Microchip Technology DM164134 Data Sheet

DS41159E-page 57

PIC18FXX8

4.13

Status Register

The Status register, shown in Register 4-2, contains the
arithmetic status of the ALU. The Status register can be
the destination for any instruction, as with any other
register. If the Status register is the destination for an
instruction that affects the Z, DC, C, OV or N bits, then
the write to these five bits is disabled. These bits are set
or cleared according to the device logic. Therefore, the
result of an instruction with the Status register as
destination may be different than intended.

For example, CLRF STATUS will clear the upper three
bits and set the Z bit. This leaves the Status register as
000u u1uu

(where u = unchanged).

It is recommended, therefore, 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 from the Status
register. For other instructions which do not affect the
status bits, see Table 25-2.

REGISTER 4-2:

STATUS REGISTER

Note:

The C and DC bits operate as a Borrow
and Digit Borrow bit respectively, in
subtraction.

U-0

R/W-x

—

bit 7

bit 0

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 of the ALU
operation was negative (ALU MSb = 1).

= Result was negative

= 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.

= Overflow occurred for signed arithmetic (in this arithmetic operation)

= No overflow occurred

bit 2

Z: Zero bit

= The result of an arithmetic or logic operation is zero

= The result of an arithmetic or logic operation is not zero

bit 1

DC: Digit Carry/Borrow bit

For ADDWF, ADDLW, SUBLW and SUBWF instructions:

= A carry-out from the 4th low-order bit of the result occurred

= No carry-out from the 4th low-order bit of the result

Note:

For Borrow, the polarity is reversed. A subtraction is executed by adding the 2’s
complement of the second operand. For rotate (RRCF, RRNCF, RLCF and RLNCF)
instructions, this bit is loaded with either bit 4 or bit 3 of the source register.

bit 0

C: Carry/Borrow bit

For ADDWF, ADDLW, SUBLW and SUBWF instructions:
1

= A carry-out from the Most Significant bit of the result occurred

= No carry-out from the Most Significant bit of the result occurred

Note:

For Borrow, the polarity is reversed. A subtraction is executed by adding the 2’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.

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