Re: Data Overlays

[portal!cup.portal.com!R_Tim_Coslet@uunet.uu.net]

>     As for creating an overlay by using an address clause, here's the kind
>of problem you could get into.  Consider the following very simplified
>example:
>
>          X : Integer;
>          Y : Integer:
>          FOR X USE AT Y'Address;
>
>          X := 1;
>          ...
>          IF Y = 1 THEN ...
>
>It might be that your compiler, using standard code generation techniques,
>is using a delayed store on the value of X, which means basically that the
>1 is held in a register for a period of time rather than being immediately
>placed into the memory location of X.  As a result, Y does not have the
>expected value when the IF statement is executed.  Since the overlaying has

Or alternatively, the compiler could store the value of 1 into the overlayed
variable... but having previously worked with Y, it still resides in a
register and since Y wasn't manipulated when X was loaded with 1, the
compiler chooses to use the copy of Y "still in the register" for the
test. As a result, Y does not have the expected value when the IF 
statement is executed.

Same unexpected result, but different cause.


                                        R. Tim Coslet

Usenet: R_Tim_Coslet@cup.portal.com
        technology, n.  domesticated natural phenomena