GNU Info

(nasm.info)Section 4.1.3

---

Next: Section 4.1.4 Prev: Section 4.1.2 Up: Section 4.1

---

Enter node , (file) or (file)node

4.1.3. Concatenating Single Line Macro Tokens: `%+'
---------------------------------------------------

   Individual tokens in single line macros can be concatenated, to
produce longer tokens for later processing. This can be useful if there
are several similar macros that perform similar functions.

   As an example, consider the following:

     %define BDASTART 400h                ; Start of BIOS data area

     struc   tBIOSDA                      ; its structure
             .COM1addr       RESW    1
             .COM2addr       RESW    1
             ; ..and so on
     endstruc

   Now, if we need to access the elements of tBIOSDA in different
places, we can end up with:

             mov     ax,BDASTART + tBIOSDA.COM1addr
             mov     bx,BDASTART + tBIOSDA.COM2addr

   This will become pretty ugly (and tedious) if used in many places,
and can be reduced in size significantly by using the following macro:

     ; Macro to access BIOS variables by their names (from tBDA):

     %define BDA(x)  BDASTART + tBIOSDA. %+ x

   Now the above code can be written as:

             mov     ax,BDA(COM1addr)
             mov     bx,BDA(COM2addr)

   Using this feature, we can simplify references to a lot of macros
(and, in turn, reduce typing errors).