Re: Types defining h/w r/o or w/o registers

[orville@weyrich.UUCP (Orville R. Weyrich)]

In article <9105311025.aa04299@PARIS.ICS.UCI.EDU> Jose Aleman Duarte <jduarte@liege.ICS.UCI.EDU> writes:
>> Is there some way to define a type such that all variables of that
>> type can be only read or only written?  For instance, I have a type that
>> defines the status register of some peripheral which can only be
>> read.  It would be best if the compiler could flag any assignments
>> to variables of such a type as errors.
>
>> Is there some other means for commonly handling this problem?  The
>> compiler being used is VADSWorks, if an implementation-defined solution
>> is required.
>
>Alternatively, you can define a "type" and then define a constant variable
>of that type for read only variables...
>
>subtype XYZ is INTEGER;
>NY_REG: constant XYZ := 0; -- define a  variable that can only be read
>

NO NO NO!

The original poster wants to describe a memory location that can be modified
by an entity outside the control of the program. In C (and many DEC language
extensions) this would be a VOLATILE memory location. If you tell the compiler
that it is constant, the optimizer may copy the value into a register and use
the copy, or do constant folding and substitute the value 0 for the memory
reference.

Can you say BUG?  :-)



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Orville R. Weyrich, Jr., Ph.D.                   Certified Systems Professional
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