GNU Info

(nasm.info)Section 3.6

---

Next: Section 3.7 Prev: Section 3.5.7 Up: Chapter 3

---

Enter node , (file) or (file)node

3.6. `SEG' and `WRT'
====================

   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.