Accessing fixed addresses

[Rolf Fiedler]

> 
> I've seen structures defined for devices with large register sets.  Then
> an instance of that structure is defined as extern for each device.
> These go in a header file.
> 
> eg.
> typedef struct device
> {
>     unsigned short    reg1;
>     unsigned long      reg2;
> }
> 
> extern device    device1;
> extern device    device3;
> extern device    device2;
> 
> The address of each device is then defined/set in a linker command file.
> (eg. linkcmds).  The symbols can also be set by linker flags/switches on
> the CC or LD command.
> 
> I think this will do what you want.

I do not think this is a good idea at all. The compiler may put that
structure
packed or unpacked into memory, so this is highly compiler dependent. 
And then you start messing around with #pragma pack and all that
nonsense.
I have even seen this kind of code for network applications, for
structures
of data that is going to be sent via the interface... yak.

Furthermore there should at least be a volatile, otherwise you may end
up not reading the chips register but the copy in a CPU register.

why not have something like:

volatile const unsigned long *chip_base=(const unsigned char
*)0x12345678;

It is smart not to use int for long when accessing hardware - if you
want your code
to be portable :-).

enum offsets {
    FIRST_REG,
    SECOND_REG,
    etc.
};

and access by referencing chip_base[SECOND_REG]

Problem here is that is you want to use the same definition in more than
one C-File you have to create a header-file and a seperately linked C
file
for it, or make the pointer static.

-- 
+-----------------+--------------------------------------------------+
|    _____        |  Rolf Fiedler                                    |
|   / ___/        |  Electronic Design Engineer                      |
|  / _/           |  Ferrari electronic AG                           |
| /_/e/r/r/a/r/i/ |  phone: +49 3328 4559 0   fax: +49 3328 4559 60  |
|   electronic    |  E-Mail: Rolf.Fiedler@Ferrari.DE                 |
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