Hi Maziar,
functions cannot be located at an absolute address by the _at_ keyword.
To locate procedures at an fix place in code memory is necessary to use CODE control of the BL51 linker.

You can also use wildcards in BL51 v5.00 (I am not sure for erlier versions) such way:

BL51 myfile.obj CODE (?PR?*?myfile (0x1000))

Read Keil User's Guides for more detail.

Vaclav