Ben Eater's Homebuilt Computer

[Bob Kuczewski]

Registers

In this video Ben tests one of his "normal" (A or B) registers. Ben also introduces his instruction register and demonstrates how each of the three registers (A, B, and Instruction) can communicate with each other through the bus.

https://www.youtube.com/watch?v=9WE3Obdjtv0

[Bob Kuczewski]

Arithmetic and Logic Unit (ALU)

In this video Ben demonstrates a binary adding circuit made from simple logic gates. The circuit can add two 4-bit numbers to produce a 5-bit result.

https://www.youtube.com/watch?v=wvJc9CZcvBc

[Bob Kuczewski]

Arithmetic and Logic Unit (ALU)

In this video, Ben describes how negative numbers can be represented to support both addition and subtraction. In particular, Ben describes the "2's complement" system used in most modern computers. With 2's complement, a binary number is negated by flipping all of its bits and adding one. This will be used in the upcoming ALU design.

https://www.youtube.com/watch?v=4qH4unVtJkE

[Bob Kuczewski]

Arithmetic and Logic Unit (ALU)

In this video Ben begins to describe the Arithmetic and Logic Unit (ALU). The ALU will use two 74LS283 chips which are each 4-bit adders chips. The two 74LS283 chips will be combined to do 8 bit addition and subtraction. The subtraction is supported by optionally inverting the B register input with eight Exclusive Or (XOR) gates and then adding one through the carry input to the low order 4-bit adder. This works because negative binary numbers are inverted versions of positive binary numbers with 1 added (invert and add 1). The carry input to low order 74LS283 chip provides the "carry in" input that does the "add 1" portion of the 2's complement while it does the adding.

https://www.youtube.com/watch?v=mOVOS9AjgFs

[Bob Kuczewski]

Arithmetic and Logic Unit (ALU)

In this video Ben builds the ALU designed earlier. He uses the same Tri-State 74LS245 used for the registers. Ben also wires the ALU up to the two existing registers A and B.

https://www.youtube.com/watch?v=S-3fXU3FZQc

[Bob Kuczewski]

Arithmetic and Logic Unit (ALU)

In this video Ben diagnoses the ALU to find 2 mis-wiring errors. This video demonstrates the logical process needed to isolate and diagnose problems in a complicated circuit.

https://www.youtube.com/watch?v=U7Q8-2YZTUU

[Bob Kuczewski]

Arithmetic and Logic Unit (ALU)

In this video Ben demonstrates the operation of the ALU. The ALU is shown to both add and subtract.

https://www.youtube.com/watch?v=4nCMDvnR2Fg

[Bob Kuczewski]

Random Access Memory (RAM)

In this video Ben describes how a full 16 byte memory could be constructed from logic gates, flip flops, or other technologies. For this design, Ben chose to use two 74LS189 chips which each provide sixteen 4-bit memory registers. Ben's design will combine them to produce sixteen 8-bit registers which will be the memory for this simple computer.

https://www.youtube.com/watch?v=FnxPIZR1ybs

[Bob Kuczewski]

Random Access Memory (RAM)

In this video Ben constructs the actual RAM module designed earlier. As with the other modules, the RAM module is isolated from the bus with a 74LS245 Tri-State buffer.

https://www.youtube.com/watch?v=uYXwCBo40iA

[Bob Kuczewski]

Random Access Memory (RAM)

In this video Ben builds and demonstrates the circuit to address memory via DIP switches during programming mode and to separately address the memory in running mode.

https://www.youtube.com/watch?v=KNve2LCcSRc