PIC18CXX2
Register 4-1: STKPTR - Stack Pointer Register
R/C-0
STKFUL STKUNF
bit7
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
R
= Readable bit
W = Writeable bit
C
U
6
5
4
3
2
1
bit0
= Clearable bit
= Unimplemented bit,
Read as ‘0’
- n = Value at POR reset
bit 7(1): STKFUL: 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.
FIGURE 4-3: RETURN ADDRESS STACK AND ASSOCIATED REGISTERS
Return Address Stack
11111
11110
11101
STKPTR<4:0>
TOSU
0x00
TOSH
0x1A
TOSL
0x34
00010
00011
0x001A34 00010
0x000D58 00001
00000
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 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.
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 under-
flow 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.
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.
DS39026B-page 36
Preliminary
7/99 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
