*8-bit Arithmetic and Logic Unit (ALU)*

The 8-bit Arithmetic and Logic Unit (ALU) is a fundamental building block of a processor responsible for performing arithmetic and logic operations. The ALU takes two 8-bit operands (`Operand1`

and `Operand2`

) and an opcode (`Opcode`

) as inputs. The opcode selects the operation to be performed on the operands. The result of the operation is provided through the 16-bit `Result`

port. Additionally, there are two flags `flagC`

and `flagZ`

, which indicate the carry-out and zero result conditions, respectively.

**The Arithmetic Unit compromises three functions. They are:**

- Addition
- Subtraction
- Multiplication

**The Logical Unit compromises five functions. They are:**

- Bitwise AND
- Bitwise OR
- Bitwise NAND
- Bitwise NOR
- Bitwise XOR

**Flags**

ALU updates the conditional flags, which are used by the processor to perform other operations like condition checking and branching. In this example, two flags are implemented. They are:

- Zero – If all the bits of the result data are zero then the zero flag is set
- Carry out – If the addition of the two operands gives the carry out this flag is set

**Truth Table**

Figure 2 shows the truth table of the 8-bit ALU. The ALU works on 8-bit operands. It supports 8 instructions which are selected by the 3-bit Opcode.

**Verilog Module: 8-bit ALU**

The Verilog module of the 8-bit ALU is shown below. It implements the arithmetic functions of addition, subtraction, and multiplication, as well as the logical functions of bitwise AND, OR, NAND, NOR, and XOR.

**Verilog Code for the 8-bit ALU**

module ALU8bit(

input [2:0] Opcode,

input [7:0] Operand1,

input [7:0] Operand2,

output reg [15:0] Result = 16’b0,

output reg flagC = 1’b0,

output reg flagZ = 1’b0

);

parameter [2:0] ADD = 3’b000,

SUB = 3’b001,

MUL = 3’b010,

AND = 3’b011,

OR = 3’b100,

NAND = 3’b101,

NOR = 3’b110,

XOR = 3’b111;

always @ (Opcode or Operand1 or Operand2)

begin

case (Opcode)

ADD: begin

Result = Operand1 + Operand2;

flagC = Result[8];

flagZ = (Result == 16’b0);

end

SUB: begin

Result = Operand1 – Operand2;

flagC = Result[8];

flagZ = (Result == 16’b0);

end

MUL: begin

Result = Operand1 * Operand2;

flagZ = (Result == 16’b0);

end

AND: begin

Result = Operand1 & Operand2;

flagZ = (Result == 16’b0);

end

OR: begin

Result = Operand1 | Operand2;

flagZ = (Result == 16’b0);

end

NAND: begin

Result = ~(Operand1 & Operand2);

flagZ = (Result == 16’b0);

end

NOR: begin

Result = ~(Operand1 | Operand2);

flagZ = (Result == 16’b0);

end

XOR: begin

Result = Operand1 ^ Operand2;

flagZ = (Result == 16’b0);

end

default: begin

Result = 16’b0;

flagC = 1’b0;

flagZ = 1’b0;

end

endcase

end

endmodule

**Verilog Test Bench for 8-bit ALU**

The Verilog test bench for the 8-bit ALU is shown below. It checks the functionality of the ALU by providing stimulus to the input ports (`Opcode`

, `Operand1`

, and `Operand2`

) and observing the output ports (`Result`

, `flagC`

, and `flagZ`

).

module ALU8bit_tb; // Inputs reg [2:0] Opcode; reg [7:0] Operand1; reg [7:0] Operand2; // Outputs wire [15:0] Result; wire flagC; wire flagZ; // Temporary variable reg [2:0] count = 3'd0; // Instantiate the Unit Under Test (UUT) ALU8bit uut ( .Opcode(Opcode), .Operand1(Operand1), .Operand2(Operand2), .Result(Result), .flagC(flagC), .flagZ(flagZ) ); initial begin // Initialize Inputs Opcode = 3'b0; Operand1 = 8'd0; Operand2 = 8'd0; // Wait 100 ns for global reset to finish #100; // Add stimulus here Operand1 = 8'hAA; Operand2 = 8'h55; for (count = 0; count < 8; count = count + 1'b1) begin Opcode = count; #20; end end endmodule

**Timing Diagram for 8-bit ALU**

The following figure presents the timing diagram of the 8-bit ALU, illustrating the waveforms of the output `Result`

, `flagC`

, and `flagZ`

as different operations are performed with different operand values.

The provided Verilog module and test bench ensure the correct functionality of the 8-bit ALU for various arithmetic and logical operations.

You may also likes following Verilog Code

Full Adder |

D Flip-Flop |

FIFO Buffer |

LIFO Buffer |

Barrel Shifter |

3-to-8 Decoder |

8-to-1 Multiplexer |

4-bit Carry Ripple Adder |

8-bit Magnitude Comparator |

Related Posts

Analog and Memory Layout Design Forum |

Physical Layout Design Forum |

RTL & Verilog Design Forum |

Semiconductor Forum |

Analog Layout Design Interview Questions |
Memory Design Interview Questions |

Physical Design Interview Questions |
Verilog Interview Questions |

Digital Design Interview Questions |
STA Interview Questions |