Which is the fastest?

XOR, MOV or INC/DEC

I figure that XOR should be the fastest because
bit-twiddling is a lot faster than MOVing a
part of the memory or a still number to a register.

Correct me if I'm wrong.
Please, reply by MAIL as I don't visit the newsgroup
often.

I assume that XOR and MOV refer to clearing a register, so lets consider the
various ways to clear a register -

              486  386  286  86   (size)   example
MOV r8,im8     1    2    2    4      2     mov AL,0
MOV r16,im16   1    2    2    4      3     mov AX,0
no flags altered

XOR r8,r8      1    2    2    3      2     xor AL,AL
XOR r16,r16    1    2    2    3      2     xor,AX,AX
affects O,S,Z,A,P,C

SUB r8,r8      1    2    2    3      2     sub AL,AL
SUB r16,r16    1    2    2    3      2     sub AX,AX
affects O,S,Z,A,P,C

lea r16,m      1    2    2   10      4     lea AX,[DS:0]
no flags altered

I tossed the last one in for the fun of it.  

Anyway, the differences you assumed will only exist in the 8086.  All other
versions of the x86 require the same number of clock cycles across the board.

Actual speed is based upon a lot more.  The above times assume:
1) the instruction has been pre-fetched, decoded, and is ready for execution.
2) Bus cycles do not require wait states.
3) There are no bus HOLD requests delaying the processor.
4) No exceptions are detected during instruction execution.
5) Memory operands are aligned.

A series of fast executing (fewer than two cycles per opcode byte) instructions
can drain the prefetch queue and increase execution time.  With typical
instruction mixes, actual time will be within 5-10% of the sum of the individual
times.  Of course the 8k onboard cache of the 486 would tend to limit penalties
for fast executing instructions.

From the above you can see that most of the time you spend worrying about which
instructions execute faster would be better spent on other activities.  Even the
1-cycle difference in the 8086 XOR and MOV instructions can easily be canceled
when used with too many fast instructions that drain the 4-byte 8088, 6-byte
8086, ... prefetch queue.  Then again, 1 single interrupt can upset the delicate
balance of things.

So when you see myself or others using "xor AL,AL" or "sub AL,AL" to zero a
register it may simply be from many years of habit, or it could really be an
efficient way to clear the register and the carry flag (e.g. mov AL,0; clc).

MOV DX,3c8h ; dx = 3c8
...
XOR DX,1    ; dx = 3c9
...

You might not think it necessary if you're the only one to see the code and are
comfortable with the method, but before you decide, think about how easy or
difficult things will be when you come back to the code some 5, 10, or 20 years
from now.

The following table lists AX and AL separately since these registers often use a
different machine instruction.

   mov r16,im16   1    2    3    4      3
   xor r16,im8    1    2    3    4      3
   xor r16,im16   1    2    3    4      4
   add r16,im8    1    2    3    4      3
   add r16,im16   1    2    3    4      4
   inc/dec r16    1    2    2    3      1

   mov AL,im8     1    2    2    4      2
   add AL,im8     1    2    3    4      2
   xor AL,im8     1    2    3    4      3
   inc/dec AL     1    2    2    3      2

   mov r8,im8     1    2    3    4      2
   xor r8,im8     1    2    3    4      3
   add r8,im8     1    2    3    4      3
   inc/dec r8     1    2    2    3      2

FWIW, I had a real mode program execute 5% faster on a 386 when routines pushed
and popped segment registers (2-bytes, 9-cycles) rather than load them through a
general purpose register (4-bytes, 4-cycles).

To me, "not visiting", seems to imply that you aren't willing to put that much
effort into obtaining an answer, are unappreciative of the effort spent on your
behalf, and hints at no one here has anything worthwhile to say. :-)

On Wed, 05 Mar 1997 17:02:01 +1000, Emil Mikulic

There are more people here who'll be interested in this topic.
There are more topics here you'll be interested in if you read the group.

(That means, if all people who ask here first read the group for awhile,
there'd be much less questions, esp. FAQs.)

BTW: Nice to see that you use your real name now. Bur please cut down
your signature to max. 4 lines. I think most people here don't like
Ascii "graphics", esp. when they see them in every posting.