PIC16LF18854 Datasheet, PDF (85/668 Pages) Microchip Technology – C Compiler Optimized RISC Architecture

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3.4 Stack
All devices have a 16-level x 15-bit wide hardware
stack (refer to Figure 3-4 through Figure 3-7). The
stack space is not part of either program or data space.
The PC is PUSHed onto the stack when 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
bit is programmed to ‘0‘ (Configuration Words). 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.
Note 1: 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.
3.4.1 ACCESSING THE STACK
The stack is available through the TOSH, TOSL and
STKPTR registers. STKPTR is the current value of the
Stack Pointer. TOSH:TOSL register pair points to the
TOP of the stack. Both registers are read/writable. TOS
is split into TOSH and TOSL due to the 15-bit size of the
PC. To access the stack, adjust the value of STKPTR,
which will position TOSH:TOSL, then read/write to
TOSH:TOSL. STKPTR is five bits to allow detection of
overflow and underflow.
Note: Care should be taken when modifying the
STKPTR while interrupts are enabled.
During normal program operation, CALL, CALLW and
interrupts will increment STKPTR while RETLW,
RETURN, and RETFIE will decrement STKPTR. At any
time, STKPTR can be inspected to see how much
stack is left. The STKPTR always points at the currently
used place on the stack. Therefore, a CALL or CALLW
will increment the STKPTR and then write the PC, and
a return will unload the PC and then decrement the
STKPTR.
Reference Figure 3-4 through Figure 3-7 for examples
of accessing the stack.
FIGURE 3-4:
ACCESSING THE STACK EXAMPLE 1
TOSH:TOSL
TOSH:TOSL
0x0F
0x0E
0x0D
0x0C
0x0B
0x0A
0x09
0x08
0x07
0x06
0x05
0x04
0x03
0x02
0x01
0x00
0x1F
0x0000
Rev. 10-000043A
7/30/2013
Stack Reset Disabled
STKPTR = 0x1F _{(STVREN = 0)}
Initial Stack Configuration:
After Reset, the stack is empty. The
empty stack is initialized so the Stack
Pointer is pointing at 0x1F. If the Stack
Overflow/Underflow Reset is enabled, the
TOSH/TOSL register will return ‘0’. If the
Stack Overflow/Underflow Reset is
disabled, the TOSH/TOSL register will
return the contents of stack address
0x0F.
STKPTR = 0x1F
Stack Reset Enabled
(STVREN = 1)
 2016 Microchip Technology Inc.
Preliminary
DS40001824A-page 85