Is there a list of deprecated x86 instructions?

Question

I'm taking an x86 assembly language programming class and know that certain instructions shouldn't be used anymore -- because they're slow on modern processors; for example, the loop instruction.

I haven't been able to find any list of instructions that are considered deprecated and should be avoided; any guidance would be appreciated.

Answer 1

Oh, but there still might be a good reason to use the loop instruction. For example, loop label only requires two bytes. As opposed to dec cx followed by jnz label requires three bytes. Sometimes code size is more important than speed.

I would suggest, however, that if you're just learning x86 assembly--especially if this is your first foray into assembly language--that you first concentrate on how to do things. Once you've gotten a better feel for how things work, then worry about making them faster.

Answer 2

If you what to know what to avoid, go directly to the processor manufacturers, both intel and amd have manuals for the instruction sets their processors support and to what degree they support them, your best bet if probably the optimization volumes, but if your only just starting out, take Jim's advice, get the thing working first before you worry about speed

Answer 3

All CPU instructions are 100% functional to reach compatibility with older CPUs. So why to avoid some instruction? There is no realy deprecated x86 instructions! But we can say:

1)All string istructions like rep movsb are slower.

2) xlat is slow and very rare in use.

3)Also the use of stack frame functions ENTER and LEAVE is slow.

4)Uder Windows (XP, vista...) the deprecated instructions are IN and OUT, but only under CPU ring 2 (aplication level), also the int nn is deprecated, except int3 (debugger trap).

EDIT: added simple speed test to check strings instruction rep cmp on different versions of CPUs.

Test is made under Delphi IDE but the asm part is very easy to translate in any other IDE.

program ProjectTest;

{$APPTYPE CONSOLE}

uses SysUtils, windows;

const
  ArraySize = 50000;

var
  StartTicks    :int64;
  EndTicks      :int64;
  arA           :array [0..ArraySize - 1]of byte;
  arB           :array [0..ArraySize - 1]of byte;

begin
  FillChar(ArA, SizeOf(ArA), 255);          //Set all bytes to 0xFF
  FillChar(ArB, SizeOf(ArB), 255);          //Set all bytes to 0xFF

repeat
  Sleep(100);       //Calm down
  asm
//Save  StartTicks
    rdtsc
    mov         dword ptr [StartTicks], eax
    mov         dword ptr [StartTicks + 4], edx
//Test LOOP
    push        edi
    mov         ecx, -ArraySize
    mov         edi, offset arA + ArraySize
    mov         esi, offset arB + ArraySize
@loop:
    mov         al,[esi + ecx]
    cmp         [edi + ecx], al
    jnz         @exit
    inc         ecx
    jnz         @loop
@exit:
    pop         edi
//Save  EndTicks
    rdtsc
    mov         dword ptr [EndTicks], eax
    mov         dword ptr [EndTicks + 4], edx
  end;

  WriteLn('Loop ticks : ' + IntToStr(EndTicks - StartTicks));

  Sleep(100);       //Calm down
  asm
//Save  StartTicks
    rdtsc
    mov         dword ptr [StartTicks], eax
    mov         dword ptr [StartTicks + 4], edx
//Test REP
    push        edi
    cld
    mov         ecx, ArraySize
    mov         edi, offset arA
    mov         esi, offset arB
    repe        cmpsb
    pop         edi
//Save  EndTicks
    rdtsc
    mov         dword ptr [EndTicks], eax
    mov         dword ptr [EndTicks + 4], edx
  end;

  WriteLn('Rep ticks  : ' + IntToStr(EndTicks - StartTicks));

  ReadLn                    //Wait keyboard
until false;

end.

TESTs for ArraySize = 50000

Average results...

1)My Intel single core CPU Pentium 4 results: Loop ticks : 232000; Rep ticks : 233000

2)My Intel Core 2 Quad CPU results: Loop ticks : 158000; Rep ticks : 375000

Answer 4

Your best bet is to consult Intel's official optimization guide.

This an other manuals can be found here.