38.5. String Substitution

A subset of allowable symbols (which we'll call subsyms) may be assigned arbitrary string values. This is roughly equivalent to C preprocessor #define macros. When as encounters one of these symbols, the symbol is replaced in the input stream by its string value. Subsym names must begin with a letter.

Subsyms may be defined using the .asg and .eval directives (refer to Section 38.9 Directives and Section 38.9 Directives.

Expansion is recursive until a previously encountered symbol is seen, at which point substitution stops.

In this example, x is replaced with SYM2; SYM2 is replaced with SYM1, and SYM1 is replaced with x. At this point, x has already been encountered and the substitution stops.

 .asg   "x",SYM1
 .asg   "SYM1",SYM2
 .asg   "SYM2",x
 add    x,a             ; final code assembled is "add  x, a"

Macro parameters are converted to subsyms; a side effect of this is the normal as '\ARG' dereferencing syntax is unnecessary. Subsyms defined within a macro will have global scope, unless the .var directive is used to identify the subsym as a local macro variable Section 38.9 Directives.

Substitution may be forced in situations where replacement might be ambiguous by placing colons on either side of the subsym. The following code:

 .eval  "10",x
LAB:X:  add     #x, a

When assembled becomes:

LAB10  add     #10, a

Smaller parts of the string assigned to a subsym may be accessed with the following syntax:

:symbol(char_index):

Evaluates to a single-character string, the character at char_index.

:symbol(start,length):

Evaluates to a substring of symbol beginning at start with length length.