[RFC] Gcc and harvard architectures (towards AVR)

[Svein E. Seldal]

Denis Chertykov wrote:

>>I have a technical suggestion for how to implement this, but I would
>>like to discuss the issue in a general basis before entering the
>>detailed discussion.
> 
> 
> Look to archive.
> Subject: Segment register support for the i386
> Date: 31 Dec 1999

Yes, I've seen this before. But it still is an unresolved issue IMHO. 
For the avr-gcc this is a obvious limiation, as it required more 
resources to do something that is simpler if gcc had the page support.

My suggestion is as follows:

A target has two address-spaces; the data (the default) and the code. 
Consider the following example:

Accesses to the data-space is done as normal:

   int a = 1234; // Declares a data storeage in data-space
   int *p = &a;	// Declare a pointer (in data-space) to a data-space
                 // storeage
   x = *p;       // Will fill x with 1234

Accesses using the code-space can be done as follows:

(Please note that code-space is usually read-only, and for this to work 
you'd have to declare the variables and pointers as const - but I'll 
skip this for now)

   // Declare a datastoreage in code-space
   int __attribute__((codespace)) b = 5678;

   // Declare a pointer in data-space which points to a code-space
   // address
   int __attribute__((codespace)) * q = &b;

   // Declare a pointer in code-space which points to a code-space
   // address
   int __attribute__((codespace)) * __attribute__((codespace)) r = &b;

   // Will read the contents of the code-space datastorage 'b' and
   // place it into x, which in this example is 5678
   x = *q;

   // This will fail because 'q' is a codespace pointer, while 'p' is
   // dataspace pointer
   q = p;

*) For those familiar with the avr target this is the same as the 
'progmem' attribute. I just want to use the codespace name as a 
demostration of how to do it.

Would this be a feasable implemenation of address-spaces?

What global (gcc) changes would a implementation like this imply?


Svein