A Forth for the R216 computer

What?

This project aims to implement the Forth programming language on the R216 computer by LBPHacker. The computer is implemented completely in the video game Powder Toy, so this Forth will be one of the few systems out there that target a fictional computer. Since Forth is extremely easy to port, it only takes a couple of primitive routines to get started writing the whole thing again.

This is the first system I know of in Powder Toy that allows interactive programming within Powder Toy itself (i.e. without needing an assembler), and the first non-assembly language to be ported to a Powder Toy computer.

The default program forth.asm reads up to 128 characters of input, then starts the interpreter up, which performs the standard Forth loops until a NUL byte is read. The interpreter echos back what it's reading for debugging purposes, according to the following table.

Here are some example programs that you can type at the REPL.

." hello, world!"
\ => hello, world ok

: stars 0 do star loop ;
10 stars
\ => ********** ok

: factorial dup 0= if drop 1 else dup 1- recurse * then ;
5 factorial .
\ => 120 ok

Here is a list of words defined in the system.

s0 sp@ depth .s decimal hex latest base xor and find allot here c@ c!
@ !  , +!  -!  >r r> /mod nip mod div space create immed ?immed hide [
] : ; constant value to .  d.  u.  ?dup < > = 0<> 0= <> dup 2dup drop
2drop swap over rot + - * um* 1- 1+ 2- 2+ number max min word /mod
origin page emit >dfa >cfa execute recurse puts lit tell s" ."  halt
(') ' if else then begin until again while j i repeat do loop +loop
star

Features

• The first Forth system for a TPT computer; 75 words and counting
  • An extensible, lightweight, interactive, introspective language.
• Harnesses the 16-bit power of the R216 computer, with features such as
  • 16-bit multiplication (16 * 16 -> 16 bit or 16 * 16 -> 32 bit)
  • Terminal output/input (can be generalized to other peripherals)
• Adapts to memory layout, can be flashed on the 4K or 8K variant of the R216

Quirks

• Only the first three characters and length are checked when traversing the dictionary (this was actually the case in older Forth systems such as the one used in Starting Forth)
• Runs slowly: increase the frame rate by running tpt.setfpscap(2) unless you're prepared to wait minutes for simple programs

Why?

Fictional computers are great. One can learn a lot from both implementing and playing around with them. But when was the last time you saw a Forth REPL in fictional computer? What about a Forth REPL in a fictional computer implemented in a powder simulation game? Well, now you have! From the relative success of my previous project, a Forth-based operating system for TI-84+ calculators, I thought it would be fun to write another one. From initially knowing nothing about the architecture, Within two days I got a REPL working with compile/interpret states, and now it's just a matter of porting all the other Forth words we know and love.

Building and running R216 Forth

Ensure you have a recent version of Powder Toy (tested on version 94.1).

Clone the assembler and this repository using Git. Open the file r216-forth.cps in Powder Toy, you'll see a computer, screen and keyboard. Open the Lua console by pressing ~ and type the following:

r2asm = loadfile("<path to repo>/r2asm.lua")

Where <path to assembler repo> is the absolute path to the cloned assembler. Then, type in

a = "<path to this repo>/forth.asm"

To define a variable a to shorten lines (Powder Toy truncates long commands). Finally, you can assemble forth.asm by running:

r2asm(a, 0xDEAD, "<path to logfile>")

Where <path to logfile> is the path to a log file which you can open to see if the assembling worked. Close the console by pressing ~, and click on the sign to begin using R216 Forth!

Future plans

• Add ability to read numbers
  • Relies on multiplication routine
• Add string words .", s"
• Add exceptions, catch, throw
• Add ?do, fix leave
  • Requires rewriting implementation of do, loop and +loop