[AX] is an invalid memory operand specification.
The valid 16-bit ones are:
[constant]
[BX]
[SI]
[DI]
[BX+constant]
[BP+constant]
[SI+constant]
[DI+constant]
[BX+SI]
[BX+DI]
[BP+SI]
[BP+DI]
[BX+SI+constant]
[BX+DI+constant]
[BP+SI+constant]
[BP+DI+constant]
[BP] is formally invalid, but many assemblers will quietly convert it into [BP+0].
See the CPU manual for memory operand encodings and the ModR/M and SIB bytes.
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