Re: How do I write directly to a memory address?

[Simon Clubley <clubley@remove_me.eisner.decus.org-Earth.UFP>]

On 2011-02-07, Randy Brukardt <randy@rrsoftware.com> wrote:
> "Simon Clubley" <clubley@remove_me.eisner.decus.org-Earth.UFP> wrote in 
> message news:iiopbr$bs7$1@news.eternal-september.org...
>
> You mean a numeric literal (to use the Ada terms).
>

Yes. I was in too much of a rush when I wrote the original posting.

>> In the statement:
>>
>> c = (4 * i) + 2
>>
>> the compiler would allocate memory locations containing the values 4 and 
>> 2.
>> These memory locations would then be used in the generated code whenever 
>> the
>> values of 2 or 4 were found. In other words, all access, including 
>> literals,
>> was by reference.
>>
>> This was a long time ago however.
>
> Must have been. I remember hearing this story in college. And it supposely 
> was old then. I've always thought of it as a sort of urban legend. But I 
> suspect that it has more to do with the instruction set of the target 
> machine. On the Intel architectures that I've worked on my entire working 
> life, it wouldn't make sense to have a global memory location holding a 
> literal (the literals can be folded into instructions at very little cost). 
> But I can imagine cases where that wouldn't be true.
>

As someone else has commented in another posting, something like this also
happened to them, but I can see how it might be considered a urban legend if
you have only used more mainstream hardware.

In my case, this was during the 1980's while I was still in the sixth form
(the US equivalent of the sixth form would be high school) and was taking
advanced (for the time :-)) programming classes at the local higher education
establishment as part of my sixth form education.

The miniframe which was installed there was a obscure miniframe from
Sperry/Unisys and not something mainstream like a PDP-11. The operating
system and compilers were quite limited even by the standards of what was
available in those days and given these other limitations, this global
sharing and allowed overwriting of literals by the generated compiler
code is not a surprise in hindsight.

Simon.

-- 
Simon Clubley, clubley@remove_me.eisner.decus.org-Earth.UFP
Microsoft: Bringing you 1980s technology to a 21st century world