RISC-iest Tape Computer

General purpose computer using Tape as system memory and an ATMega as a head (only using its processing capabilities and registers, no SRAM used). Fully Implemented in AVR assembly.

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RISC-iest Tape Computer

A Turing Complete Tape & ATMega general purpose computer. The tape is a physical strip of paper with binary nibbles written on it using our own (extra-reduced) instruction set, detailed at bottom. The ATMega executes interpeted opcodes & operands using its registers and the motors to move the tape.

Key Points

  • System memory is all on Tape - program & data stored as sets of nibbles.
  • Only used ATMega as Head, using only processing power & registers.
    (no SRAM)
  • Implemented fully in AVR assembly

High-Level Overview

[ refer to High-Level Diagram ]

The RISC-iest Tape Computer uses a tape where LDRs capture blocked light as the stepper motors rolls the tape backwards and forwards through the head.

DEMO

A caesar cipher implemented using general purpose (extra-reduced) instruction set.

Abstract

  1. Fetch next data item
    a. if character, decrypt and display to 7-segment display
    b. if number != 0, set as cipher key
    c. if number == 0, halt program

Example

Input Data: 12 'V' 'S' 'Z' 'Z' 'C' 0
  • cipher key = 12
  • cipher-text data = "VSZZC"
  • halt key = 0
Output on 7-segment Display: "HELLO"

Tape Structure

The high level layout of the tape is as follows. It contains an initial section of the tape which is the program followed by a section for the data.

[ refer to Tape structure diagram ]

The information on the tape itself is broken down into 1 byte per cell. For 1 instruction using our instruction set, it takes 2 bytes(2 cells). The first 4 bits represent the opcode(instruction) and there are 2 operands which are 6 bits each; together, the opcode and operands make the 2 bytes.

[ refer to instruction & nibble, and Tape nibble representation diagrams ]

Below is a picture specifically looking at 1 cell of the tape and how it's broken down.

[ refer to Hardware Pic ]

Innovations

  • 4x Denser - Initially we planned to fit 1 bit per cell, using Lego, we found a reliable way to densely pack each cell with 8 bits thus allowing more information to be stored in smaller tape length.
  • General Purpose structuring - Setting the opcode to 4 bits and each operand to 6 bits allowed us to have the full instruction set with upto 16 instructions as well as ample range for the operands. This was a key innovation as it allowed us to have a more general purpose instruction set.
  • Reliable Bit-marking - Initially we planned to punch holes in the tape to allow light to pass through to signify a bit. Although, we couldn't find a reliable method to do this. Instead of letting the light pass through, we decided to block the light with a black marker to signify a bit.

Limitations

  • The tape is read only and cannot be written to. This means that the program cannot be changed once it has been loaded onto the tape. We are limited by the number of available registers on the ATMega to store and manipulate data.
  • The program & data have to be manually translated into binary and drawn onto the tape, which is a time consuming process and suceptible to human-error. <!-- - Currently, there is potential for expanding the instruction set with 2 more instructions. In the future, if we wanted to implement more instructions, we would have to completely restructure the bit allocation for the opcodes and operands. -->

FWKB-I (Instruction set)

The following is a table of our instruction set, and the binary representation of each instruction. It's composed of 14 instructions and is Turing complete.

Instruction 4-bit Binary Description
HALT 0000 Halt the program
LOAD 0001 Load the value into the specified register
ADD 0010 Add the values of the 2 specified register
ADDK 0011 Add constant k to the value of the specified register
SUB 0100 Subtract the values of the 2 specified register
SUBK 0101 Subtract constant k from the value of the specified register
COMP 0110 Compare constant k to the specified register
iLESS 0111 If the specified register is less than k, jump to the next instruction
iEQIV 1000 If the specified register is equal to k, jump to the next instruction
iGEQ 1001 If the specified register is greater than or equal to k, jump to the next instruction
JUMP 1010 Jump to the specified location
COPY 1011 Copy the value from one register to another
LOADK 1100 Load constant k into the specified register
DISP 1101 Display the value of the specified register on the 7-segment display

Files

  1. main.asm The assembly code for running the paper machine

  2. caesar.asm The assembly code for the program on the tape

  3. nibbles.bin is the binary code from tape broken down into nibbles (Binary)

  4. tape.bin is the binary code for the physical tape which represents the program and the data (Binary)

  5. abstract.py The initial workspace for conceptualising the program (Python)

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