Re: gcc for any microcontroller?

["Svein E. Seldal" <Svein dot Seldal at solidas dot com>]

tm_gccmail at mail dot kloo dot net wrote:
> o Special areas of memory which can only be accessed by special addressing
>   modes and/or banked memory
>
>   This usually applies to DSPs and DSP-like processors which have internal
>   memory which can be accessed in parallel with the external bus, or to
>   funky processors that have been extended past their normal lifespan by
>   artificially extending the address space.
>
>   Applies to: 8051, etc

One major drawback with GCC is that is does not support multiple 
memory-space addressing. Some microcontrollers and DSP's have separate 
memory-space for code and for data (for performance reasons). E.g. 
address 0x1000 can both refer to a point in the code memory (usually 
flash or similar) and to a point into data memory. These processors 
usually uses different op-codes and addressing methods to access the 
correct address and memory-space.

Applies to: AVR, 8051, high-performance DSP's.

Today, gcc selects one of these spaces (usually the data-space) and 
sticks to it. If the user needs to access another memory-space, e.g. 
constants in flash, he needs to do it manually himself (usually by 
inline assembly).


Technical about gcc's limitation:
---------------------------------

If it would be possible to declare attributes on pointers in gcc, this 
limitation could be solved in a rather simple way. Example:

1)	int in_flash __attribute__((progmem)) = 1234;
2)	int in_data = 5678;
3)	int * __attribute__((progmempointer)) code_pointer = &in_flash;
4)      int * data_pointer = &in_data;
5)      printf("%d %d\n",*code_pointer);
6)      data_pointer = code_pointer;

1) stores some information in the flash/code memory (this works today 
with the AVR port).
2) stores some information in data memory
3) declares a pointer to code memory using the proposed attribute
4) declares a pointer to data memory
5) will generate the target-specific opcodes required to access the data 
from progmem (because of the progmempointer attribute)
6) would fail because the types are different.

But as the operation in 3) is not possible with gcc, this need work with 
gcc. You can declare attributes on storages and implementations, but not 
on pointers. Gcc need to have these new features:

- Possibility to declare attributes of pointers. I.e. the attribute 
becomes a part of the type definition.

- For those targets with multiple memory-spaces, add the new 
memory-access methods. (E.g. in the AVR use 'lpm' to access code-memory, 
and 'lds' to access data-memory.)


Svein