# Build An 8-Bit Computer by Ben Eater (Part 2) - Registers and Arithmetic Logic Units (ALU)

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## Build An 8-Bit Computer by Ben Eater - Registers and Arithmetic Logic Units (ALU) (Kit 2 of 4)

Registers: Most CPUs have several registers which store small amounts of data the CPU is processing. In our simple breadboard CPU, we'll build three 8-bit registers: A, B, and IR. The A and B registers are general-purpose registers. IR (the instruction register) works similarly, but we'll only use it for storing the current instruction that is being executed. Register build videos below.

Arithmetic Logic Unit (ALU): The arithmetic logic unit (ALU) part of a CPU is usually capable of performing various arithmetic, bitwise and comparison operations on binary numbers. In our simple breadboard CPU, the ALU is only able to add and subtract. It is connected to the A and B registers, and it outputs either the sum of A+B or the difference of A-B. ALU build videos below.

### Theory

Before diving into building the ALU, check out these two videos (below). The first shows how we can use hardware to add numbers in binary. The second video explains how negative numbers work. Between the two videos, you'll know how to add negative numbers which means you'll know how to subtract!

### Kit Includes:

 Part No. Qty. Description 151116 10 Capacitor Ceramic Disc 0.1µF 25V ±20% 2153705 1 22 AWG 6-Color Solid Tinned-Copper Hook-Up Wire 25 Foot 2234071 5 LED Blue Diffused 5mm T-1 3/4 470nm 350mcd 2283767 4 830 Point Solderless Plug-in Breadboard with 4 Power Rails 21-26AWG ABS 333973 30 LED Red Diffused 5mm (T-1 3/4) 643nm 75mcd 36VA 46922 6 IC 74LS173 4-BIT D-TYPE REGISTER 3-STATE OUTPUT 47212 4 IC 74LS245 Tri-State Octal Bus Transceiver 47423 2 IC 74LS283 4-BIT BINARY FULL ADD FAST CARRY 48098 2 IC 74LS86 QUAD 2-INPUT EXCLUSIVE OR GATE 690700 40 Carbon Film Resistor 220 Ohm 1/4 Watt 5% 697549 10 LED Yellow Diffused 5mm (T-1 3/4) 588nm 29mcd 50VA

### Register Videos

8-bit Register (part 1) - Bus Architecture and How Register Transfers Work

8-bit Register (part 2) - Tri-State Logic: Connecting Multiple Outputs Together

8-bit Register (part 3) - Designing and Building a 1-bit Register

8-bit Register (part 4) - Building and 8-bit Register

8-bit Register (part 5) - Testing Our Computer's Registers

### ALU Introduction Videos

Twos Complement: Negative Numbers in Binary

### Arithmetic Logic Unit Design & Construction Videos

ALU Design

Building the ALU

Troubleshooting the ALU

Testing the Computer's ALU

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Design LevelIntermediate Kit

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