PIC24F Software Stack?
If you look at page 37 of the PIC24F16KA102 data sheet (PDF) (a typical small PIC24 device) you will see a description of the software stack: the W15 register is used as a stack pointer.

"The pointer always points to the first available free word and grows from lower to higher addresses. It pre-decrements for stack pops and post-increments for stack pushes, as depicted in Figure 4-4."

Page 70 of the 16-bit MCU and DSC Programmer’s Reference Manual (PDF) has some more information:

"The 16-bit MCU and DSC devices feature a software stack which facilitates function calls and exception handling. W15 is the default Stack Pointer (SP) and after any Reset, it is initialized to 0x0800 (0x1000 for PIC24E and dsPIC33E devices). This ensures that the SP will point to valid RAM and permits stack availability for exceptions, which may occur before the SP is set by the user software. The user may reprogram the SP during initialization to any location within data space."