MilliForth-6502, A Forth For The 6502 CPU

"A Forth in 328 bytes — the smallest real programming language ever, as of yet."

The milliForth¹ is a review of sectorForth², and smaller than sector Lisp³

The miniForth⁴ is another Forth to use in a boot sector of less than 512 bytes.

we are in hackaday

we are alive

( this repo needs a review )

Yes, bytes. But those are for a x86 16-bit CPU. How minimal could be it for a classic 6502 8-bit CPU ?

Two essentially different CPUs, a 16-bit x86 based on complex registers and opcodes, and a 8-bit 6502 using page zero as common registers and page one as hardware stack.

The FIG-Forth (1980) for 6502 uses Indirect Thread Code (ITC) as inner interpreter.

The miniForth, sectorforth and milliForth use Direct Thread Code (DTC)

This Forth for 6502, will be was done using two models:

(later we will compare both, but DTC will win for less size)

This project is also used to verify standart Direct Thread Code against variations of Minimum Thread Code.

Minimal Thread Code is an alternative model for inner interpreter, where the dictionary is organized with the primitives words grouped together before the compound words, defining a "tipping point", from where could decide if the reference of a word will be to executed or be pushed into return stack.

PS. MTC is a reduced form of TachyonForth inner interpreter.

Focus in size not performance.

The compilation wqas done with ca65 V2.19 - Git 7979f8a41.

The emulation of 6502 CPU was done with run6502

The way at 6502 is use a page zero and lots of lda/sta bytes.

Both DTC and MTC runs with my_hello_world.FORTH, with these numbers.

(*) Thanks to wise changes from peterferrie !

• as Forth-1994⁵: FALSE is $0000 and TRUE is $FFFF ;

• all tib (80 bytes), pic (16 cells), data (36 cells) and return (36 cells) stacks are in page $200 ;

• tib and pic grows forward, stacks grows backwards ;

• only immediate flag used as $80, no more flags ;

• no line editor, no backspace, no cancel line, no low ASCII verify ;

• no stacks overflow or underflow checks ;

• 6502 is a byte processor, no need 'pad' at end of even names ;
• hardware stack (page $100) not used as forth stack, free for use ;
• uses 32 bytes of page zero ;
• no multiuser, no multitask, no faster ;
• only update latest at end of word definition ;
• redefine a word does not change previous uses ;
• stacks moves like the hardware stack ;
• words must be between spaces, before and after ever ;
• better use 7-bit ASCII characters ;
• approuch as ANSI X3.215-1994⁵

Look up at Notes⁶ for more.

24/08/2024 merge of dtc and mtc code into a flagged sector-6502.s file

16/08/2024: both models DTC and MTC, works with my_hello_world.FORTH;

11/08/2024: return to direct thread code;

11/06/2024: adapt for minimal thread indirect code;

24/01/2024: write using standart direct thread code;

14/11/2023 code for 6502 sized to 624 bytes, no ascii-7, no key, no emit, no 2/, many errors

This version includes:

" When heap moves forward, move the stack backward "

Push is 'store and decrease'. Pull is 'increas and fetch'.

A common memory model organization of Forth:

then backward stacks allow to use the slack space ...

This 6502 Forth memory model is blocked in pages of 256 bytes:

at page2, without 'rush over'

"SectorFORTH was an extensive guide throughout the process of implementing milliFORTH, and milliFORTH's design actually converged on sectorFORTH unintentionally in a few areas. That said, the language implemented is intentionally very similar, being the 'minimal FORTH'."

For Forth language primer see Starting Forth

For Forth from inside howto see JonasForth

** 16/08/2024 DTC and MTC models operational, using lib6502 for emulation and tests **

A crude small script for compile with ca65 is included.

the size is take from main: to ends: using the sector-6502.lbl file

the originals files are edited for lines with less than 80 bytes

the bf.FORTH and hello_world.FORTH are from original milliForth¹

the my_hello_world.FORTH is adapted for miiliforth-6502

1. The original milliForth: https://github.com/fuzzballcat/milliForth ↩ ↩²

2. The inspirational sectorForth: https://github.com/cesarblum/sectorforth/ ↩

3. Mind-blowing sectorLISP: https://justine.lol/sectorlisp2/, https://github.com/jart/sectorlisp ↩

4. The miniforth: https://github.com/meithecatte/miniforth ↩

5. Forth standart ANSI X3.215-1994: http://www.forth.org/svfig/Win32Forth/DPANS94.txt ↩ ↩²

6. Notes and Times: https://github.com/agsb/milliForth-6502/blob/acc2f8ddc6aafb2dec6346e90f5372ee16b38c8c/docs/Notes.md ↩