PIC16F1847_13 Datasheet, PDF (39/440 Pages) Microchip Technology – 18/20/28-Pin Flash Microcontrollers with XLP Technology

PIC16(L)F1847
3.5 Stack
All devices have a 16-level x 15-bit wide hardware
stack (refer to Figures 3-4 through 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 = 0 (Configuration Words). This means that after the
stack has been PUSHed sixteen times, the seven-
teenth 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.
3.5.1 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 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
STKPTR = 0x1F ^{Stack Reset Disabled}
(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 registers will return ‘0’. If
the Stack Overflow/Underflow Reset is
disabled, the TOSH/TOSL registers will
return the contents of stack address 0x0F.
Stack Reset Enabled
STKPTR = 0x1F _{(STVREN = 1)}
 2011-2013 Microchip Technology Inc.
Preliminary
DS40001453D-page 39