Access to fixed memory locations, head const, ...

[Kuba Ober]

>> I'm contemplating using D for an embedded project where system 
>> configuration registers have fixed memory locations.
>>
>> One way of doing it would be to have a constant pointer to a structure 
>> with manually aligned members that match the register map, and access 
>> them like that. This becomes cumbersome, as all accesses look like
>>
>> struct Regs {
>>   ...
>> };
>>
>> Regs* cfg = cast(Regs*)0xF00;
>>
>> cfg.reg0 = ...
>>
>> This is also where D's const-transitivity becomes problematic. Since
>> the configuration registers are not going anywhere, ideally you'd want
>> a way of telling the compiler that the *pointer* is constant, but not
>> the pointed-to stuff.
>>
>> For a systems language, this seems like a serious drawback. Why can't
>> I tell the compiler that noone should muck with the cfg pointer's
>> value? I presume there can be ways around it via delegates, but this
>> seems like a C++ approach: instead of making it simple, it becomes
>> convoluted...
>>
>>
>> OTOH, there's a C hack that allows you to access variables at fixed
>> memory locations "directly":
>>
>> #define CFGREG0 (*(unsigned short*)0xBEEF)
>>
>> I'm an absolute noob to D, and so far I found no way of approximating
>> this behavior -- of bringing the configuration registers at a fixed
>> memory location into the scope as an identifier that can be used 
>> naked, like
>>
>> CFGREG0 = 0xFF;
>> if (CFGREG0 != 3) { ... }

> i think you should be able to do what you want with the linker (ld).
> i did this one time to fix my page directory address, inserting a symbol 
> at a certain address.
> 
> . = ALIGN(4K);
> .data.pd :
> {
>     _pdpr = .;
>     . = . + 0x1000;
> }
> 
> And then doing something like:
> 
> struct PageDirectory
> {
>     ...
> }
> 
> extern (C) char _pdpr;
> auto pdtr = cast (PageDirectory*) &_pdpr;
> 
> This was working...
> 
> Even if it does not resolve your problem completly maybe you can do 
> something like:
> 
> extern (C) PageDirectory _pdpr;

The main reason I'm looking at D is to avoid hacks like this.
So the short answer is: no, D does not support it "natively",
without some extra syntactic sugar. And all I was really after
is to avoid needless sugar in the first place...

Cheers, Kuba