Object Address Plus Symbol '@@'

The Object Address Plus Symbol operator returns the value of the symbol following it plus the current object's program base address. Object Address Plus Symbol can only be used in variable expressions. 

This operator is useful when creating a table of offset addresses, then at run time, using those offsets to reference the absolute run-time addresses they represent. For example, a DAT block may contain a number of strings to which you want both direct and indirect access. Here's an example DAT block containing strings.

DAT
  Str1  byte  "Hello.", 0
  Str2  byte  "This is an example", 0
  Str3  byte  "of strings in a DAT block.",0 

At run time we can access those strings directly, using @Str1, @Str2, and @Str3, but accessing them indirectly is troublesome because each string is of a different length; making it difficult to use any of them as a base for indirect address calculations.

The solution might seem to be within reach by simply making another table of the addresses themselves, as in:

DAT
  StrAddr  word  @Str1, @Str2, @Str3 

This creates a table of words, starting at StrAddr, where each word contains the address of a unique string. Unfortunately, for compile-time constants (like those of the StrAddr table), the address returned by @ is only the compile-time offset address, rather than the run-time absolute address, of the symbol. To get the true, run-time address, we need to add the object's program base address to the symbol's offset address. That is what the Object Address Plus Symbol operator does. Example:

REPEAT Idx FROM 0 TO 2
  PrintStr(@@StrAddr[Idx])

The above example increments Idx from 0 through 2. The StrAddr[Idx] statement retrieves the compile-time offset of the string stored in element Idx of the StrAddr table. The @@ operator in front of the StrAddr[Idx] statement adds the object's base address to the compile-time offset value that was retrieved, resulting in a valid run-time address of the string. The PrintStr method, whose code is not shown in this example, can use that address to process each character of the string.