PIC18F6520-I Datasheet, PDF (45/380 Pages) Micrel Semiconductor – 64/80-Pin High-Performance, 256 Kbit to 1 Mbit Enhanced Flash Microcontrollers with A/D

PIC18F6520/8520/6620/8620/6720/8720
REGISTER 4-2: STKPTR REGISTER
R/C-0
R/C-0
U-0
STKFUL⁽¹⁾ STKUNF⁽¹⁾
—
bit 7
R/W-0
SP4
R/W-0
SP3
R/W-0
SP2
R/W-0
SP1
R/W-0
SP0
bit 0
bit 7
bit 6
bit 5
bit 4-0
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
Unimplemented: Read as ‘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
- n = Value at POR
W = Writable bit
‘1’ = Bit is set
U = Unimplemented bit, read as ‘0’
‘0’ = Bit is cleared x = Bit is unknown
FIGURE 4-3:
RETURN ADDRESS STACK AND ASSOCIATED REGISTERS
TOSU
0x00
TOSH
0x1A
Return Address Stack
TOSL
0x34
11111
11110
11101
STKPTR<4:0>
00010
00011
Top-of-Stack 0x001A34 00010
0x000D58 00001
00000
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
execution, 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 current 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
instruction 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
appropriate STKFUL or STKUNF bit, but not cause a
device Reset. When the STVREN bit is enabled, 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 only cleared by the user
software or a POR Reset.
 2004 Microchip Technology Inc.
DS39609B-page 43