From: andrew@nospicedham.cam.ac.uk   
      
   On 13/04/2023 12:05 pm, Paul Edwards wrote:   
   > / enable interrupts and then halt until interrupt hit   
   > _hltintgo:   
   >         sti   
   > hloop:   
   > / I believe hlt will be interrupted by other interrupts, like   
   > / the timer interrupt, so we need to do it in a loop   
   >         hlt   
   >         jmp     hloop   
   > _hltinthit:   
   > / remove return address, segment and flags from the stack as we   
   > / do not intend to return to the jmp following the hlt instruction   
   > / that was likely interrupted   
   >         add     %esp, 12   
   > / note that interrupts will be disabled again (I think) by virtue   
   > / of the fact that an interrupt occurred. The caller would have   
   > / disabled interrupts already, so we are returning to the same   
   > / disabled state.   
   >         ret   
      
   This is why things get stuck.   
      
   The first time through, you hlt with interrupts enabled, so will wake up   
   on the fist interrupt.   
      
   But when you loop, you'll hlt again, this time with interrupts disabled,   
   and will never ever wake up again (other than for an NMI).   
      
   Your loop needs to read:   
      
   _hltintgo:   
   	sti   
   	hlt   
   	cli   
   	jmp _hltintgo   
   _hltinthit:   
   	...   
      
   to make things work as you intend.   
      
   sti;hlt as a pair is important for getting the blocked-by-STI shadow to   
   cover you into the hlt state, but it only works on the rising edge of   
   IF, so you need to explicitly clear interrupts in order to make the   
   sti;hlt on the second loop iteration work.   
      
   ~Andrew   
      
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