PIC17C4X
6.2.2.1
ALU STATUS REGISTER (ALUSTA)
It is recommended, therefore, that onlyBCF, BSF, SWAPF
and MOVWF instructions be used to alter the ALUSTA
register because these instructions do not affect any
status bit. To see how other instructions affect the sta-
tus bits, see the “Instruction Set Summary.”
The ALUSTA register contains the status bits of the
Arithmetic and Logic Unit and the mode control bits for
the indirect addressing register.
As with all the other registers, the ALUSTA register can
be the destination for any instruction. If the ALUSTA
register is the destination for an instruction that affects
the Z, DC or C bits, then the write to these three bits is
disabled. These bits are set or cleared according to the
device logic. Therefore, the result of an instruction with
the ALUSTA register as destination may be different
than intended.
Note 1: The C and DC bits operate as a borrow
out bit in subtraction. See the SUBLWand
SUBWFinstructions for examples.
Note 2: The overflow bit will be set if the 2’s com-
plement result exceeds +127 or is less
than -128.
Arithmetic and Logic Unit (ALU) is capable of carrying
out arithmetic or logical operations on two operands or
a single operand. All single operand instructions oper-
ate either on the WREG register or a file register. For
two operand instructions, one of the operands is the
WREG register and the other one is either a file register
or an 8-bit immediate constant.
For example, CLRF ALUSTAwill clear the upper four bits
and set the Z bit. This leaves the ALUSTA register as
0000u1uu(where u= unchanged).
FIGURE 6-7: ALUSTA REGISTER (ADDRESS: 04h, UNBANKED)
R/W - 1 R/W - 1 R/W - 1 R/W - 1 R/W - x R/W - x R/W - x R/W - x
R = Readable bit
W = Writable bit
-n = Value at POR reset
(x = unknown)
FS3
FS2
FS1
FS0
OV
Z
DC
C
bit7
bit0
bit 7-6: FS3:FS2: FSR1 Mode Select bits
00 = Post auto-decrement FSR1 value
01 = Post auto-increment FSR1 value
1x = FSR1 value does not change
bit 5-4: FS1:FS0: FSR0 Mode Select bits
00 = Post auto-decrement FSR0 value
01 = Post auto-increment FSR0 value
1x = FSR0 value does not change
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 (bit7) to change state.
1 = Overflow occurred for signed arithmetic, (in this arithmetic operation)
0 = No overflow occurred
bit 2:
bit 1:
Z: Zero bit
1 = The result of an arithmetic or logic operation is zero
0 = The results of an arithmetic or logic operation is not zero
DC: Digit carry/borrow bit
For ADDWFand ADDLWinstructions.
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
Note: For borrow the polarity is reversed.
bit 0:
C: carry/borrow bit
For ADDWFand ADDLWinstructions.
1 = A carry-out from the most significant bit of the result occurred
Note that a subtraction is executed by adding the two’s complement of the second operand. For rotate
(RRCF, RLCF) instructions, this bit is loaded with either the high or low order bit of the source register.
0 = No carry-out from the most significant bit of the result
Note: For borrow the polarity is reversed.
DS30412C-page 36
1996 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
