LOCATED AT

A module variable can include LOCATED AT to indicate that it is stored at a specific memory address. For example, here are two BYTE variables located at I/O addresses $D0_0300 and $D0_0301:

MODULE IO
  . . .
  # PALIX VIDEO SYSTEM
  VAR BACKGROUND_COLOR: BYTE                  LOCATED AT $D0_0300
  VAR MATTE_COLOR: BYTE                       LOCATED AT $D0_0301
  . . .
END MODULE

Example usage:

MODULE MAIN
  FUNC START()
    # STORE A BYTE AT MEMORY ADDRESS $D0_0300;
    # THE BACKGROUND COLOR CHANGES TO GREEN (#19)
    19 -> IO:: BACKGROUND_COLOR
  END FUNC
END MODULE

It is possible for two variables to be mapped to the same location. As a hypothetical example, we could also define:

MODULE EXAMPLE
  VAR COMBINED_COLOR: PAIR                    LOCATED AT $D0_0300
END MODULE

Reading EXAMPLE::COMBINED_COLOR would read both bytes into a 16-bit PAIR variable. BACKGROUND_COLOR would be in the high bits, and MATTE_COLOR would be in the low bits.

Rules for LOCATED AT

Rules that the compiler enforces for LOCATED AT:

• The LOCATED AT variable must be a member of a MODULE, not a CLASS.
• If the variable has an array or class type, it must be marked as INSET.
• If the variable has a class type, the class must not have a vtable.
• The address must be in the RAM segment or I/O segment. (This restriction may be relaxed in the future.)
• If the LOCATED AT address is in the RAM segment, then the variable must have a caret type; for any other address, it must not have a caret type.