itsy-linux

32-bit Linux port of Itsy Forth by John Metcalf — a minimal Forth used to bootstrap a larger system.

Original articles:

• Itsy Forth 1K Tiny Compiler
• Dictionary and Inner Interpreter
• Primitives
• Compiler
• Bit-twiddling notes

The original 8086 code (.com) lives in msdos/. This tree is a Linux ELF32 port (x86, int 0x80 syscalls).

Build

make
# or: nasm itsy-linux.asm -fbin -o itsy-linux && chmod +x itsy-linux

Requires NASM. Produces a standalone ELF binary (about 1.8 KB).

What's in the kernel

Direct-threaded Forth with a small set of primitives, including:

Area	Words
Stack	dup drop swap rot >r r>
Memory	@ ! c@ c! , count
Arithmetic / logic	+ - * = and or invert 0<
Control / compile	lit 0branch branch : ; create execute exit
Outer interpreter	word find >number interpret abort
I/O	accept (buffered, line-oriented) type bye
Variables	state >in #tib dp base last tib

Higher-level words (if/then, emit, variable, comments, …) are loaded from Forth sources.

Bootstrap with extend.4

./itsy-linux < extend.4          # batch only
cat extend.4 - | ./itsy-linux    # then type interactively

extend.4 defines immediate, \, (, arithmetic helpers, emit (via type), control flow, ', [char], etc.

Example session:

$ cat extend.4 - | ./itsy-linux
65 emit 10 emit
A
bye
# press Ctrl+D so `cat` can exit (see below)

Pipeline note (cat file - |)

bye exits Itsy only. With cat extend.4 - | ./itsy-linux, cat is still blocked on the terminal. After bye, press Ctrl+D (EOF) so cat finishes and the shell regains the prompt. A second bye only unblocks cat (SIGPIPE); it is not required by Itsy.

Tests

cat extend.4 tester-min.fr | ./itsy-linux

Expect 27 asterisks (*) and exit status 0. tester-min.fr is a small Hayes-style suite (stack, arithmetic, memory, colon definitions, if/begin).

Files

File	Role
itsy-linux.asm	Kernel sources
extend.4	Bootstrap Forth (Core subset)
tester-min.fr	Minimal tests
say_hi.f	Tiny sample
msdos/	Original Itsy for DOS

Design notes

• Threading: direct-threaded; esi = IP, esp = data stack, ebp = return stack, ebx = TOS.
• Input: private kernel buffer + line-oriented ACCEPT (handles long pipes, CRLF, EOF as −1).
• Output: TYPE is the primitive; EMIT is defined in extend.4 using a one-cell buffer.
• Size: kernel is intentionally small; prefer new words in Forth (extend.4) over new primitives unless they are required early or are performance-critical.

License / credit

Based on John Metcalf's Itsy Forth. Linux port and later fixes/extensions are part of this repository.