RRISC (Reduced Reduced Instruction Set Computer)

My primary goal for this project was to make a computer from "scratch" (simple logic gates and memory, no embedded microcontrollers/FPGAs/etc) to learn more about Computer Engineering as a "learn by doing" approach.

Since it's only made out of simple logic gates and hand-soldered, it needs to be simple in design (no microcode/pipelining) and in implementation (trying to aim for around 50 logic chips, none of which are more complex than a comparator).

This project includes the Assembler, LogiSim simulator/emulator, schematic, and PCB. This is the computer at every level of abstraction, and as such this is Open Source Hardware.

What is RISC? What does this computer do?

A RISC computer commonly has simple operations to have higher instructions per second. My computer has extremely simple operations to have extreme efficiency (ideally). My goal for this computer is for it to run at about 20MHz.

This is a General Purpose Von-Neumann-Architecture computer with 8-bit instructions. It has 64Kb of RAM and all 16 instructions follow a quick 3-stage IF/EX/WB cycle.

Instructions are broken up into two parts. Here's the entire Instruction Set Architecture:

ID	Opcode	Description	Argument Type
_	4 bits	_	4 bits
0x0	ADD	Add to R0	Register(1)
0x1	ADDI	Add to R0	Immediate(2)
0x2	SUB	Subtract from R0	Register(1)
0x3	SUBI	Subtract from R0	Immediate(2)
0x4	SKIP	Skip next instruction	Condition(3)
0x5	JUMP	Jump to MEM	Condition(3)
0x6	XOR	Xor	Register(1)
0x7	XORI	Xor	Immediate(2)
0x8	AND	And	Register(1)
0x9	ANDI	And	Immediate(2)
0xA	SET	Move ACC to Reg	Register(1)
0xB	RB	Read from memory to Reg	Register(1)
0xC	GET	Move Reg to ACC	Register(1)
0xD	WB	Write from reg to memory	Register(1)
0xE	OR
0xF	ORI

Register(1)

The last 4 bits of this argument point to one of the 16 registers.

ACC (AKA R0) is the accumulator register.

R1-RC General Purpose Registers.

RMH,RML point in memory (Register Memory High, Register Memory Low).

RCMP is used in conditional operations³

Immediate(2)

Condition(3)

Assembler

Given an assembly file (Ex: code.asm), it turns it into a hex file (Ex: code.asm.hex) that can be loaded either onto the real computer, or the simulated computer in LogiSim.

Simulation

This represents the computer's overall layout. It is both a logical diagram for the computer and a convenient simulator for programs.

Open this with Logisim Evolution. I use "logisim-evolution-3.4.1-all.jar". Upload the hex from the assembler into the RAM, start the controller, and watch it run!

Hardware

The strict hardware requirement of this computer is that everything except the memory-mapped-IO has to be made from 7400 logic.

This is made in KiCad, and contains the schematic (local layout of how things connect) and the PCB design (how to physically connect everything together).

The plan is to have the submodules used in LogiSim (registers, ALU, RAM, etc) be a daughter board for the main PCB.

The plan is to design/order/build the PCBs in this order:

1. Program Counter
   • Logic for jump/skip decisions
2. Registers
   • Might use either a literal register file (select MUX, asynchronous D-latch, 8x 16:1 demux) or a 74xx register file
3. ALU
4. RAM/ROM/Programmer
5. Motherboard/Controller
6. Memory-Mapped IO
   • Memory Mapped UART
   • Memory Mapped 7-segment Display

This way, I can test them all separately, and revisions can be made more easily.