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

4.8 SOFTWARE STACK
Run Time Environment
The section attribute specifies that the function should be placed in a section
named.myDataSection, rather than the default .data section. It does not specify
where the user-defined section is to be located. Again, 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:
.myDataSection 0x1000 :
{
*(.myDataSection);
} >data
This specifies that the output file should contain a section named.myDataSection
starting at location 0x1000 and containing all input sections named.myDataSection.
Since, in this example, there is a single variable Mabonga in that section, then the
variable will be located at address 0x1000 in data memory.
The dsPIC DSC device dedicates register W15 for use as a software Stack Pointer. All
processor stack operations, including function calls, interrupts and exceptions, use the
software stack. The stack grows upward, towards higher memory addresses.
The dsPIC DSC device also supports stack overflow detection. If the Stack Pointer
Limit register, SPLIM, is initialized, the device will test for overflow on all stack operations.
If an overflow should occur, the processor will initiate a stack error exception. By
default, this will result in a processor reset. Applications may also install a stack error
exception handler by defining an interrupt function named _StackError. See Chapter
8. “Interrupts” for details.
The C run-time startup module initializes the Stack Pointer (W15) and the Stack Pointer
Limit register during the startup and initialization sequence. The initial values are
normally provided by the linker, which allocates the largest stack possible from unused
data memory. The location of the stack is reported in the link map output file.
Applications can ensure that at least a minimum-sized stack is available with the
--stack linker command-line option. See the “MPLAB ® Assembler, Linker and
Utilities for PIC24 MCUs and dsPIC ® DSCs User’s Guide” (DS51317) for details.
Alternatively, the stack of specific size may be allocated with a user-defined section in
a custom linker script. In the following example, 0x100 bytes of data memory are
reserved for the stack. Two symbols are declared, __SP_init and __SPLIM_init,
for use by the C run-time startup module:
.stack :
{
__SP_init = .;
. += 0x100;
__SPLIM_init = .;
. += 8;
} >data
__SP_init defines the initial value for the Stack Pointer (W15) and __SPLIM_init
defines the initial value for the Stack Pointer Limit register (SPLIM). The value of
__SPLIM_init should be at least 8 bytes less than the physical stack limit, to allow
for stack error exception processing. This value should be decreased further to account
for stack usage by the interrupt handler itself, if a stack error interrupt handler is
installed. The default interrupt handler does not require additional stack usage.
© 2008 Microchip Technology Inc. DS51284H-page 69