PIC18F2220/2320/4220/4320
REGISTER 5-1:
STKPTR REGISTER
R/C-0 R/C-0
U-0
—
R/W-0
SP4
R/W-0
SP3
R/W-0
SP2
R/W-0
SP1
R/W-0
SP0
STKFUL STKUNF
bit 7
bit 0
bit 7(1)
bit 6(1)
STKFUL: Stack Full Flag bit
1= Stack became full or overflowed
0= Stack has not become full or overflowed
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
‘1’ = Bit is set
U = Unimplemented
‘0’ = Bit is cleared
C = Clearable only bit
x = Bit is unknown
- n = Value at POR
5.2.3
PUSH AND POP INSTRUCTIONS
5.2.4
STACK FULL/UNDERFLOW RESETS
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.
These Resets are enabled by programming the
STVREN bit in Configuration Register 4L. When the
STVREN bit is cleared, a full or underflow condition will
set the appropriate STKFUL or STKUNF bit but not
cause a device Reset. When the STVREN 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 POR Reset.
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 POPinstruction. The POPinstruc-
tion discards the current TOS by decrementing the
stack pointer. The previous value pushed onto the
stack then becomes the TOS value.
2003 Microchip Technology Inc.
DS39599C-page 55
MICROCHIP [ MICROCHIP ]
