Add DEFINE preprocessor directive

README.md

@@ -12,7 +12,7 @@ language, or keep on reading!
 ## From Text To Runtime Behaviour
 
 In order to turn the source text into executable code we use 3 passes:
-- Pass 1: tokenization (syntax check)
+- Pass 1: tokenization (syntax check) and preprocessing (substitution)
 - Pass 2: interpretation (semantics check)
 - Pass 3: execution (runtime check)
 
@@ -69,7 +69,6 @@ The following characters are used as identifiers:
 - semicolon (`;`) for statement termination
 - hash (`#`) for comments
 - square brackets (`[` and `]`) for addressing memory
-- double quotes (`"`) for string values
 
 ## Memory Model
 
@@ -87,12 +86,16 @@ second byte of location `$900`).
 All symbols are reserved keywords and can therefore NOT be used as labels.
 There is currently no strict checking, so be careful.
 
-## Directives
+## Preprocessor
 
-- `DECLARE` declares the first label argument to equal the second, immediate
-	value, argument and is used to declare a constant for the virtual machine.
-- `STRING` puts the string value declared as the second argument in the memory
-	memory location of the first immediate argument
+All preprocessor directives are prefixed by a `#`. Ill formed preprocessor
+directives do not halt compilation, they are merely reported and then ignored.
+
+- `DEFINE` replaces any occurrence of the first argument by the second argument.
+	The second argument may be empty, effectively deleting occurences of argument
+	one. Quotes are currently not supported and arguments are separated by
+	whitespace. If multiple defines exist for the same substitution the first
+	declared is used.
 
 ### Operands