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PIC24FJ256GA705 FAMILY

DS30010118D-page 40

 2016-2018 Microchip Technology Inc.

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

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:

32-bit signed/16-bit signed divide

32-bit unsigned/16-bit unsigned divide

16-bit signed/16-bit signed divide

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

MULTIBIT SHIFT SUPPORT

The PIC24F ALU supports both single bit and single-

cycle, multibit arithmetic and logic shifts. Multibit 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 multibit 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 MULTIBIT SHIFT OPERATION

Instruction

Description

ASR

Arithmetic Shift Right Source register by one or more bits.

Shift Left Source register by one or more bits.

LSR

Logical Shift Right Source register by one or more bits.

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