[Dlang-internal] Does D spec require arbitrary memory access?

[Paulo Torrens via Dlang-internal]

According to the C language spec, casting from integer to 
pointers is implementation defined. Though one would expect it to 
work, the spec does not require compilers to allow it.

For example, for accessing VGA text video memory, one would write 
(in D):

byte *video = cast(byte *)0xB8000;

As a systems' programming language, existing D compilers surely 
do allow that. But does the spec actually require it, or simply 
allow it?