One of the huge advantages of Flat Assemblers over other assemblers for the Intel platform is its macro engine. In addition to being able to perform its original task--substituting macro instructions with their definitions--it is able to perform relatively complex computations, and I would dare to call it an additional programming language. The preceding examples only utilize a tiny fraction of what FASM's macro processor is capable of. While we only used a set of if clauses and a variable, we may, in necessary cases, use loops (with while or repeat statements). For example, imagine a string of characters that you want to keep encrypted:

my_string  db 'This string will be encrypted',0x0d, 0x0a, 0x00
my_string_len = $ - my_string

Here, my_string_len is the length of the string.

A simplistic XOR encryption may be applied with just a four-line macro:

repeat my_string_len
   load b byte from my_string + % - 1
   store byte b xor 0x5a at my_string + % - 1
end repeat

The % symbol here denotes the current iteration and the -1 value is needed because the count of iterations starts at 1.

This is just a short and primitive example of what the macro engine of FASM is able to do, and there is a lot more. However, this book, though it uses FASM as a primary assembler, is dedicated to Intel Assembly language rather than to specific dialect, so this additional information goes beyond its scope. I strongly recommend that you refer to the FASM documentation available at http://flatassembler.net.