On 22/07/2010 20:28, alan.b.pearce@stfc.ac.uk wrote:

[snip]
>> void OpenSPI1
>> (
>> unsigned char sync_mode,
>> unsigned char bus_mode,
>> unsigned char smp_phase
>> );
>> unsigned char WriteSPI1(unsigned char data_out );
>>
>> void OpenSPI2
>> (
>> unsigned char sync_mode,
>> unsigned char bus_mode,
>> unsigned char smp_phase
>> );
>> unsigned char WriteSPI2(unsigned char data_out );
>>
>> void Open1USART
>> (
>> unsigned char config,
>> unsigned int spbrg
>> );
>>     =20
> Note that the variables declared inside each function are not declared
> 'static' so each time the function gets called they are recreated on the
> stack, and are unique to that function. So the variables inside OpenSPI1
> are different variables to OpenSPI2, even though they have the same
> name. The scope of each name is limited to the code inside that
> function. The variables are cleared to all zeros on creation on the
> stack if I understand the C standard correctly, but may need correction
> here. If you attempt to look at the values of these variables with a
> debugger such as MPLAB/ICD combination, you get strange results unless
> you are single stepping through the code inside that function.
>
> When the function terminates by executing a 'return' the variables are
> removed from the stack, and no longer exist. At this stage your debugger
> will again go strange with the values of these variables (IIRC it gives
> an error message that says they are 'Out of Scope'). If the other
> routine is now called the variables for that function are automatically
> created on the stack, and are unique to that function, even if they
> happened to reside at the same position in the stack.
>
> If the variables are declared 'static' in addition to the other
> attributes they have, then they are created in the normal RAM area, and
> are persistent between calls to the function, i.e. they keep their last
> value from the last time the function was called, or are uninitialized
> before the first call of the function unless initialisation values are
> declared. But they are still separate variables even though they have
> the same name. I don't know how the debugger identifies which one you
> wish to watch, I haven't had occasion to go down this road at this
> stage.
>   =20
Thanks Alan, this really hits the root of my question. Very helpful indeed.

Regards,
Matt
--=20
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