Directives are to control the behaviour of the compiler (assembler), when compiling (assembling, translating from source to machine code). It means, that some parts of the source will be translated and other won't. For example (with freely adopted syntax, this might not work with the metalink assembler):

$SET FASTCODE
(... no other $SET directives ...)
$IF FASTCODE
  blahblah1
$ELSE
  blahblah2
$ENDIF
(...)
$IF SLOWCODE
  blahblah3
$ELSE
  blahblah2
$ENDIF
(...)

Here, blahblah1 will be translated, blahblah2 won't; blahblah3 won't, blahblah4 will.

On the other hand, you want to tell the processor, that he needs to compare A and B, so you definitively want that part to be translated (assembled), and you want that the processor makes the check. You need to choose then the instructions the processor knows - and those you'll find listed in the mentioned "bible".

Jan Waclawek