Datasheet

PIC18F2480/2580/4480/4580
DS39637D-page 94 © 2009 Microchip Technology Inc.
6.3.5 STATUS REGISTER
The STATUS register, shown in Register 6-2, contains
the arithmetic status of the ALU. As with any other SFR,
it can be the operand for any instruction.
If the STATUS register is the destination for an instruc-
tion that affects the Z, DC, C, OV or N bits, the results
of the instruction are not written; instead, the status is
updated according to the instruction performed. There-
fore, the result of an instruction with the STATUS
register as its destination may be different than
intended. As an example, CLRF STATUS will set the Z
bit and leave the remaining Status bits unchanged
(‘000u u1uu’).
It is recommended that only BCF, BSF, SWAPF, MOVFF
and MOVWF instructions are used to alter the STATUS
register, because these instructions do not affect the Z,
C, DC, OV or N bits in the STATUS register.
For other instructions that do not affect Status bits, see
the instruction set summaries in Table 26-2 and
Table 26-3.
Note: The C and DC bits operate as the borrow
and digit borrow bits respectively in
subtraction.
REGISTER 6-2: STATUS REGISTER
U-0 U-0 U-0 R/W-x R/W-x R/W-x R/W-x R/W-x
— — —NOVZDC
(1)
C
(2)
bit 7 bit 0
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
bit 7-5 Unimplemented: Read as ‘0’
bit 4 N: Negative bit
This bit is used for signed arithmetic (2’s complement). It indicates whether the result was negative
(ALU MSB = 1).
1 = Result was negative
0 = Result was positive
bit 3 OV: Overflow bit
This bit is used for signed arithmetic (2’s complement). It indicates an overflow of the 7-bit magnitude
which causes the sign bit (bit 7) to change state.
1 = Overflow occurred for signed arithmetic (in this arithmetic operation)
0 = No overflow occurred
bit 2 Z: Zero bit
1 = The result of an arithmetic or logic operation is zero
0 = The result of an arithmetic or logic operation is not zero
bit 1 DC: Digit carry/borrow
bit
(1)
For ADDWF, ADDLW, SUBLW and SUBWF instructions:
1 = A carry-out from the 4th low-order bit of the result occurred
0 = No carry-out from the 4th low-order bit of the result
bit 0 C: Carry/borrow
bit
(2)
For ADDWF, ADDLW, SUBLW and SUBWF instructions:
1 = A carry-out from the Most Significant bit of the result occurred
0 = No carry-out from the Most Significant bit of the result occurred
Note 1: For borrow,
the polarity is reversed. A subtraction is executed by adding the two’s complement of the
second operand. For rotate (RRF, RLF) instructions, this bit is loaded with either the bit 4 or bit 3 of the
source register.
2: For borrow,
the polarity is reversed. A subtraction is executed by adding the two’s complement of the
second operand. For rotate (RRF, RLF) instructions, this bit is loaded with either the high or low-order bit of
the source register.