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# What is Arithmetic Logic Shift Unit in Computer Architecture?

A computer systems create use of several storage registers that are linked to a typical operational unit is known as the arithmetic and logic unit (ALU). ALU is the central and one of the most essential units internal the CPU of the computer.

All the logical and numerical operations of a computer are implemented here. The contents of a particular register are arranged in the input of ALU. ALU implements the given operation and then transfers it to the target register.

The one stage of an arithmetic logic shift unit is demonstrated in the figure. The subscript i nominate a frequent stage. There are two inputs are used for both the arithmetic and logic units.

A specific micro-operation is choosing with inputs S_{1} and S_{0} A 4 x 1 multiplexer at the output select between an arithmetic output inand a logic output in . The data in the multiplexer are chosen with inputs S_{3} and S_{2}.

There are two data inputs to the multiplexer receive inputs A_{i-1} for the shift-right operation and A_{i+1} for the shift-left operation. The circuit should be continued n times for an n-bit ALU.

The output carry C_{i+1} of a given arithmetic stage should be linked to the input carry C_{i} of the next stage in the series. The input carries to the first stage is the input carry C_{in} It can support a selection variable for the arithmetic operations.

The circuit whose one stage is determined in the diagram. It provides eight arithmetic operations, four logic operations, and two-shift operations. Each operation is selected with the five variables S_{3},S_{2},S_{1},S_{0}The input carries C_{i}∧C_{in} can be used for selecting an arithmetic operation only.

**Functional Table for Arithmetic Logic Shift Unit**

S_{3} | S_{2} | S_{1} | S_{0} | C_{in} | Operation | Function |
---|---|---|---|---|---|---|

0 | 0 | 0 | 0 | 0 | F = A | Transfer A |

0 | 0 | 0 | 0 | 1 | F = A + 1 | Increment A |

0 | 0 | 0 | 1 | 0 | F = A + B | Addition |

0 | 0 | 0 | 1 | 1 | F = A + B + 1 | Add with carry |

0 | 0 | 1 | 0 | 0 | F = A + B’ | Subtract with borrow |

0 | 0 | 1 | 0 | 1 | F = A + B’ + 1 | Subtraction |

0 | 0 | 1 | 1 | 0 | F = A – 1 | Decrement A |

0 | 0 | 1 | 1 | 1 | F = A | Transfer A |

0 | 1 | 0 | 0 | X | F = A $\wedge$ B | AND |

0 | 1 | 0 | 1 | X | F = A $\vee$ B | OR |

0 | 1 | 1 | 0 | X | F = A$\oplus$B | XOR |

0 | 1 | 1 | 1 | X | F = A’ | Complement A |

1 | 0 | X | X | X | F = shr A | Shift right A into F |

1 | 1 | X | X | X | F = shl A | Shift Left A into F |

This table shows the 14 operations of the ALU. The first eight are arithmetic operations and are selected with S_{3}S_{2} = 00. The next four are logic operations and are selected with S_{3}S_{2} = 01. The final two operations are shift operations and are selected with S_{3}S_{2} = 10 and 11. The other three selection inputs do no influence the shift.

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