64GA110 datasheet(34/330 Pages) MICROCHIP

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PIC24FJ256GA110 FAMILY

DS39905E-page 34

 2010 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 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. Sixteen-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 below in Table 3-2.

TABLE 3-2:

INSTRUCTIONS THAT USE THE SINGLE AND MULTI-BIT 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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