9.5 Stack

All devices have a 16-level by 15-bit wide hardware stack. The stack space is not part of either program or data space. The PC is PUSHed onto the stack when the CALL or CALLW instructions are executed or an interrupt causes a branch. The stack is POPed in the event of a RETURN, RETLW or a RETFIE instruction execution. PCLATH is not affected by a PUSH or POP operation.

The stack operates as a circular buffer if the STVREN Configuration bit is programmed to ‘0’. This means that after the stack has been PUSHed sixteen times, the seventeenth PUSH overwrites the value that was stored from the first PUSH. The eighteenth PUSH overwrites the second PUSH, and so on. The STKOVF and STKUNF flag bits will be set on an Overflow/Underflow, regardless of whether the Reset is enabled.

If the STVREN bit is programmed to ‘1’, the device will be reset if the stack is PUSHed beyond the sixteenth level or POPed beyond the fist level, setting the appropriate bits (STKOVF or STKUNF, respectively).
Important: There are no instructions/mnemonics called PUSH or POP. These are actions that occur from the execution of the CALL, CALLW, RETURN, RETLW and RETFIE instructions or the vectoring to an interrupt address.