Processor and its components
A processor consists of several components including a control unit, an arithmetic logic unit, registers, and some sort of CPU interconnection. All these components help organize and allow data processing by the computer
What is an ALU?
The arithmetic logic unit is the main functional component of a processor that performs all the data processing. It can perform arithmetic operations as well as logical operations.
We'll try to build a very basic ALU that can perform data processing on 4 bits of data, using Logisim.
Design
We'll design an ALU that can perform four types of operations on two inputs each consisting of 4 bits. We can use a 2-bit op-code to represent the operation we want to perform.
Operations
- 00 - Logical AND
- 01 - Logical OR
- 10 - Addition
- 11 - Subtraction
Components
We'll divide our ALU into following smaller blocks.
- AND block
- OR block
- Arithmetic block
- 3-1 Multiplexer
Flags
We'll also have two output flags
- Bit overflow - active when addition yields a result that can't be represented by a 4-bit output
- Equal flag - active when two numbers (inputs) are equal
Arithmetic Unit
We'll start from very basic circuits and then create an arithmetic unit that can perform addition and subtraction.
Full Adder
A full adder can add 2 single-bit numbers along with a carry and generate the sum and a carry out which can be passed to the next adder
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4 bit adder
Using full adder we can build a circuit which can add up to four bits
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Arithmetic Unit
An adder can be used to perform subtraction using the technique called 2's complement.
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Logical Unit
Logical units are fairly simple and they perform a logical operation on a single bit from each input.
OR Block
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AND Block
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Flags
Bit Overflow Flag
The flag is only active when the op-code is 10 (ADD) and the output can't be represented by a 4-bit number.
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Equal Flag
The flag is only active when the op-code is 11 (SUB) and both inputs are equal
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Multiplexer
We can generate a single output from multiple outputs from our different blocks based on our op-code. We can use a 3-1 multiplexer
2-1 MUX
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3-1 MUX
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ALU
Now, let's plug everything together
Conclusion
Most modern ALUs can perform way many operations on high number of bits. We can upgrade our ALU to perform other operations as well. This is just a very basic representation of how we can create our circuit that may perform very basic operations.
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