Datasheet

PIC18FXX2
DS39564C-page 38 © 2006 Microchip Technology Inc.
REGISTER 4-1: STKPTR REGISTER
FIGURE 4-3: RETURN ADDRESS STACK AND ASSOCIATED REGISTERS
4.2.3 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 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.
4.2.4 STACK FULL/UNDERFLOW RESETS
These resets are enabled by programming the
STVREN configuration bit. When the STVREN bit is
disabled, a full or underflow condition will set the appro-
priate STKFUL or STKUNF bit, but not cause a device
RESET. When the STVREN bit is enabled, a full or
underflow will set the appropriate STKFUL or STKUNF
bit and then cause a device RESET. The STKFUL or
STKUNF bits are only cleared by the user software or
a POR Reset.
R/C-0 R/C-0 U-0 R/W-0 R/W-0 R/W-0 R/W-0 R/W-0
STKOVF STKUNF
— SP4 SP3 SP2 SP1 SP0
bit 7 bit 0
bit 7
(1)
STKOVF: 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 can only be cleared in user software or by a POR.
Legend:
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
00011
0x001A34
11111
11110
11101
00010
00001
00000
00010
Return Address Stack
Top of Stack
0x000D58
TOSLTOSHTOSU
0x340x1A0x00
STKPTR<4:0>