MOV — Move

1. In 64-bit mode, r/m8 can not be encoded to access the following byte registers if a REX prefix is used: AH, BH, CH, DH.

2. In 32-bit mode, the assembler may insert the 16-bit operand-size prefix with this instruction (see the following “Description” section for further information).

3. The moffs8, moffs16, moffs32, and moffs64 operands specify a simple offset relative to the segment base, where 8, 16, 32, and 64 refer to the size of the data. The address-size attribute of the instruction determines the size of the offset, either 16, 32, or 64 bits.

Instruction Operand Encoding ¶

Description ¶

Copies the second operand (source operand) to the first operand (destination operand). The source operand can be an immediate value, general-purpose register, segment register, or memory location; the destination register can be a general-purpose register, segment register, or memory location. Both operands must be the same size, which can be a byte, a word, a doubleword, or a quadword.

The MOV instruction cannot be used to load the CS register. Attempting to do so results in an invalid opcode exception (#UD). To load the CS register, use the far JMP, CALL, or RET instruction.

If the destination operand is a segment register (DS, ES, FS, GS, or SS), the source operand must be a valid segment selector. In protected mode, moving a segment selector into a segment register automatically causes the segment descriptor information associated with that segment selector to be loaded into the hidden (shadow) part of the segment register. While loading this information, the segment selector and segment descriptor information is validated (see the “Operation” algorithm below). The segment descriptor data is obtained from the GDT or LDT entry for the specified segment selector.

A NULL segment selector (values 0000-0003) can be loaded into the DS, ES, FS, and GS registers without causing a protection exception. However, any subsequent attempt to reference a segment whose corresponding segment register is loaded with a NULL value causes a general protection exception (#GP) and no memory reference occurs.

Loading the SS register with a MOV instruction suppresses or inhibits some debug exceptions and inhibits interrupts on the following instruction boundary. (The inhibition ends after delivery of an exception or the execution of the next instruction.) This behavior allows a stack pointer to be loaded into the ESP register with the next instruction (MOV ESP, stack-pointer value) before an event can be delivered. See Section 6.8.3, “Masking Exceptions and Interrupts When Switching Stacks,” in the Intel^® 64 and IA-32 Architectures Software Developer’s Manual, Volume 3A. Intel recommends that software use the LSS instruction to load the SS register and ESP together.

When executing MOV Reg, Sreg, the processor copies the content of Sreg to the 16 least significant bits of the general-purpose register. The upper bits of the destination register are zero for most IA-32 processors (Pentium Pro processors and later) and all Intel 64 processors, with the exception that bits 31:16 are undefined for Intel Quark X1000 processors, Pentium, and earlier processors.

In 64-bit mode, the instruction’s default operation size is 32 bits. Use of the REX.R prefix permits access to additional registers (R8-R15). Use of the REX.W prefix promotes operation to 64 bits. See the summary chart at the beginning of this section for encoding data and limits.

Operation ¶

DEST := SRC;
Loading a segment register while in protected mode results in special checks and actions, as described in the following listing. These
checks are performed on the segment selector and the segment descriptor to which it points.
IF SS is loaded
    THEN
        IF segment selector is NULL
            THEN #GP(0); FI;
        IF segment selector index is outside descriptor table limits
        OR segment selector's RPL ≠ CPL
        OR segment is not a writable data segment
        OR DPL ≠ CPL
            THEN #GP(selector); FI;
        IF segment not marked present
            THEN #SS(selector);
            ELSE
                SS := segment selector;
                SS := segment descriptor; FI;
FI;
IF DS, ES, FS, or GS is loaded with non-NULL selector
THEN
    IF segment selector index is outside descriptor table limits
    OR segment is not a data or readable code segment
    OR ((segment is a data or nonconforming code segment) AND ((RPL > DPL) or (CPL > DPL)))
        THEN #GP(selector); FI;
    IF segment not marked present
        THEN #NP(selector);
        ELSE
            SegmentRegister := segment selector;
            SegmentRegister := segment descriptor; FI;
FI;
IF DS, ES, FS, or GS is loaded with NULL selector
    THEN
        SegmentRegister := segment selector;
        SegmentRegister := segment descriptor;
FI;

Flags Affected ¶

None.

Protected Mode Exceptions ¶

Real-Address Mode Exceptions ¶

Virtual-8086 Mode Exceptions ¶

Compatibility Mode Exceptions ¶

Same exceptions as in protected mode.

64-Bit Mode Exceptions ¶