From: nowhere@never.at   
      
   James Van Buskirk wrote:   
      
   >> Kind of funny that Intel should list RAX, RCX, RDX, RBX, RSP, RBP, RSI,   
   >> and RDI   
   >> in the wrong order in Fig. 1 :)   
   >   
   > Just to follow up on my own message, it may seem that the order only   
   > has meaning for really low-level code where the programmer is looking   
   > at the machine language, but consider what happens with 64-bit   
   > addressing: format MS64   
      
   [list of opcodes ..]   
      
   not much changed from 16-bit to 32-bit to 64-bit opcodes:   
      
   register numbers within opcodes remained (except for specials)   
   r/eax=0 r/ecx=1 r/edx=0 r/ebx=3 r/esp=4 r/ebp=5 r/esi=6 r/edi=7   
   the specials are:   
   06 in 16 bit mode means [address]   
   05 in 32 bit mode means [address], RIP in 64 bit mode   
   04 in 32/64 modes mean SIB   
   and 64 bit mode allow also Zxd 32 or full 64-bit beside access to   
   all the low bytes from GP-regs.   
      
   > the above listing generated by FASM, it can be seen that   
   > we can't address off of RSP without an SIB byte because the   
   > mod r/m encoding that would correspond to RSP instead   
   > call out the presence of an SIB byte, so an SIB byte is required   
   > even for an effective address of [RSP+offset].  This is echoed   
   > with R12.   
      
   Have you ever tried 48 8b 24 24  MOV RSP,[RSP] ? (64 bit)   
      
   > Similarly, the encoding for an effective address of [RBP]   
   > ends up being occupied by [RIP+offset] so you need to   
   > specify an effective address of [RBP+0] to encode [RBP].   
   > Again, this is echoed with R13.   
      
   yes, but we can have both:   
      
   48 8b 45 00  MOV RAX,[RBP+0]  almost same issue as 16 bit   
   8b 46 00     MOV ax,[bp+0]   
      
   48 8b 05 04 03 02 01    MOV RAX,[RIP+0x01020304] in 64bit   
   8b 05 01 02 03 04       MOV eax,[0x04030201]   
      
   > If you didn't know the register order, you wouldn't know   
   > that addressing off of R12 and R13 is a little funky like   
   > addressing off of RSP and RBP, so you might and end with   
   > some longer encodings than you would if you planned   
   > register allocations taking this into account.   
      
   Yes, so whenever you program for 64-bit you should know the   
   difference to 32bit code.   
   smart tools will anyway show what's possible and what's not.   
   __   
   wolfgang   
      
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