how the kernel configures the interrupt controllers at boot time.
When a machine is first started, there is a designated processor that is used for bootstrapping. It is the responsibility of the OS to configure the multiprocessor hardware accordingly. On PC platforms this would involve reading information about the multiprocessor configuration from in-memory tables provided by the boot firmware. This data would likely conform to either the ACPI or the Intel multiprocessor specifications. The kernel then uses that date to configure the APIC hardware accordingly.
There is no x86 instruction that disables HyperThreading or additional cores. But, there is BIOS settings that can turn off these features. Because it can be set in BIOS, it requires rebooting, and generally it's beyond OS control. There is Windows booting option that limits the number of active core, but HyperThreading can be turn on/off only by BIOS. Current Intel's HyperThreading implementation doesn't allow dynamic turn on and off (and it won't be easily implemented in a near time).
I have assumed 'multithreading' in your question as 'hardware multithreading' which is technically identical to HyperThreading. However, if you really intended software-level multithreading (i.e., multitasking), then it's totally different question. It is (almost) impossible for modern operating systems since they are by default supports multitasking. And, this question actually doesn't make sense. It can make sense if you want to run MS-DOS (as real mode of x86, where a single task can be done).
p.s. Please note that 'multithreading' can be either hardware or software. Also I agree all others' answers such as processor/thread affinity.