XPost: comp.lang.forth   
   From: terje.mathisen@nospicedham.tmsw.no   
      
   JJ wrote:   
   > On Mon, 09 Mar 2020 14:10:13 GMT, Stephen Pelc wrote:   
   >>   
   >> In AMD64, the 4x instructions are *all* REX prefices. The 48   
   >> instruction is NOT a DEC reg instruction, it's a REX prefix.   
   >>   
   >> Stephen   
   >   
   > That's interresting. I didn't realize that there are two sets of opcode for   
   > `INC/DEC ` instructions in x86: `4x` and `FF Cx`. Is there any article   
   > about it? Why there's two set of them? And if one was added later, when was   
   > that?   
      
   I suspect they first defined the single-byte opcodes for things like   
   PUSH/POP reg, INC/DEC/RET/IRET/CLD/STD/CLC etc, many/most of them   
   inherited from the 8080, then we got the two-byte opcodes for an   
   accumulator machine, with almost everything going through AL/AX, along   
   with the Jcc/JMP short branches. Finally they added a much more modern   
   reg/mem decoding engine which as a side-effect meant that most of the   
   reg-reg forms with AX as the target would duplicate the short form.   
      
   I once stumbled over this when challenging Mike Abrash to improve some   
   code I had written and was very proud of:   
      
   He pointed out that I had written something like 'CMP BX,AX' instead of   
   the opposite 'CMP AX,BX' and the AX target form would be a byte shorter   
   and therefore also  a few percent faster.   
      
   Terje   
      
   --   
   -    
   "almost all programming can be viewed as an exercise in caching"   
      
   --- SoupGate-Win32 v1.05   
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