movbe

Move Big-Endian

MOVBE r, m

Moves data swapping bytes (Big Endian load/store).

Pseudocode Operation

if (dest is register) {
  dest ← byte_reverse(src);
} else {
  [dest] ← byte_reverse(src);
}

Example

MOVBE rax, [rbp-8]

Encoding

Binary Layout

Format Legacy

Opcode 0F 38 F0

Extension MOVBE

Operands

dest
General-purpose register
src
Memory operand

Reference

View in Intel® 64 and IA-32 Architectures SDM ↗ Intel® SDM

Instruction Forms

Opcode	Instruction	Op/En	64/32-bit Mode	Description
0F 38 F0 /r	MOVBE	RM	V/V	Reverse byte order in m16 and move to r16. MOVBE r16, m16
0F 38 F0 /r	MOVBE	RM	V/V	Reverse byte order in m32 and move to r32. MOVBE r32, m32
REX.W + 0F 38 F0 /r	MOVBE	RM	V/N.E.	Reverse byte order in m64 and move to r64. MOVBE r64, m64
0F 38 F1 /r	MOVBE	MR	V/V	Reverse byte order in r16 and move to m16. MOVBE m16, r16
0F 38 F1 /r	MOVBE	MR	V/V	Reverse byte order in r32 and move to m32. MOVBE m32, r32
REX.W + 0F 38 F1 /r	MOVBE	MR	V/N.E.	Reverse byte order in r64 and move to m64. MOVBE m64, r64

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 location; 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 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

TEMP := SRC
IF ( OperandSize = 16)
THEN
DEST[7:0] := TEMP[15:8];
DEST[15:8] := TEMP[7:0];
ELES IF   ( OperandSize = 32)
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.

Exceptions

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[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[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[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[20] = 0.

Compatibility Mode Exceptions

Same exceptions as in protected mode. MOVBE-Move Data After Swapping Bytes Vol. 2B 4-46

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[22] = 0. If the LOCK prefix is used. If REP (F3H) prefix is used. MOVBE-Move Data After Swapping Bytes Vol. 2B 4-47