When writing large 16-bit programs, which must be split into multiple segments, it is often necessary to be able to refer to the segment part of the address of a symbol. NASM supports the SEG operator to perform this function.

The SEG operator returns the preferred segment base of a symbol, defined as the segment base relative to which the offset of the symbol makes sense. So the code

        mov ax, seg symbol
        mov es, ax
        mov bx, symbol

will load es:bx with a valid pointer to the symbol symbol.

Things can be more complex than this: since 16-bit segments and groups may overlap, you might occasionally want to refer to some symbol using a different segment base from the preferred one. NASM lets you do this, by the use of the WRT (With Reference To) keyword. So you can do things like

        mov ax, weird_seg       ; weird_seg is a segment base
        mov es, ax
        mov bx, symbol wrt weird_seg

to load es:bx with a different, but functionally equivalent, pointer to the symbol symbol.

NASM supports far (inter-segment) calls and jumps by means of the syntax call segment:offset, where segment and offset both represent immediate values. So to call a far procedure, you could code either of

        call (seg procedure):procedure
        call weird_seg:(procedure wrt weird_seg)

(The parentheses are included for clarity, to show the intended parsing of the above instructions. They are not necessary in practice.)

NASM supports the syntax call far procedure as a synonym for the first of the above usages. JMP works identically to CALL in these examples.

To declare a far pointer to a data item in a data segment, you must code

        dw symbol, seg symbol

NASM supports no convenient synonym for this, though you can always invent one using the macro processor.