Reserving a fixed memory location for serial numbering.

[h.a.j.molenaar]

Hi all. I want to use a serial number which must be read by the firmware itself. My programmer (BeeProg) can insert an incremental number in the program memory.

I want to do like this (PIC12F683).
unsigned long int const value = 0x0000;

I want this on a fixed memory location, preferably at the end of the map.
This is at 07FD.

How do I do this or is there a more better way?

Thanks, Hugo

[Haplo]

From the manual:

[Ttelmah]

Another poster has already given you details of how to reserve a location, and write something at it.
Unfortunately, you cannot access this like a const variable though. The problem is that when you declare a const variable, the compiler actually inserts a 'header', which then allows the data to be read, in front of the data. A const variable uses significantly more memory, than the actual size of the value to be stored. the nature of this, depends on the chip involved (more latter...)

So you can declare a block of memory at a location, using #ROM, like:
#ROM 0x7FD = {12,34,56,78}

Four bytes will be stored at 0x7FD. However to access it, you will need to use the 'read_program_memory' function to read the data.
There are some 'caveats' though. On a 18chip, ROM stores 16bit values at each location. On smaller chips the size of each value is limited by the chip. The read function similarly only returns whatever data size actually exists at the location.

The big problem for you, is that the read function requires a chip that supports direct reading of the program memory using the registers for internal flash programming. Your chip does not support this. On chips without this ability, a 'const', is stored using the upper part of each instruction, combined with a 'RETLW' instruction. So when you declare a four byte constant, these values are not directly stored in the chip. Instead three instructions set the bank registers, then the offset of the required byte is added to the PC, to trigger a jump to the required return instruction, and the values are stored as a 'table' of RETLW instructions in the next few bytes. The instruction, then returns to the caller, with the required value. This means that you can only store byte values that contain the RETLW instruction. If a programmer changed one of these into another instruction, the code will fail...

So, though your programmer can insert an incremental number into program memory, this feature is not useable with the chip you have!. However there is one possibility. If your programmer can insert the value at the addresses for the internal EEPROM (0x2100 to 0x21FF), this is accessible by the read_program_eeprom instruction.

Best Wishes

[PCM programmer]

[Ttelmah]

Yes.
Slip of the keyboard.

Best Wishes

[h.a.j.molenaar]

I like to thank the contributors for their response.