from-machine-to-code

Explore and program a tiny microprocessor starting from a single gate. An explorable explanation that shows how a CPU is built from AND, OR, NOT, and XOR gates. In the style of http://explorableexplanations.com.

Installation and deployment

Requires Node v6.x.

Build and deploy

• edit url paths in client/styles/base/_fonts.scss to include <post-name> in path
• npm run build
• copy client/index.html to s3 bucket at <root>/<post-name>/index.html
• copy dist folder to s3 bucket at <root>/<post-name>/dist
• copy assets folder to s3 bucket at <root>/<post-name>/assets

Outline

Meet the gates

• Gates are electrical
• Gates have inputs and outputs
• Inputs and outputs can be on or off, like a switch
• AND, OR, NOT

Gates can be combined

• If it's hot outside and it's not windy and it's not raining and we've got sunscreen, then let's go to the beach. We can use a few gates to build a circuit that'll help us decide whether to go to the beach.

Gates can remember things

• 1 bit register
• Feeding an output back to the input of a gate lets us build a circuit that remembers it's input.

ON / OFF switches can count

• relationship between ON / OFF and a bit
• With many bits we can count
• Visualize as abstract circle / bulbs that are on or off.
• Below is binary representation of 0 and 1s
• Below is decimal representation

Meet a new gate, the XOR gate

How to add 1-bit: adding and carrying

• adding and carrying on a table
• Add with XOR
• Carry with AND

Gates can make a circuit that adds numbers

• 4 bit adder

Two's complement lets us keep track of positive and negative numbers

• Our 4 bit adder still works for two's complements numbers

Gates can make a circuit that remembers many things

• concept of a register file
• one 2 bit input selects which cubby hole to use
• another 1 bit input is the number to remember
• another bit that controls reading / writing
• we can visualize this as individual gates
• we can also visualize this as a box "register file"

Talk about clocks and edge triggered registers?

Register files can have many cubbies and remember numbers

• 4 bit address, 4 bit data register file
• don't show as gates (probably too complicated)
• show as box with address and data and out

With an adder and a register file we can make a calculatr

• adder has register file at output which feeds back to one of the adders inputs
• start seeding intuition data paths and control paths
• start seeding intuition about need for a clock

... more lessons ...

• build up to super simple CPU that you can program with a simple assembly and see some kind of program output on a tiny low pixel count screen
• is there some out output (lights, moving a character around) that's simpler than a screen but complex enough that it makes sense to program a simple CPU with assembly?

Eventual tiny cpu built

• 16 bit data path?
• no distinction between register file, cache, memory (large memory addressable in 1 cycle)
• registers that feed two ALU inputs and register that captures output
• Load / store instructions that load from memory into one of two registers
• what should a fun out put be? Tiny display mapped into memory
• no interrupts, but registers that capture some simple inputs like mouse and keyboard