NOP

From Asmpedia

No operation

Intel	AMD
+	+

Definition:

NOP

One-byte or two-byte (with 0x66 prefix) instruction is equivalent to XCHG (E)AX, E(AX)
Multi-byte (>=3 bytes) NOP is supported on processors with CPUID[0x01][EAX:8:11] == 0110 || 1111

Operation in pseudo code:

nop()
{
  if (opcode==0x0F1F && processor doesn't support multi-byte NOP)
  {
     throw #UD; // invalid opcode exception 
  }
}

Opcodes (x86/x64):

90	-	NOP	-
0F 1F	mod (xx) reg (xxx) r/m (xxx)	NOP r/m16/32	-

EFLAGS

:31	:30	:29	:28	:27	:26	:25	:24	:23	:22	ID :21	VIP :20	VIF :19	AC :18	VM :17	RF :16	:15	NT :14	IOPL :13:12	OF :11	DF :10	IF :9	TF :8	SF :7	ZF :6	:5	AF :4	:3	PF :2	:1	CF :0
0	0	0	0	0	0	0	0	0	0	-	-	-	-	-	-	0	-	-	-	-	-	-	-	-	0	-	0	-	1	-

Annotated WinDbg disassembly (x86/x64):

90              nop
6690            xchg    ax,ax ; 66: switch to 16-bit operand 90: opcode 
0f1f00          nop     dword ptr [eax] ; 0f1f: 2-byte opcode 00: mod=00 reg=000 rm=000 [EAX]
0f1f4000        nop     dword ptr [eax] ; 0f1f: 2-byte opcode 40: mod=01 reg=000 rm=000 [EAX+0x00]
0f1f440000      nop     dword ptr [eax+eax] ; 0f1f: 2-byte opcode 44: mod=01 reg=000 rm=100 SIB + 0x00
660f1f440000    nop     word ptr [eax+eax] ; 66: switch to 16-bit operand 0f1f: 2-byte opcode 44: mod=01 reg=000 rm=100 SIB + 0x00
0f1f8000000000  nop     dword ptr [eax] ; 0f1f: 2-byte opcode 80: mod=10 reg=000 rm=000 [EAX+0x00000000]