4.3.11 Arithmetic Logic Unit (ALU)

The dsPIC33A ALU is 32 bits wide and is capable of addition, subtraction, single-bit shifts and logic operations. Unless otherwise mentioned, arithmetic operations are a two’s complement in nature. Depending on the operation, the ALU can affect the values of the following bits in the STATUS Register:

Carry (C)
Zero (Z)
Negative (N)
Overflow (OV)

The ALU can perform 8/16-bit or 32-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.

Note: Byte operations use the 16-bit ALU and can produce results in excess of eight bits. However, to maintain backward compatibility with PIC^® MCU devices, the ALU result from all byte operations is written back as a byte (i.e., the MSB is not modified) and the STATUS Register is updated based only upon the state of the LSB of the result.