Valid index register combinations are [BX], [BP], [SI], [DI], [BX+SI], [BX+DI], [BP+SI], and [BP+DI]. Other index register combinations, such as [AX], [BP+BX], and [SI+DX], are not allowed.

Local variables are always allocated on the stack and accessed via the BP register. The assembler automatically adds [BP] in references to such variables, so even though a construct like Local[BX] (where Local is a local variable) appears valid, it is not since the final operand would become Local[BP+BX].