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
