PIC18F6585 Datasheet, PDF (57/496 Pages) Microchip Technology – 64/68/80-Pin High-Performance, 64-Kbyte Enhanced Flash Microcontrollers with ECAN Module

PIC18F6585/8585/6680/8680
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:
C = Clearable bit
- n = Value at POR
R = Readable bit
‘1’ = Bit is set
U = Unimplemented bit, read as ‘0’ W = Writable bit
‘0’ = Bit is cleared
x = Bit is unknown
FIGURE 4-4:
RETURN ADDRESS STACK AND ASSOCIATED REGISTERS
TOSU
00h
TOSH
1Ah
Return Address Stack
TOSL
34h
11111
11110
11101
STKPTR<4:0>
00010
Top-of-Stack 001A34h
000D58h
00011
00010
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 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
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.
DS30491C-page 55