PIC18F6525/6621/8525/8621
REGISTER 4-2:
STKPTR: STACK POINTER 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(1) STKUNF(1)
bit 7
bit 0
bit 7
bit 6
STKFUL: Stack Full Flag bit(1)
1= Stack became full or overflowed
0= Stack has not become full or overflowed
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
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 4-4:
RETURN ADDRESS STACK AND ASSOCIATED REGISTERS
Return Address Stack
11111
11110
11101
STKPTR<4:0>
TOSU
0x00
TOSH
0x1A
TOSL
0x34
00010
00011
00010
00001
00000
0x001A34
0x000D58
Top-of-Stack
4.2.3
PUSH AND POP INSTRUCTIONS
4.2.4
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
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.
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 Power-on 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 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.
2005 Microchip Technology Inc.
DS39612B-page 43
MICROCHIP [ MICROCHIP ]
