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Fixed the article by mostly re-writting it. PLEASE STOP USING AI FOR THE LOVE OF ALL THAT'S HOLY. IF YOU CAN'T EDIT WITHOUT AI, DON'T EDIT!!
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[[File:ALU.gif|thumb|A symbolic representation of an ALU. The arrows represent inputs and outputs.]]
{{Stub}}[[File:ALU.gif|thumb|A symbolic representation of an ALU. The arrows represent inputs and outputs.]]
[[File:Alu-8-bit.png|thumb|An 8-bit ALU with inputs A, B, !A, !B, Carry, XOR, AND, OR.]]
[[File:Alu-8-bit.png|thumb|An 8-bit ALU with inputs A, B, !A, !B, Carry, XOR, AND, OR.]]An Arithmetic Logic Unit (ALU) is a circuit that performs basic arithmetic and logic operations, usually used in CPUs. They usually have "flags" to keep track of different things during execution, like if the result overflowed, or the result was zero. Flags are usually implemented with flip-flops. Operations that an ALU can perform include arithmetic operations such as addition and subtraction, as well as bitwise operations like XOR, AND, and OR. 


{{ALU}}
The different kinds of flags an ALU could keep track of are:
:It may be called the "brains" of the [[CPU|central processor]]. 
* If  "A" is equal to "B"
An ALU also has "flags" to keep track of different situations during computation. 
* If "A" is lesser than "B"
Operations that an ALU can perform include arithmetic operations such as addition and subtraction, as well as ''bitwise'' operations like XOR, NOR, AND, and OR.
* If "A" is greater than "B"
* If the carry bit was set during execution.


The status flags it keeps track of are, for example:
The carry bit is usually the "Carry-out" part of an [[Full Adder|adder]].
* ''A = B''
* ''A > B''
* ''A < B''
* Carry
* Others


ALUs can have very different designs with varying specifications, such as bit size, possible operations, and speed. In principle, you don't need to implement every possible operation - a few operations are sufficient to make the system Turing complete - although additional operations make it more convenient and often faster.  
{{Todo|Improve this section, also add better images.}}


== Example ==
== ALU Designs and operations ==
Below is an example of possible operations in an ALU:
ALUs can have many different designs with varying specifications, such as bit width, operations, size, and speed. Below are a few operations an ALU could have:


{| class="wikitable"
{| class="wikitable"
! Name
! Operation
! Opcode
! Description
! Description
|-
|-
| Add
| Add
| 000
| Adds A and B
| Adds A and B
|-
|-
| Sub
| Sub
| 001
| Subtracts B from A
| Subtracts B from A
|-
|-
| AND
| AND
| 010
| Outputs true if both A and B are true, otherwise false
| Outputs true if both A and B are true, otherwise false
|-
|-
| NAND
| NAND
| 011
| Inverse of AND. If A and B are true, output is false
| Inverse of AND. If A and B are true, output is false
|-
|-
| OR
| OR
| 100
| Outputs true if either A or B is true
| Outputs true if either A or B is true
|-
|-
| NOR
| NOR
| 101
| Inverse of OR. If either A or B is true, output is false
| Inverse of OR. If either A or B is true, output is false
|-
|-
| XOR
| XOR
| 110
| Outputs true if exactly one of A or B is true
| Outputs true if exactly one of A or B is true
|}
|}
== ALU Designs ==
{{Todo|Improve this section, without ai.
Their are two main alu Designs:
man I am so tired its 6:18am and i just want to go to bed.}}
 
* Mux Based ALU's
* Control Line Based ALU's
=== Mux Based ALU's ===
[[File:Muxbased alu.jpg|thumb|266x266px|Simple Mux Based ALU]]
Mux based ALU's uses a [[Multiplexer]] to select outputs from gates or operations
 
Pros:


* They are simple to make
== Multiplexer based ALU's ==
* They are easy to understand
[[File:Muxbased alu.jpg|thumb|266x266px|Simple multiplexer based ALU]]
Multiplexer based ALU's use a [[Multiplexer]] to select outputs from their different operations.


Cons:
They are fairly easy to make, and usually easy to understand, but can take up a lot of space, and are limited in operations.


* They take up a lot of space
{{Todo|Explain mux ALUs more. WITHOUT THE USE OF AI.}}
* They are limited in operations


=== Control Line Based ALU's ===
== Control line based ALU's ==
Unlike Mux Based ALU's, Control Line Based ALU's don't have a mux instead they have inputs like:
Unlike Mux Based ALU's, Control Line Based ALU's don't have a multiplexer, instead they have inputs like:


* Carry
* Carry
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* NOR
* NOR


They can do lots of operations. They don't have a sub mode input, instead you can turn on invert B (!B) and carry to preform subtraction.
They can do lots of operations. They don't have a subtract mode input, instead you can turn on "Invert B" (!B) and carry to perform subtraction.


