MPLAB C Compiler for PIC24 MCUs and dsPIC DSCs ... - Microchip

16-Bit C Compiler User’s Guide
4.7 LOCATING CODE AND DATA
As described in Section 4.3 “Address Spaces”, the compiler arranges for code to be
placed in the .text section, and data to be placed in one of several named sections,
depending on the memory model used and whether or not the data is initialized. When
modules are combined at link time, the linker determines the starting addresses of the
various sections based on their attributes.
Cases may arise when a specific function or variable must be located at a specific
address, or within some range of addresses. The easiest way to accomplish this is by
using the address attribute, described in Section 2.3 “Keyword Differences”. For
example, to locate function PrintString at address 0x8000 in program memory:
int __attribute__ ((address(0x8000))) PrintString (const char *s);
Likewise, to locate variable Mabonga at address 0x1000 in data memory:
int __attribute__ ((address(0x1000))) Mabonga = 1;
Another way to locate code or data is by placing the function or variable into a
user-defined section, and specifying the starting address of that section in a custom
linker script. This is done as follows:
1. Modify the code or data declaration in the C source to specify a user-defined
section.
2. Add the user-defined section to a custom linker script file to specify the starting
address of the section.
For example, to locate the function PrintString at address 0x8000 in program
memory, first declare the function as follows in the C source:
int __attribute__((__section__(".myTextSection")))
PrintString(const char *s);
The section attribute specifies that the function should be placed in a section named
.myTextSection, rather than the default .text section. It does not specify where
the user-defined section is to be located. That must be done in a custom linker script,
as follows. Using the device-specific linker script as a base, add the following section
definition:
.myTextSection 0x8000 :
{
*(.myTextSection);
} >program
This specifies that the output file should contain a section named .myTextSection
starting at location 0x8000 and containing all input sections named.myTextSection.
Since, in this example, there is a single function PrintString in that section, then the
function will be located at address 0x8000 in program memory.
Similarly, to locate the variable Mabonga at address 0x1000 in data memory, first
declare the variable as follows in the C source:
int __attribute__((__section__(".myDataSection"))) Mabonga =
1;
DS51284H-page 68 © 2008 Microchip Technology Inc.