ARITHMETIC AND LOGIC UNIT DESIGN (ALU)

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ARITHMETIC AND LOGIC UNIT DESIGN (ALU)
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ALU Design

• 1-bit Full Adder
• 4-bit Arithmetic circuits
• Add/Subtract/Increament/Decrement Circuit
• Arithmetic and Logic Unit
• Flags
• Carry-Out, Sign, Zero, Overflow
• Shift and Rotate Operations

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Arithmetic and Logic Unit Design (ALU)

• The Arithmetic and Logic Unit (ALU) is a
  combinational digital circuit, within the CPU,
  responsible for performing all arithmetic and
  logic operations.
• The operation of the arithmetic unit of the ALU
  is based on the Full Adder, a circuit that adds
  two bits (A and B) and the carry out generated by
  the addition of the two bits in the previous
  column.
• To design an ALU, for an N-bit processor, we
  first design the ALU for 1 bit, and then we
  cascade N 1-bit ALUs to obtain the N-bit ALU.

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1-bit Full Adder
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4-bit Arithmetic Circuit - Four signals (F3, F2,
F1, F0)

• full adders and multiplexers, a 4-bit arithmetic
  unit .
• Four signals (F3, F2, F1, F0) are used to specify
  the function to be performed.

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4-bit Arithmetic Circuit - Four signals (F3, F2,
F1, F0) (cont)

• Truth table for the inputs
  of the full adders

Control Inputs Control Inputs Control Inputs Control Inputs Output Output
F3 F2 F1 F0 Output Output
0 0 0 0 A B Addition
0 0 0 1 A 1 Increment
0 0 1 0 A 1 Decrement
0 0 1 1 A B Subtraction
0 1 0 0 B A Subtraction
0 1 0 1 B 1 Decrement
0 1 1 0 B 1 Increment
0 1 1 1 - A Negate (Minus)
1 X 0 0 A Complement (Not)
1 X 0 1 A and B And
1 X 1 0 A or B Or
1 X 1 1 A ex-or B Exclusive Or
Control Inputs Control Inputs Control Inputs Control Inputs Output Full Adder Inputs Full Adder Inputs Full Adder Inputs
F3 F2 F1 F0 Output A B Cin
0 0 0 0 A B A B 0
0 0 0 1 A 1 A 0 1
0 0 1 0 A 1 A 1 0
0 0 1 1 A B A B 1
0 1 0 0 B A A B 1
0 1 0 1 B 1 1 B 0
0 1 1 0 B 1 0 B 1
0 1 1 1 - A A 0 1
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4-bit Arithmetic Circuit - Two signals - inputs
(F1, F0) 4-bit arithmetic unit, including the
Sign, Zero, and Carry Out flags.

• Full adders and multiplexers, a 4-bit arithmetic
  unit .
• Two signals (F1, F0) are used to specify the
  function to be performed.