|Red Hat Enterprise Linux 3: Using as, the Gnu Assembler|
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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.
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:
Evaluates to a single-character string, the character at char_index.
Evaluates to a substring of symbol beginning at start with length length.