PIC17C75X Datasheet, PDF (47/320 Pages) Microchip Technology – High-Performance 8-Bit CMOS EPROM Microcontrollers

PIC17C75X
7.2.2.1 ALU STATUS REGISTER (ALUSTA)
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.
For example, CLRF ALUSTA will clear the upper four
bits and set the Z bit. This leaves the ALUSTA register
as 0000u1uu (where u = unchanged).
It is recommended, therefore, that only BCF, 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 status bits, see the “Instruction Set Sum-
mary.”
Note 3: The C and DC bits operate as a borrow
and digit borrow bit, respectively, in sub-
traction. See the SUBLW and SUBWF
instructions for examples.
Note 4: The overflow bit will be set if the 2’s com-
plement result exceeds +127 or is less
than -128.
The Arithmetic and Logic Unit (ALU) is capable of car-
rying out arithmetic or logical operations on two oper-
ands or a single operand. All single operand
instructions operate either on the WREG register or the
given 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.
FIGURE 7-6: ALUSTA REGISTER (ADDRESS: 04h, UNBANKED)
R/W - 1 R/W - 1 R/W - 1 R/W - 1 R/W - x
FS3 FS2 FS1 FS0
OV
bit7
R/W - x
Z
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
R/W - x R/W - x
DC
C
bit0
R = Readable bit
W = Writable bit
-n = Value at POR reset
(x = unknown)
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:
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
bit 1:
DC: Digit carry/borrow bit
For ADDWF and ADDLW 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
Note: For borrow the polarity is reversed.
bit 0:
C: carry/borrow bit
For ADDWF and ADDLW instructions.
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.
© 1997 Microchip Technology Inc.
Preliminary
DS30264A-page 47