Atari Programming Overview

Programming the FujiNet on the Atari 8-bit platform revolves around the SIO (Serial Input/Output) bus, the same communication layer used by disk drives, printers, and modems. The FujiNet appears as one or more SIO devices, and programs interact with it through the Atari’s standard CIO (Central I/O) subsystem or directly via SIO calls.

Architecture

The Atari’s I/O stack is layered. User programs issue CIO calls, which the operating system translates into low-level SIO bus transactions. The FujiNet N: device handler (loaded at boot or from the CONFIG program) sits in this stack and translates CIO operations into the appropriate SIO commands for the FujiNet hardware.

graph TD
    A["User Program<br/>(BASIC / Assembly / cc65)"] -->|"CIO Calls<br/>OPEN, GET, PUT, XIO"| B["Atari OS CIO Subsystem"]
    B -->|"Device Handler Dispatch"| C["N: Device Handler<br/>(FujiNet Handler in RAM)"]
    C -->|"SIO Bus Commands"| D["FujiNet Hardware<br/>(ESP32 on SIO Bus)"]
    D -->|"Network Protocols<br/>HTTP, TCP, UDP, etc."| E["Internet / Network"]

    style A fill:#4a90d9,color:#fff
    style B fill:#7b68ee,color:#fff
    style C fill:#e07020,color:#fff
    style D fill:#d94040,color:#fff
    style E fill:#50a050,color:#fff

Device IDs

The FujiNet uses the following SIO device IDs:

Device ID	Purpose
`$70`	FujiNet Control Device – adapter configuration, WiFi management, host/device slot operations, clock, app keys, and other system-level functions
`$71` - `$78`	N: Network Devices – up to eight simultaneous network connections, each supporting protocols such as HTTP, HTTPS, TCP, UDP, FTP, TNFS, SMB, and more

The FujiNet also emulates standard Atari SIO devices (disk drives at $31-$38, printers, RS-232, etc.), but the $70 control device and $71-$78 network devices are unique to FujiNet.

Programming Approaches

Atari BASIC

The most accessible way to use the FujiNet is through Atari BASIC using OPEN, PRINT#, INPUT#, GET#, and XIO commands with the N: device. The N: handler must be loaded first (it loads automatically when booting from FujiNet CONFIG).

Quick Start – Reading a Mastodon Post:

0 DIM A$(256)
1 TRAP 91
2 POKE 756,204
10 OPEN #1,12,0,"N:HTTPS://OLDBYTES.SPACE/api/v1/timelines/public?limit=1"
20 XIO 252,#1,12,1,"N:":REM SET CHANNEL MODE TO JSON
30 XIO ASC("P"),#1,12,0,"N:":REM PARSE THE JSON
40 XIO ASC("Q"),#1,12,3,"N:/0/account/display_name"
50 INPUT #1,A$:? A$
60 XIO ASC("Q"),#1,12,3,"N:/0/created_at"
70 INPUT #1,A$:? A$
80 XIO ASC("Q"),#1,12,3,"N:/0/content"
90 GET #1,A:? CHR$(A);:GOTO 90
91 CLOSE #1:? :?
100 POKE 18,0:POKE 19,0:POKE 20,0
110 IF PEEK(19)<30 THEN 110
120 GOTO 1

This example opens an HTTPS connection, switches the channel to JSON mode, parses the response, queries specific JSON fields, and prints the results.

Assembly Language

For maximum performance and control, programmers can issue SIO calls directly from 6502 assembly. This bypasses CIO overhead and communicates with FujiNet device IDs $70-$78 at the bus level. The SIO command frame specifies the device ID, command byte, auxiliary bytes, buffer address, and transfer length.

cc65 (C Cross-Compiler)

The cc65 cross-compiler for 6502 targets provides a practical middle ground. The fujinet-lib project offers a C library that wraps the SIO interface, making FujiNet programming accessible from C while retaining good performance.

CIO Commands

The N: device handler implements the following standard CIO commands, allowing BASIC and other high-level languages to perform network I/O as if it were file I/O:

CIO Command	Operation
OPEN (`$03`)	Open a network connection using a URL
GET RECORD (`$05`)	Read a line of text (up to EOL)
GET CHARACTERS (`$07`)	Read a specified number of bytes
PUT RECORD (`$09`)	Write a line of text with EOL
PUT CHARACTERS (`$0B`)	Write a specified number of bytes
CLOSE (`$0C`)	Close the network connection
STATUS (`$0D`)	Get connection status and bytes waiting

XIO Commands

The XIO statement provides access to extended operations beyond standard CIO:

XIO Command	Description
`$0F`	Flush write buffer – forces buffered data to the network
`$41` (`'A'`)	Accept a pending TCP client connection
`$FC` (252)	Set channel mode (e.g., JSON mode)
`$50` (`'P'`)	Parse JSON data
`$51` (`'Q'`)	Query a JSON element by path
`$FF`	Reset FujiNet

XIO Syntax in BASIC:

XIO command,#iocb,aux1,aux2,"N:"

Where command is the XIO code, #iocb is the I/O channel (1-7), and aux1/aux2 provide additional parameters.

SIO Command Reference

For low-level programming (assembly, cc65, or advanced BASIC using USR routines), the full SIO command tables are essential references:

Device $70 (FujiNet Control) – WiFi management, host/device slots, clock, app keys, boot mode, hashing, Base64, QR codes, and more. See the SIO Commands for Device $70 reference.
Devices $71-$78 (N: Network) – Open, Close, Read, Write, Status, JSON parse/query, channel mode, login/password, file operations, and protocol-specific commands. See the SIO Commands for Devices $71-$78 reference.

URL Format

All N: device URLs follow this format:

N:PROTO://[HOSTNAME][:PORT]/[PATH]

Where:

PROTO is the protocol (HTTP, HTTPS, TCP, UDP, TNFS, FTP, SMB, SSH, etc.)
HOSTNAME is the destination host (can be omitted for listening sockets)
PORT is the port number (optional for most protocols, required for listening sockets)
PATH is the resource path (optional)

FujiNet Documentation