Datasheet

 2010 Microchip Technology Inc. DS39616D-page 63
PIC18F2331/2431/4331/4431
When the stack has been popped enough times to
unload the stack, the next pop will return a value of zero
to the PC and set the STKUNF bit, while the Stack
Pointer remains at zero. The STKUNF bit will remain
set until cleared by software or a POR occurs.
FIGURE 6-3: RETURN ADDRESS STACK AND ASSOCIATED REGISTERS
Note: Returning a value of zero to the PC on an
underflow has the effect of vectoring the
program to the Reset vector, where the
stack conditions can be verified and
appropriate actions can be taken. This is
not the same as a Reset as the contents
of the SFRs are not affected.
REGISTER 6-1: STKPTR: STACK POINTER REGISTER
R/C-0 R/C-0 U-0 R/W-0 R/W-0 R/W-0 R/W-0 R/W-0
STKFUL
(1)
STKUNF
(1)
— SP4 SP3 SP2 SP1 SP0
bit 7 bit 0
Legend: C = Clearable bit
R = Readable bit W = Writable bit U = Unimplemented bit, read as ‘0’
-n = Value at POR ‘1’ = Bit is set ‘0’ = Bit is cleared x = Bit is unknown
bit 7 STKFUL: Stack Full Flag bit
(1)
1 = Stack became full or overflowed
0 = Stack has not become full or overflowed
bit 6 STKUNF: Stack Underflow Flag bit
(1)
1 = Stack underflow occurred
0 = Stack underflow did not occur
bit 5 Unimplemented: Read as ‘0’
bit 4-0 SP<4:0>: Stack Pointer Location bits
Note 1: Bit 7 and bit 6 are cleared by user software or by a POR.
00011
001A34h
11111
11110
11101
00010
00001
00000
00010
Return Address Stack
To p -o f- St a ck
000D58h
TOSLTOSHTOSU
34h1Ah00h
STKPTR<4:0>