Yannik Nelson

Overview

When I become interested in a topic I often go majorly overboard with my enthusiasm and background knowledge in an attempt to be as good at it as I possibly can, this is what happened with programming and this project. I had the thought that maybe if I understood more about how the actual bare metal of a computer worked I'd be able to write better code, as it turns out this holds some truth, but not as much as I'd hoped.

This started with taking the nand2tetris course online and working throught it all, but at the end of it I decided I wanted more practical experience with computer architecture, that's where this project and Ben Eater came in. Ben Eater is the youtuber who designed this CPU and produced a series of videos explaining the logic (which I already understood thanks to nand2tetris) and the electrical sides to the CPU (which I was sorely lacking knowledge of). And so I decided to follow along, as I watched I made a list of all the parts needed for each 'module' and took notes of how it all worked making circuit diagrams where necessary and sourced and bought all of the chips etc... . It turned out a couple months after I'd finished the project Ben Eater started selling kits with all the parts needed to make this, oh well.

Clock

The clock Consists of 2 modes, an auto run mode whose frequency can be changed using a potentiometer and a step mode where each clock cycle is a press of a button. You can switch between these two modes using a toggle switch, as a note both the button and toggle switch are de-bounced using 555-timers to avoid accidental cycles beign performed. The chosen clock signal then goes through an edge detector before being sent to the other modules. The clock lines are shown using white wire.

Instruction Cycles

The instuctions for this CPU use a maximum of 6 clock cycles, this is achieved using a 4-bit counter that indicated which stage of an instruction the CPU is currently executing and is incremented every (inverted) clock cycle (shown by the blue LED). We pass the first three bits (shown by the red LEDS) to the decode ROM and to a demultiplexer which defaults high and has the input pulled low so that all except the selected output will be high (shown by the green LEDS) and the 7th output is connected to the rest of the counter so that once the final instruction cycle is used the counter is reset.

Instruction Register

The instuction register pulls its value from the bus and can be broken down into two parts, the instruction bits (shown with blue LEDS) and the address bits (shown with yellow LEDS). The 4 instruction bits determine which instruction is to be executed, resulting in there being a maximum of 16 different instructons, and the 4 address bits can be ouput back onto the bus by instructions that require this, for exmple for loading a literal into the A register or for accessing memory.

Control Lines

The value of the instruction bits + the three bits from the counter are then used as addresses in the decode ROM which will then output the desired control lines. The control lines are shown using yellow wires and the values of the lines are displayed using a series blue LEDS.

Note: Some of the actual values required for the control lines are inverted, these are not shown in the series of blue LEDS, instead if a line is active the line from the decode ROM is set to high and th put through its corresponding LED and then put through an inverter before going to its corresponding module.

Memory Modes

The RAM for this Computer is included as it's own module as well with two modes, Programming Mode(red LED) and Execution Mode(Green LED), which can be selected using a toggle switch.

Execution Mode works excatly as you'd expect, the memory address (shown by the yellow LEDs) is determined by a 4-bit register that is set either by the program counter or a literal from an instruction and the value at that address (shown by the red LEDs) is out put when the memory output control line is active.

Programming Mode causes the address to be determined by a 4-position DIP switch allowing the programmer to access memory by hand. Then an 8-position DIP switch is used to determine the desired value at a memory address and that value is written with the push of a button (not automatically), note that writing memory using this button is disabled in Execution Mode.

ALU

The ALU has two registers an A register (on top) and B register (bellow) and always automatically sums them but only outputs to the result to the bus when the ALU output control line is active.

The ALU also has 'extra' arichecture for zero and carry bits. The zero bit is found by nanding the bits of the alu output in pairs and then anding those results until a single bit is produced. This can be seen at the bottom right of the module and the blue LED there shows the result of that process. The carry flag is simply the top carry from the adder circuit and is shown by the blue LED just bellow the A regsiter LED display. These are then connected to extra circuitry used to determin wheter a conditional jump will occur (the green LEDS, left is the carry bit, right is the zero bit).

Display

The display always automatically displays whatever is in its register. Multiplexing is used to display values greater than 1 digit, this required the display to have its own clock.

To determine which display was to be used for each 'display-clock' cycle a similar method to the instruction cycles was used, except with a two bit coutner. The output from the multiplexer connected to the enable pins for each of the displays so for that each 'display-clock' cycle only one of the 7-segment displays is on.

A decode ROM is then used to determine which of the pins for the 7-segment display should be active in order to 'draw' a number. This ROM used the value in the display register along with the value of the 2-bit counter as addresses. An extra address bit was available after this which was used as a 2's complement bit as seen to the left.