From: alex@nospicedham.rivadpm.com   
      
   On 07-Mar-19 06:35, Terje Mathisen wrote:   
   > On Wednesday, March 6, 2019 at 2:47:41 PM UTC-5, James Harris wrote:   
   >> A small programming challenge if you are interested, just for fun.   
   >   
   > What happens if you do:   
   >   
   >    neg eax   
   >    mov eax,0   
   >    adc eax,eax   
   >   
   > This does not use any extra regs, and the NEG and the MOV should still   
   > run in the same cycle even if they both use EAX as the target. This is   
   > because the MOV form is known to not depend on the previous value in   
   > EAX, so it can be started even before the NEG.   
   >   
   > Terje   
   >   
   >   
      
   If you're willing to accept 0 or -1 (all bits set if EAX <> 0) then   
      
          add     eax,-1   
          sbb     eax,eax   
      
   The opposite test (return all bits set if EAX == 0) is   
      
          sub     eax,1   
          sbb     eax,eax   
      
   Some other neat comparison operations are   
      
   Unsigned EAX < EDX (swap the regs in CMP for unsigned EAX > EDX)   
      
          cmp     edx,eax   
          sbb     eax,eax   
      
   The following are 3 instruction forms; I don't think there are 2   
   instruction forms of these tests.   
      
   EAX == EDX   
      
          sub     eax,edx   
          sub     eax,1   
          sbb     eax,eax   
      
   EAX <> EDX   
      
          sub     eax,edx   
          add     eax,-1   
          sbb     eax,eax   
      
      
   --   
   Alex   
      
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