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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

Resources & Downloads

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


Learn How Computers Add Numbers and Build a 4-bit Adder Circuit


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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