Note they don't have opcode input like mux based alus so you have to make a ROM that turns on specific pins like:
Note that they don't have an opcode input like multiplexer based ALUs, so you have to make a ROM that turns on specific pins like:
{| class="wikitable"
{| class="wikitable"
|+
|+
!Name
!Operation
!Opcode
!Pins
!Pins
!Description
!Description
|-
|-
|Add
|Add
|0000
|None
|None
|Adds A and B
|Adds A and B
|-
|-
|Sub
|Sub
|0001
|Carry, !B
|Carry, !B
|Subtracts B from A
|Subtracts B from A
|-
|-
|AND
|AND
|0010
|AND
|AND
|Outputs true if both A and B are true, otherwise false
|Outputs true if both A and B are true, otherwise false
|-
|-
|NAND
|NAND
|0011
|!A, !B, AND
|!A, !B, AND
|Inverse of AND. If A and B are true, output is false
|Inverse of AND. If A and B are true, output is false
|-
|-
|OR
|OR
|0100
|OR
|OR
|Outputs true if either A or B is true
|Outputs true if either A or B is true
|-
|-
|NOR
|NOR
|0101
|!A, !B, OR
|!A, !B, OR
|Inverse of OR. If either A or B is true, output is false
|Inverse of OR. If either A or B is true, output is false
|-
|-
|XOR
|XOR
|0110
|XOR
|XOR
|Outputs true if exactly one of A or B is true
|Outputs true if exactly one of A or B is true
|-
|-
|XNOR
|XNOR
|0111
|!A, !B, XOR
|!A, !B, XOR
|Inverse of XOR Outputs true if exactly one of A or B is false
|Inverse of XOR Outputs true if exactly one of A or B is false
|-
|-
|INC
|INC
|1000
|B = 1
|B = 1
|Increments A by 1
|Increments A by 1
|-
|-
|DEC
|DEC
|1001
|Carry, !B, B = 1
|Carry, !B, B = 1
|Decrements A by 1
|Decrements A by 1
|}
|}
Pros:
They allow for complex operations, and are usually small, but can be complex and difficult to build.{{Todo|Improve this section, WITHOUT AI.}}
 
* They allow for complex operations.
* They are small
 
Cons:
 
* They are complex

Revision as of 10:21, 14 March 2026

This article is a stub. Please help expand it by adding content.
A symbolic representation of an ALU. The arrows represent inputs and outputs.
An 8-bit ALU with inputs A, B, !A, !B, Carry, XOR, AND, OR.

An Arithmetic Logic Unit (ALU) is a circuit that performs basic arithmetic and logic operations, usually used in CPUs. They usually have "flags" to keep track of different things during execution, like if the result overflowed, or the result was zero. Flags are usually implemented with flip-flops. Operations that an ALU can perform include arithmetic operations such as addition and subtraction, as well as bitwise operations like XOR, AND, and OR.

The different kinds of flags an ALU could keep track of are:

  • If "A" is equal to "B"
  • If "A" is lesser than "B"
  • If "A" is greater than "B"
  • If the carry bit was set during execution.

The carry bit is usually the "Carry-out" part of an adder.


TODO: Improve this section, also add better images.

ALU Designs and operations

ALUs can have many different designs with varying specifications, such as bit width, operations, size, and speed. Below are a few operations an ALU could have:

Operation Description
Add Adds A and B
Sub Subtracts B from A
AND Outputs true if both A and B are true, otherwise false
NAND Inverse of AND. If A and B are true, output is false
OR Outputs true if either A or B is true
NOR Inverse of OR. If either A or B is true, output is false
XOR Outputs true if exactly one of A or B is true

TODO: Improve this section, without ai. man I am so tired its 6:18am and i just want to go to bed.

Multiplexer based ALU's

Simple multiplexer based ALU

Multiplexer based ALU's use a Multiplexer to select outputs from their different operations.

They are fairly easy to make, and usually easy to understand, but can take up a lot of space, and are limited in operations.


TODO: Explain mux ALUs more. WITHOUT THE USE OF AI.

Control line based ALU's

Unlike Mux Based ALU's, Control Line Based ALU's don't have a multiplexer, instead they have inputs like:

  • Carry
  • !A
  • !B
  • OR
  • XOR
  • AND
  • NOR

They can do lots of operations. They don't have a subtract mode input, instead you can turn on "Invert B" (!B) and carry to perform subtraction.

Note that they don't have an opcode input like multiplexer based ALUs, so you have to make a ROM that turns on specific pins like:

Operation Pins Description
Add None Adds A and B
Sub Carry, !B Subtracts B from A
AND AND Outputs true if both A and B are true, otherwise false
NAND !A, !B, AND Inverse of AND. If A and B are true, output is false
OR OR Outputs true if either A or B is true
NOR !A, !B, OR Inverse of OR. If either A or B is true, output is false
XOR XOR Outputs true if exactly one of A or B is true
XNOR !A, !B, XOR Inverse of XOR Outputs true if exactly one of A or B is false
INC B = 1 Increments A by 1
DEC Carry, !B, B = 1 Decrements A by 1

They allow for complex operations, and are usually small, but can be complex and difficult to build. TODO: Improve this section, WITHOUT AI.

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