By default, all displacement-only values provide offsets into the data segment. If you want to provide an offset into a different segment, you must use a segment override prefix before your address. For example, to access location 1234h in the extra segment (es) you would use an instruction of the form mov ax,es:[1234h]. Likewise, to access this location in the code segment you would use the instruction mov ax, cs:[1234h]. The ds: prefix in the previous examples is not a segment override. The CPU uses the data segment register by default. These specific examples require ds: because of MASM's syntactical limitations.