GNU Info

(nasm.info)Section 6.2.8

---

Next: Section 6.3 Prev: Section 6.2.7 Up: Section 6.2

---

Enter node , (file) or (file)node

6.2.8. `obj' Extensions to the `COMMON' Directive
-------------------------------------------------

   The `obj' format allows common variables to be either near or far;
NASM allows you to specify which your variables should be by the use of
the syntax

     common  nearvar 2:near   ; `nearvar' is a near common
     common  farvar  10:far   ; and `farvar' is far

   Far common variables may be greater in size than 64Kb, and so the OMF
specification says that they are declared as a number of _elements_ of
a given size. So a 10-byte far common variable could be declared as ten
one-byte elements, five two-byte elements, two five-byte elements or one
ten-byte element.

   Some `OMF' linkers require the element size, as well as the variable
size, to match when resolving common variables declared in more than one
module. Therefore NASM must allow you to specify the element size on
your far common variables. This is done by the following syntax:

     common  c_5by2  10:far 5        ; two five-byte elements
     common  c_2by5  10:far 2        ; five two-byte elements

   If no element size is specified, the default is 1. Also, the `FAR'
keyword is not required when an element size is specified, since only
far commons may have element sizes at all. So the above declarations
could equivalently be

     common  c_5by2  10:5            ; two five-byte elements
     common  c_2by5  10:2            ; five two-byte elements

   In addition to these extensions, the `COMMON' directive in `obj'
also supports default-`WRT' specification like `EXTERN' does (explained
in *Note Section 6.2.7::). So you can also declare things like

     common  foo     10:wrt dgroup
     common  bar     16:far 2:wrt data
     common  baz     24:wrt data:6