The XLAT instruction changes the AL register from the table index to the table entry. The AL register should be the unsigned index into a table addressed by the DS:BX register pair (for an address-size attribute of 16 bits) or the DS:EBX register pair (for an address-size attribute of 32 bits).
The operand to the XLAT instruction allows for the possibility of a segment override. The XLAT instruction uses the contents of the BX register even if they differ from the offset of the operand. The offset of the operand should have been moved into the BX or EBX register with a previous instruction.
The no-operand form, the XLATB instruction, can be used if the BX or EBX table will always reside in the DS segment.