Well... it's a text substitution. At its simplest:
%define BUFSIZ 128
... and then you use "BUFSIZ" instead of "128" throughout your program - makes it easy to change, and "128" doesn't have much meaning without some "context". Typically, there are a flock of these stuffed into a "win32.inc" or "linux.inc" which you "%include". Then you can use "WM_MESSAGEBOX_OK" or "O_RDONLY" without having to remember the numbers (both 0 :). I sometimes call this "long names for small integers". Nasm knows a bunch of "built in" macros - Nasm version, bits, output format, date and time...
More complex macros emit more than one instruction - perhaps different instructions under different conditions - sometimes nothing. For example:
%macro dbgdump 0 ; no parameters
%ifdef DEBUG
call dumpregs
call dumpstack
%endif
; else do nothing
%endmacro
; code sprinkled with "dbgdump"
%ifdef DEBUG
; code for "dumpregs" and "dumpstack"
%endif
Now if we assemble this with "nasm -f ??? -dDEBUG myprog.asm", we'll "tell everything we know", and if we assemble without "-dDEBUG", that code completely disappears. Macros that don't emit anything are my favorite kind! :)
Macros can be used to emit code to emulate "high-level constructs", IF/ELSE, WHILE/ENDWHILE, REPEAT/UNTIL, SWITCH/CASE... things like that. The famous "PROC" macro mostly defines convenient names for parameters which you can use instead of "ebp + ???" (Nasm remembers what the "???" is, so we don't have to). "INVOKE" replaces "push/push/push/call" and perhaps cleans up the stack afterwards... I really feel that a person should learn to code these things "by hand" before resorting to macros, but they simplify the code... once you understand what they're "hiding".
There's a considerable section in the Friendly Manual on the subject:
http://www.nasm.us/doc/nasmdoc4.htmlI suspect you'll need to read it more than once, interspersed with some observation of example macros and plenty of trial and error, to get anywhere with it. I find it "difficult", anyway. But it's just text substitution...
Did you have some particular macro in mind?
Best,
Frank
