PIC18F010 datasheet(42/176 Pages) MICROCHIP

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PIC18F010/020

DS41142A-page 40

Preliminary

 2001 Microchip Technology Inc.

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. There-

fore, the result of an instruction with the STATUS regis-

ter 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 STA-

TUS register. For other instructions not affecting any

status bits, see Table 13-2.

STATUS REGISTER

Note:

The C and DC bits operate as a borrow and

digit borrow bit respectively, in subtraction.

U-0

R/W-x

—

NOV

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 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 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.

bit 0

C: Carry/borrow bit

For ADDWF, ADDLW, SUBLW, and SUBWF instructions

= 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 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.

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

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