Example Programs for Calcutron-33

Erik Engheim

Aug 3, 2020·3 min read

This is a follow up story on my story about inventing a RISC CPU called Calcutron-33 which has a decimal based number system rather than a binary number system.

Obviously this has no practical application for actual implementation. Rather the intention here is to come up with a simple model of a CPU, which is easy to teach.

The idea here is to play with some example programs which can be useful for a beginner to implement. What I am interested in figuring out, is what a minimal set of features are to be able to do something interesting. Such as how many instructions do we need to support? How many registers do we need at minimum?

Simple Adder

Get two inputs, add them together and write them to output

loop:
    INP x1
    INP x2
    ADD x1, x2
    OUT x1
    BRA loop

Doubling Inputs

Repeatedly double inputs and write to output.

loop:
    INP x1
    ADD x1, x1
    OUT x1
    BRA loop

Reverse Inputs

Take two inputs and reverse their order

loop:
    INP x1
    INP x2
    OUT x2
    OUT x1
    BRA loop

Multiply by 8

Multiply each input by 8. This is the first step to understand how addition can be used for multiplication.

loop:
    INP x1
    ADD x1, x1
    ADD x1, x1
    ADD x1, x1
    OUT x1
    BRA loop

Multiply by 3

Sometimes you need multiple registers to achieve multipliation

loop:
    INP x2
    ADD x1, x2, x2
    ADD x1, x2
    OUT x1
    BRA loop

Zero Exterminator

Remove all the zeros. Early way to learn the use of branching

loop:
    INP x1
    BRZ x1, loop
    OUT x1
    BRA loop

Maximizer

Pick the largest of two numbers

loop:
    INP x1
    INP x2
    SUB x1, x1, x2
    BGT x1, first

second:
    OUT x2
    BRA loop
    
first:
    OUT x1
    BRA loop

Simple Multiplier

The most primitive form of general multiplier. It picks two numbers at a time, multiplies them and outputs result.

loop:
    INP x1
    INP x2
    CLR x3
    
multiply:
    ADD x3, x1
    DEC x2
    BGT x2, multiply
    OUT x3
    
    BRA loop

Fast Multiplier

The is program from original story, for how to multiple numbers.

Unlike the simple multiplier above we use a lot fewer iterations by using shifts. This speeds up the multiplication.

    INP x2  // first number to multiply. This will get added
    INP x3  // second number. Treated as counter
    CLR x1  // accumulator for result. Clear it out.
    
nextdigit:
    RSH x4, x3, 1  // Push right most digit of x3 into x4
    
multiply:
    ADD x1, x1, x2 // Add first input to accumulator     
    DEC x4         // Decrement counter for number of additions
    BGT x4, multiply  // Repeat while x4 > 0
    
    LSH x2, x2, 1     // Left shift. x2 made 10x larger
    BGT x3, nextdigit // check if all digits have been processed
    
    OUT x1

Implementation of Calcutron-33

This made up CPU instruction set architecture now has a Julia package, Calcutron33.jl offering an assembler, disassembler and simulator.

It is still in early stages, but several of these example programs should work.