Datasheet

© 2007 Microchip Technology Inc. DS39605F-page 43
PIC18F1220/1320
REGISTER 5-1: STKPTR REGISTER
5.2.3 PUSH AND POP INSTRUCTIONS
Since the Top-of-Stack (TOS) is readable and writable,
the ability to push values onto the stack and pull values
off the stack, without disturbing normal program execu-
tion, is a desirable option. To push the current PC value
onto the stack, a PUSH instruction can be executed.
This will increment the Stack Pointer and load the cur-
rent PC value onto the stack. TOSU, TOSH and TOSL
can then be modified to place data or a return address
on the stack.
The ability to pull the TOS value off of the stack and
replace it with the value that was previously pushed
onto the stack, without disturbing normal execution, is
achieved by using the POP instruction. The POP instruc-
tion discards the current TOS by decrementing the
Stack Pointer. The previous value pushed onto the
stack then becomes the TOS value.
5.2.4 STACK FULL/UNDERFLOW RESETS
These Resets are enabled by programming the STVR
bit in Configuration Register 4L. When the STVR bit is
cleared, a full or underflow condition will set the appro-
priate STKFUL or STKUNF bit, but not cause a device
Reset. When the STVR bit is set, a full or underflow
condition will set the appropriate STKFUL or STKUNF
bit and then cause a device Reset. The STKFUL or
STKUNF bits are cleared by the user software or a
Power-on Reset.
R/C-0 R/C-0 U-0 R/W-0 R/W-0 R/W-0 R/W-0 R/W-0
STKFUL STKUNF
— SP4 SP3 SP2 SP1 SP0
bit 7 bit 0
bit 7
(1)
STKFUL: Stack Full Flag bit
1 = Stack became full or overflowed
0 = Stack has not become full or overflowed
bit 6
(1)
STKUNF: Stack Underflow Flag bit
1 = Stack underflow occurred
0 = Stack underflow did not occur
bit 5 Unimplemented: Read as ‘0’
bit 4-0 SP4:SP0: Stack Pointer Location bits
Note 1: Bit 7 and bit 6 are cleared by user software or by a POR.
Legend:
R = Readable bit W = Writable bit U = Unimplemented C = Clearable only bit
-n = Value at POR ‘1’ = Bit is set ‘0’ = Bit is cleared x = Bit is unknown