NASM Coding: Constants and String Length

2. Using the

   %define

   directive:

   %define name expression

   Example:

   %define WRITE_SYSCALL_NUM 1
   %define PRETTY_NUM 12345
   %define EXIT_SYSCALL mov rax, 60 \ syscall ; We can put entire code segments in the macro!!!
   %define MYREG rax                          ; We can even create register aliases!

   Explanation: we can turn any piece of NASM code into a constant. This is similar to creating a macro, whose name you just need to write somewhere in your program: you can write (repeat) it as many times as wanted! During compilation, a special software called the pre-processor will replace every instance of your constant name with the actual code behind it before converting the program into machine code (e.g., every

   WRITE_SYSCALL_NUM

   will be replaced with a

   1

   .)