assembly - How to completely suspend the processor?

Question

I'm writing a small bootloader for an x86 based PC. The problem is that the CPU is somehow still active after executing these instructions:

sti
hlt

sti is supposed to disable interrupts for the next instruction
hlt is supposed to completely halt the processor

Since they're used together, I assumed they would just 'freeze' the computer. But when I assemble it and mount it in VirtualBox as a floppy image, my CPU jumps to 100%.

What am I doing wrong?

Answer 1

I would like to add a comment about 'cli' as I have been bitten by this a couple of times in the past. The 'cli' instruction does not block all interrupts--it only blocks the maskable ones. Conceivably, the system could still be woken up due to a non-maskable interrupt (NMI).

As one of the comments indicates that the computer is ready to be turned off, I expect that there are no other threads/processes/tasks ready to run in the system (otherwise an NMI could conceivably lead to a reschedule). For the scenario you describe an NMI is unlikely; however depending upon your level of paranoia of things going wrong, you may wish to add a loop to guard against the possibility.

sysSuspend:
    cli
    hlt
    jmp sysSuspend