Forth implementation for the TI-84+ calculator

Features

• A 16-bit Forth on a 8-bit chip
  • Contains ~225 words (and counting) for everything from memory management to drawing pixels, decompilation and even playing sounds over the I/O port.
• Support for writeback (persistent data across program runs). Use SIMG (save image) and LIMG (load image) to save the words you've defined in a session.
• Highly readable and customizable implementation, see forth.asm

Getting the interpreter

Download the latest binary from the Releases page or get the bleeding edge output from the GitHub Actions CI.

The Real Thing

• A TI-84+ calculator!
• TI Connect CE
• (Optional) A 2.5 mm to 3.5 mm audio cable to connect the I/O port with a speaker.

Flash forth.8xp to your calculator. Make sure there's enough space and that you have backed up your calculator! An easy way to back up RAM contents is by creating a group, refer to the manual on how to do this.

Why TI-84+?

This is a calculator that is more or less ubiquitous among high school and university students throughout the world. It's not going extinct anytime soon (expect perhaps to newer models such as the TI-84 CE). But let's face it. TI-BASIC is not a nice language; it's slow and suffers from lack of low-level bindings. There's no REPL. We want a language that gives the programmer the full power of the calculator—treating it as the computer it is. In fact people already do, by writing assembly programs, but assembly has its share of disadvantages.

Why Forth?

Assembly is painful to program in. Programs crash at the slightest hint of error. Development is a slow process, and you have to keep reinventing the wheel with each program.

Wouldn't it be great to have a programming language on the TI-84+ that's much faster than TI-BASIC but easier to understand and as low level as assembly? Forth is just that. (Read Starting FORTH for an excellent introduction to Forth). It's low level, it's simple, but also easy to type, especially when you're on a calculator with a non-QWERTY keyboard. It is a very powerful language, allowing you to do things like change the syntax of the language itself. IF, WHILE, CONSTANT etc. statements are all implemented in Forth! Think of it as a untyped C with a REPL and the power of Lisp macros.

It's also easy to implement incrementally through continuous testing. In fact, once the base REPL was implemented, most of the programming and testing happened on the calculator itself!

Building

Nix

nix build

Mac/Linux

• spasm-ng Z80 assembler
  • If you're on a Mac you will need to install openssl as a dependency, for instance on Homebrew:

brew install openssl
cd /usr/local/include
ln -s ../opt/openssl/include/openssl .

• Compile the assembler with make (check required packages for your system).

Copy forth.asm into the cloned folder. Then run:

./spasm forth.asm forth.8xp

Emulated

There are many emulators out there, one that doesn't require installation is jsTIfied. Read the website's details for more information. You'll need to obtain a ROM image as well, which I can't provide here, but a simple web search might.

Using the Interpreter

Run the program with Asm(prgmFORTH), hit 2nd then ALPHA to enter alpha lock mode, and now you can type the characters from A-Z. Here are a couple of things to keep in mind.

• Left and right arrows are bound to character delete and space insert respectively.
• Hitting CLEAR clears the current input line.
• Hitting ENTER sends it over to the interpreter.

If you want to see the keymap, find the label key_table in forth.asm. This table maps the keys received by KEY to the appropriate character.

Typing ASCII Characters

See the 2nd or ALPHA key combos (in blue on the calculator) for information on how to type the following characters: []{}"?:.

Character	Key Sequence
;	2nd .
!	2nd PRGM
@	2nd APPS
=	2nd MATH
'	2nd +
<	2nd X,T,Θ,n
>	2nd STAT
\	2nd ÷
_	2nd VARS

Exiting the Interpreter

Type BYE and hit ENTER.

Loading Forth Programs onto the calculator

function fmake() {
    hexdump -e '".db "16/1 "$%02x, " "\n"' "$1" | sed 's/$  ,/$00,/g' | sed 's/.$//' | sed 's/$0a/$20/g' > "${1%.*}.asm" && ./spasm "${1%.*}.asm" "${1%.*}.8xp" && rm "${1%.*}.asm"
}

Once you defined this function in your shell you can just type fmake hello.fs and transfer the compiled hello.8xp to the calculator. To load this file into the interpreter you have to run LOAD HELLO in the Forth REPL.

Example Programs

See programs/ for program samples, including practical ones.

