MOVBE - Move Data After Swapping Bytes

Opcode/ Instruction

Op/ En

64/32 bit Mode Support

CPUID Feature Flag

Description

0F 38 F0 /r

MOVBE r16, m16

RM

V/V

MOVBE

Reverse byte order in m16 and move to r16.

0F 38 F0 /r

MOVBE r32, m32

RM

V/V

MOVBE

Reverse byte order in m32 and move to r32.

REX.W + 0F 38 F0 /r

MOVBE r64, m64

RM

V/N.E.

MOVBE

Reverse byte order in m64 and move to r64.

0F 38 F1 /r

MOVBE m16, r16

MR

V/V

MOVBE

Reverse byte order in r16 and move to m16.

0F 38 F1 /r

MOVBE m32, r32

MR

V/V

MOVBE

Reverse byte order in r32 and move to m32.

REX.W + 0F 38 F1 /r

MOVBE m64, r64

MR

V/N.E.

MOVBE

Reverse byte order in r64 and move to m64.

Instruction Operand Encoding

Op/En

Operand 1

Operand 2

Operand 3

Operand 4

RM

ModRM:reg (w)

ModRM:r/m (r)

N/A

N/A

MR

ModRM:r/m (w)

ModRM:reg (r)

N/A

N/A

Description

Performs a byte swap operation on the data copied from the second operand (source operand) and store the result in the first operand (destination operand). The source operand can be a general-purpose register, or memory loca- tion; the destination register can be a general-purpose register, or a memory location; however, both operands can not be registers, and only one operand can be a memory location. Both operands must be the same size, which can be a word, a doubleword or quadword.

The MOVBE instruction is provided for swapping the bytes on a read from memory or on a write to memory; thus providing support for converting little-endian values to big-endian format and vice versa.

In 64-bit mode, the instruction's default operation size is 32 bits. Use of the REX.R prefix permits access to addi- tional 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 

TEMP := SRC
IF ( OperandSize = 16)
   THEN
       DEST[7:0] := TEMP[15:8];
       DEST[15:8] := TEMP[7:0];
   ( OperandSize = 32) ELES IF
       DEST[7:0] := TEMP[31:24];
       DEST[15:8] := TEMP[23:16];
       DEST[23:16] := TEMP[15:8];
       DEST[31:23] := TEMP[7:0];
   ELSE IF ( OperandSize = 64) 
       DEST[7:0] := TEMP[63:56];
       DEST[15:8] := TEMP[55:48];
       DEST[23:16] := TEMP[47:40];
       DEST[31:24] := TEMP[39:32];
       DEST[39:32] := TEMP[31:24];
       DEST[47:40] := TEMP[23:16];
       DEST[55:48] := TEMP[15:8];
       DEST[63:56] := TEMP[7:0];
FI;

Flags Affected

None.

Protected Mode Exceptions

#GP(0)

If the destination operand is in a non-writable segment. If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit. If the DS, ES, FS, or GS register contains a NULL segment selector.

#SS(0)

If a memory operand effective address is outside the SS segment limit.

#PF(fault-code)

If a page fault occurs.

#AC(0)

If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3.

#UD

If CPUID.01H:ECX.MOVBE[bit 22] = 0. If the LOCK prefix is used. If REP (F3H) prefix is used.

Real-Address Mode Exceptions

#GP

If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit.

#SS

If a memory operand effective address is outside the SS segment limit.

#UD

If CPUID.01H:ECX.MOVBE[bit 22] = 0. If the LOCK prefix is used. If REP (F3H) prefix is used.

Virtual-8086 Mode Exceptions

#GP(0)

If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit.

#SS(0)

If a memory operand effective address is outside the SS segment limit.

#PF(fault-code)

If a page fault occurs.

#AC(0)

If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3.

#UD

If CPUID.01H:ECX.MOVBE[bit 22] = 0. If the LOCK prefix is used. If REP (F3H) prefix is used. If REPNE (F2H) prefix is used and CPUID.01H:ECX.SSE4_2[bit 20] = 0.

Compatibility Mode Exceptions

Same exceptions as in protected mode.

64-Bit Mode Exceptions

#GP(0)

If the memory address is in a non-canonical form.

#SS(0)

If the stack address is in a non-canonical form.

#PF(fault-code)

If a page fault occurs.

#AC(0)

If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3.

#UD

If CPUID.01H:ECX.MOVBE[bit 22] = 0. If the LOCK prefix is used. If REP (F3H) prefix is used.