PIC24FJ64GA705 Datasheet, PDF (40/412 Pages) –

PIC24FJ256GA705 FAMILY
3.3 Arithmetic Logic Unit (ALU)
The PIC24F ALU is 16 bits wide and is capable of addi-
tion, subtraction, bit shifts and logic operations. Unless
otherwise mentioned, arithmetic operations are 2’s
complement in nature. Depending on the operation, the
ALU may affect the values of the Carry (C), Zero (Z),
Negative (N), Overflow (OV) and Digit Carry (DC)
Status bits in the SR register. The C and DC Status bits
operate as Borrow and Digit Borrow bits, respectively,
for subtraction operations.
The ALU can perform 8-bit or 16-bit operations,
depending on the mode of the instruction that is used.
Data for the ALU operation can come from the W
register array, or data memory, depending on the
addressing mode of the instruction. Likewise, output
data from the ALU can be written to the W register array
or a data memory location.
The PIC24F CPU incorporates hardware support for
both multiplication and division. This includes a
dedicated hardware multiplier and support hardware
for 16-bit divisor division.
3.3.1 MULTIPLIER
The ALU contains a high-speed, 17-bit x 17-bit
multiplier. It supports unsigned, signed or mixed sign
operation in several multiplication modes:
• 16-bit x 16-bit signed
• 16-bit x 16-bit unsigned
• 16-bit signed x 5-bit (literal) unsigned
• 16-bit unsigned x 16-bit unsigned
• 16-bit unsigned x 5-bit (literal) unsigned
• 16-bit unsigned x 16-bit signed
• 8-bit unsigned x 8-bit unsigned
3.3.2 DIVIDER
The divide block supports 32-bit/16-bit and 16-bit/16-bit
signed and unsigned integer divide operations with the
following data sizes:
1. 32-bit signed/16-bit signed divide
2. 32-bit unsigned/16-bit unsigned divide
3. 16-bit signed/16-bit signed divide
4. 16-bit unsigned/16-bit unsigned divide
The quotient for all divide instructions ends up in W0
and the remainder in W1. The 16-bit signed and
unsigned DIV instructions can specify any W register
for both the 16-bit divisor (Wn), and any W register
(aligned) pair (W(m + 1):Wm) for the 32-bit dividend.
The divide algorithm takes one cycle per bit of divisor,
so both 32-bit/16-bit and 16-bit/16-bit instructions take
the same number of cycles to execute.
3.3.3 MULTI-BIT SHIFT SUPPORT
The PIC24F ALU supports both single bit and single-
cycle, multi-bit arithmetic and logic shifts. Multi-bit shifts
are implemented using a shifter block, capable of
performing up to a 15-bit arithmetic right shift, or up to
a 15-bit left shift, in a single cycle. All multi-bit shift
instructions only support Register Direct Addressing for
both the operand source and result destination.
A full summary of instructions that use the shift
operation is provided in Table 3-2.
TABLE 3-2: INSTRUCTIONS THAT USE THE SINGLE BIT AND MULTI-BIT SHIFT OPERATION
Instruction
Description
ASR
Arithmetic Shift Right Source register by one or more bits.
SL
Shift Left Source register by one or more bits.
LSR
Logical Shift Right Source register by one or more bits.
DS30010118B-page 40
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