Intel 64 and IA-32 Architectures Software Developers Manual Volume 2B, Instruction Set Reference, N-Z

Vol. 2B 4-409
INSTRUCTION SET REFERENCE, N-Z
XLAT/XLATB—Table Look-up Translation
Description
Locates a byte entry in a table in memory, using the contents of the AL register as a
table index, then copies the contents of the table entry back into the AL register. The
index in the AL register is treated as an unsigned integer. The XLAT and XLATB
instructions get the base address of the table in memory from either the DS:EBX or
the DS:BX registers (depending on the address-size attribute of the instruction, 32 or
16, respectively). (The DS segment may be overridden with a segment override
prefix.)
At the assembly-code level, two forms of this instruction are allowed: the “explicit-
operand” form and the “no-operand” form. The explicit-operand form (specified with
the XLAT mnemonic) allows the base address of the table to be specified explicitly
with a symbol. This explicit-operands form is provided to allow documentation;
however, note that the documentation provided by this form can be misleading. That
is, the symbol does not have to specify the correct base address. The base address is
always specified by the DS:(E)BX registers, which must be loaded correctly before
the XLAT instruction is executed.
The no-operands form (XLATB) provides a “short form” of the XLAT instructions. Here
also the processor assumes that the DS:(E)BX registers contain the base address of
the table.
In 64-bit mode, operation is similar to that in legacy or compatibility mode. AL is
used to specify the table index (the operand size is fixed at 8 bits). RBX, however, is
used to specify the table’s base address. See the summary chart at the beginning of
this section for encoding data and limits.
Operation
IF AddressSize = 16
THEN
AL ← (DS:BX + ZeroExtend(AL));
ELSE IF (AddressSize
= 32)
AL ← (DS:EBX + ZeroExtend(AL)); FI;
ELSE (AddressSize
= 64)
Opcode Instruction
64-Bit
Mode
Compat/
Leg Mode Description
D7 XLAT m8 Valid Valid Set AL to memory byte DS:[(E)BX +
unsigned AL].
D7 XLATB Valid Valid Set AL to memory byte DS:[(E)BX +
unsigned AL].
REX.W + D7 XLATB Valid N.E. Set AL to memory byte [RBX +
unsigned AL].