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MOV -- Move Data

Opcode   Instruction       Clocks        Description

88  /r   MOV r/m8,r8       2/2           Move byte register to r/m byte
89  /r   MOV r/m16,r16     2/2           Move word register to r/m word
89  /r   MOV r/m32,r32     2/2           Move dword register to r/m dword
8A  /r   MOV r8,r/m8       2/4           Move r/m byte to byte register
8B  /r   MOV r16,r/m16     2/4           Move r/m word to word register
8B  /r   MOV r32,r/m32     2/4           Move r/m dword to dword register
8C  /r   MOV r/m16,Sreg    2/2           Move segment register to r/m word
8D  /r   MOV Sreg,r/m16    2/5,pm=18/19  Move r/m word to segment register
A0       MOV AL,moffs8     4             Move byte at (seg:offset) to AL
A1       MOV AX,moffs16    4             Move word at (seg:offset) to AX
A1       MOV EAX,moffs32   4             Move dword at (seg:offset) to EAX
A2       MOV moffs8,AL     2             Move AL to (seg:offset)
A3       MOV moffs16,AX    2             Move AX to (seg:offset)
A3       MOV moffs32,EAX   2             Move EAX to (seg:offset)
B0 + rb  MOV reg8,imm8     2             Move immediate byte to register
B8 + rw  MOV reg16,imm16   2             Move immediate word to register
B8 + rd  MOV reg32,imm32   2             Move immediate dword to register
C6       MOV r/m8,imm8     2/2           Move immediate byte to r/m byte
C7       MOV r/m16,imm16   2/2           Move immediate word to r/m word
C7       MOV r/m32,imm32   2/2           Move immediate dword to r/m dword

Notes

moffs8, moffs16, and moffs32 all consist of a simple offset relative to the segment base. The 8, 16, and 32 refer to the size of the data. The address-size attribute of the instruction determines the size of the offset, either 16 or 32 bits.

Operation

DEST := SRC;

Description

MOV copies the second operand to the first operand.

If the destination operand is a segment register (DS, ES, SS, etc.), then data from a descriptor is also loaded into the register. The data for the register is obtained from the descriptor table entry for the selector given. A null selector (values 0000-0003) can be loaded into DS and ES registers without causing an exception; however, use of DS or ES causes a #GP(0), and no memory reference occurs.

A MOV into SS inhibits all interrupts until after the execution of the next instruction (which is presumably a MOV into eSP).

Loading a segment register under 80386 Protected Mode results in special checks and actions, as described in the following listing: 

IF SS is loaded;
THEN
   IF selector is null THEN #GP(0);
FI;
   Selector index must be within its descriptor table limits else
      #GP(selector);
   Selector's RPL must equal CPL else #GP(selector);
AR byte must indicate a writable data segment else #GP(selector);
   DPL in the AR byte must equal CPL else #GP(selector);
   Segment must be marked present else #SS(selector);
   Load SS with selector;
   Load SS with descriptor.
FI;
IF DS, ES, FS or GS is loaded with non-null selector;
THEN
   Selector index must be within its descriptor table limits
      else #GP(selector);
   AR byte must indicate data or readable code segment else
      #GP(selector);
   IF data or nonconforming code segment
   THEN both the RPL and the CPL must be less than or equal to DPL in
      AR byte;
   ELSE #GP(selector);
   FI;
   Segment must be marked present else #NP(selector);
   Load segment register with selector;
   Load segment register with descriptor;
FI;
IF DS, ES, FS or GS is loaded with a null selector;
THEN
   Load segment register with selector;
   Clear descriptor valid bit;
FI;

Flags Affected

None

Protected Mode Exceptions

#GP, #SS, and #NP if a segment register is being loaded; otherwise, #GP(0) if the destination is in a nonwritable segment; #GP(0) for an illegal memory operand effective address in the CS, DS, ES, FS, or GS segments; #SS(0) for an illegal address in the SS segment; #PF(fault-code) for a page fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page fault

up: Chapter 17 -- 80386 Instruction Set
prev: LTR Load Task Register
next: MOV Move to/from Special Registers