PIC18F23K22 Datasheet, PDF (72/492 Pages) Microchip Technology – 28/40/44-Pin, Low-Power, High-Performance Microcontrollers with nanoWatt XLP Technology

PIC18(L)F2X/4XK22
5.1.2.3 PUSH and POP Instructions
Since the Top-of-Stack is readable and writable, the
ability to push values onto the stack and pull values off
the stack without disturbing normal program execution
is a desirable feature. The PIC18 instruction set
includes two instructions, PUSH and POP, that permit
the TOS to be manipulated under software control.
TOSU, TOSH and TOSL can be modified to place data
or a return address on the stack.
The PUSH instruction places the current PC value onto
the stack. This increments the Stack Pointer and loads
the current PC value onto the stack.
The POP instruction discards the current TOS by decre-
menting the Stack Pointer. The previous value pushed
onto the stack then becomes the TOS value.
REGISTER 5-1: STKPTR: STACK POINTER REGISTER
R/C-0
R/C-0
U-0
STKFUL⁽¹⁾ STKUNF⁽¹⁾
—
bit 7
R/W-0
R/W-0
R/W-0
STKPTR<4:0>
R/W-0
R/W-0
bit 0
Legend:
R = Readable bit
-n = Value at POR
W = Writable bit
‘1’ = Bit is set
U = Unimplemented
‘0’ = Bit is cleared
C = Clearable only bit
x = Bit is unknown
bit 7
bit 6
bit 5
bit 4-0
STKFUL: Stack Full Flag bit⁽¹⁾
1 = Stack became full or overflowed
0 = Stack has not become full or overflowed
STKUNF: Stack Underflow Flag bit⁽¹⁾
1 = Stack Underflow occurred
0 = Stack Underflow did not occur
Unimplemented: Read as ‘0’
STKPTR<4:0>: Stack Pointer Location bits
Note 1: Bit 7 and bit 6 are cleared by user software or by a POR.
5.1.2.4 Stack Full and Underflow Resets
Device Resets on Stack Overflow and Stack Underflow
conditions are enabled by setting the STVREN bit in
Configuration Register 4L. When STVREN is set, a full
or underflow will set the appropriate STKFUL or
STKUNF bit and then cause a device Reset. When
STVREN is cleared, a full or underflow condition will set
the appropriate STKFUL or STKUNF bit but not cause
a device Reset. The STKFUL or STKUNF bits are
cleared by the user software or a Power-on Reset.
5.1.3 FAST REGISTER STACK
A fast register stack is provided for the Status, WREG
and BSR registers, to provide a “fast return” option for
interrupts. The stack for each register is only one level
deep and is neither readable nor writable. It is loaded
with the current value of the corresponding register
when the processor vectors for an interrupt. All inter-
rupt sources will push values into the stack registers.
The values in the registers are then loaded back into
their associated registers if the RETFIE, FAST
instruction is used to return from the interrupt.
If both low and high priority interrupts are enabled, the
stack registers cannot be used reliably to return from
low priority interrupts. If a high priority interrupt occurs
while servicing a low priority interrupt, the stack register
values stored by the low priority interrupt will be
overwritten. In these cases, users must save the key
registers by software during a low priority interrupt.
If interrupt priority is not used, all interrupts may use the
fast register stack for returns from interrupt. If no
interrupts are used, the fast register stack can be used
to restore the Status, WREG and BSR registers at the
end of a subroutine call. To use the fast register stack
for a subroutine call, a CALL label, FAST instruction
must be executed to save the Status, WREG and BSR
registers to the fast register stack. A RETURN, FAST
instruction is then executed to restore these registers
from the fast register stack.
Example 5-1 shows a source code example that uses
the fast register stack during a subroutine call and
return.
DS41412B-page 72
Preliminary
 2010 Microchip Technology Inc.