Arithmetic logic unit

2026-03-14T19:44:38Z

FoxFireFive:

{{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.]]An Arithmetic Logic Unit (ALU) is a circuit that performs basic arithmetic and logic operations, often 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 could perform are 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 [[Full Adder|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:

{| class="wikitable"
! 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
|}
If your ALU only has arithmatic functions, it would be an Arithmatic Unit (AU). If your ALU only has logic functions, it would be a Logic Unit (LU).
{{Todo|Improve this section, without ai.}}

== Multiplexer based ALU's ==
[[File:Muxbased alu.jpg|thumb|266x266px|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:
{| class="wikitable"
|+
!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.}}

2026-03-07T08:50:22Z

FoxFireFive:

2026-01-12T08:25:30Z

FoxFireFive: Created page for full adder

{{Stub}}

A full adder is a simple circuit that adds two bits together along with a carry bit.
[[File:FullAdderExampleFixed.png|alt=A full adder circuit built in logic world.|thumb|Example of a Full Adder.]]
They have three inputs: A, B, Carry-In (CI), and two outputs: Sum (S) and Carry-Out (CO).

{{Truth table|0 0 0 0 0|1 0 0 1 0|0 1 0 1 0|1 1 0 0 1|0 0 1 1 0|1 0 1 0 1|0 1 1 0 1|1 1 1 1 1|caption=Full Adder Truth Table|inputs=A, B, CI|outputs=S, CO}}

File:FullAdderExampleFixed.png

2026-01-12T08:15:59Z

FoxFireFive:

Example of a full adder circuit.

User:FoxFireFive

2026-01-11T09:43:36Z

FoxFireFive:

User:FoxFireFive

2026-01-11T09:43:00Z

FoxFireFive:

2026-01-03T09:19:52Z

FoxFireFive: I found out about the caution block

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[[File:MaxOnMyLap.jpg|thumb|look at this beautiful creature]]
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<syntaxhighlight lang="cpp" line="1">
#include <stdio.h>

int main(int argc, char *argv[]){
printf("GTA 6 will come out before logic world is finished\n");
return 0;
}
</syntaxhighlight>
<references />