Re: Different sized data and code pointers

[Paul Schlie <schlie at comcast dot net>]

> Thomas Gill wrote:
> I'm working on a GCC backend for a small embedded processor. We've got a
> Harvard architecture with 16 bit data addresses and 24 bit code
> addresses. How well does GCC support having different sized pointers for
> this sort of thing? The macros POINTER_SIZE and Pmode seem to suggest that
> there's one pointer size for everything.
>
> The backend that I've inherited gets most of the way with some really
> horrible hacks, but it would be nice if those hacks weren't necessary. In
> any case, the hacks don't cope with casting function pointers to integers.

With the arguable exception of function pointers (which need not be literal
address) all pointers are presumed to point to data, not code; therefore
may be simplest to define pointers as being 16-bits, and call functions
indirectly through a lookup table constructed at link time from program
memory, assuming it's readable via some mechanism; as the call penalty
incurred would likely be insignificant relative to the potential complexity
of attempting to support 24-bit code pointers in the rare circumstances
they're typically used, on an otherwise native 16-bit machine.

(and just as a heads up, there seems to be no exiting mechanism to enable
 the convenient access of static constant data stored in an orthogonal
 address space relative to read-write data memory; although suspect one
 could implement a scheme in which every address is discriminated at
 run-time based on some address range split, but likely not worth the
 run-time overhead to do so, but should work if desperate to conserve ram)