> -----Original Message-----
> From: Hu, Luhui [SMTP:Luhui_Hu@PLUGPOWER.COM]
> Sent: Tuesday, August 13, 2002 2:02 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      [PIC] MPLAB C18 Compiler data memory usage (deep design)
>
> Hi,
>
> I am using Microchip MPLAB C18 compiler for PIC18F258. I have some
> questions:
>
> (1) How can I store variables using bank selection?
>
> (2) How can I use "#pragma varlocate n"?
>
> (3) Where do the compiler store "const" variables, "#define" constants and
> "enum" member lists? In the Flash program memory or RAM data memory?
> When I look at the Assembly code (after compiling), "const" variables are
> stored in the Flash but they also use the RAM data memory. The "#define"
> constants and "enum" member lists seem that they do not take RAM data
> memory
> nor Flash memory but they are replaced by the represented values directly
> in
> the assembly code. Therefore, I should use "define" and "enum" instead of
> "const" variables when I want to use constants.
>
Sorry, I'm not familiar with the C18 compiler, but a const will always be
stored in program memory on a PIC.  #define is a pre-processor directive,
and is used for assigning a string to a literal nummber (or statement).
However, const and #define are not interchangeable.  If you have a const
variable, the variable is stored in a known memory location and can be
accessed by pointers.  This probably dosen't sound very usefull for single
bytes, but when you wish to use a lookup table (i.e. an array) then you
won't be able to use a #define.

Regards

Mike

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