Available Words

EXIT DUP + - AND OR XOR << >> INVERT DROP SWAP OVER ROT -ROT 2DROP
2DUP 2SWAP 1+ 1- 2+ 2- >R R> R@ 2>R 2R> RDROP 2RDROP LIT LITSTR S" .Q
TELL STRLEN STRCHR !  @ +!  -!  C!  C@ C@C!  CMOVE EXECUTE BASE PREC
STATE LATEST SP0 [ ] ?SE HERE DOCOL BUF BUFSZ WBUFP WBUF WBUFSZ RP0 H0
F_IMMED F_HIDDEN F_LENMASK SCR ABS PLOTSS ' , C, SP@ SP!  RP@ RP!
BRANCH 0BRANCH ?DUP = <> >= <= < > 0= KEY KEYC EMIT T.  ?  AKEY
TO_ASCII * /MOD SQRT FRAND F.  FREAD F* FSQUARE F= FDUP FDROP FSWAP F+
F/ FRCI F- FSQRT MD5 D/MOD UM* D+ M+ DS SPACE CR AT-XY PUTS PUTLN GETS
GETC UNGETC WORD ?IMMED IMMED >NFA >CFA STR= FIND WB USED SIMG LIMG
>DFA CREATE DOCOL_H : ; (DOES>) DOES> PAGE HIDDEN ?HIDDEN MOD / NEGATE
TRUE FALSE NOT LITERAL NIP TUCK ID.  HIDE IF THEN ELSE BEGIN UNTIL
AGAIN WHILE REPEAT CHAR (COMP) CONST ALLOT CELLS RECURSE VAR DO LOOP
+LOOP FORGET '0' '9' WITHIN NUM?  CFA> PICK U.  UWIDTH SPACES U.R U.
.  DEPTH .S HEX DEC SEE WORDS CASE OF ENDOF ENDCASE I J CSCR CBLK FBLK
RUN LOAD SMIT PLOT WR PN BYE STAR

Note that floating point routines are commented out by default to save on space.

Screenshots

Combine words in powerful, practical ways

Combine low-level memory words with drawing words and user input words to create an arrow-key scrollable screen for viewing RAM memory. See the 20 (or less) lines of code at programs/memview.fs.

TI-84+ inside

Load programs

Simple unfinished modal text editor with a scrollable screen.

Design Notes

Use of Macros

Judicious use of macros has greatly improved readability of the code. This was directly inspired by the jonesforth implementation (see Reading List).

Register Allocation

One notable feature of this Forth is the use of a register to keep track of the top element in the stack.

Z80 Register	Forth VM Register
DE	Instruction pointer (IP)
HL	Working register (W)
BC	Top of stack (TOS)
IX	Return stack pointer (RSP)
SP	Parameter stack pointer (PSP)

Reading List

Documentation can vary from very well-documented to resorting to having to read the source code of spasm-ng to figure out how #macro worked. See examples such as defcode and defword. I couldn't make defconst or defvar, however, but this was fixed by writing it out manually.

• General Z80 guide
• Moving Forth
• Learn TI-83 Plus Assembly In 28 Days
• KnightOS Kernel
• Starting FORTH
• Jonesforth

To be Implemented

• Ability to read/write programs
  • WB word to writeback 2048 400 (see Current Limitations) bytes of data starting from the address of SCRATCH.
  • Ability to "execute" strings (so that programs can be interpreted).
• User input
  • String reading routines
  • Number reading routines (possible with programs/number.fs)
• Output
  • Displaying strings
• Proper support for compile/interpret mode
• Assembler to convert Forth words into .dw data segments to be pasted into the program.
• Ability to switch to a "plot"
• REPL
  • Basic Read/Eval/Print/Loop
  • Allowing more than one word at a time input
  • Respect hidden flag to avoid infinite looping. (: makes the word hidden).
  • Reading numbers (support for 0-10 inclusive hardcoded, but not a general algorithm). See programs/number.fs
• Document Forth words (partially done)
• Add Z80 assembler in Forth (so ASM programs can be made!)
• Implement DOES>
• Implement SIMG (save image) and LIMG (load image) to save and load sessions.
• Add sound capabilities
• Add a way to put data on the screen as pixels (for export via screenshots).
• Add computer program to allow the user to select the words for a custom Forth system.

Current Limitations

• REPL prints out "ok" at the end of each word parsed, QUIT not implemented.
• Indirect threading means we cannot use scratch space in addreses higher than $C000 as if the program counter exceeds $C000 it crashes the OS.