Applesoft BASIC Emulator

A browser-based Applesoft BASIC emulator for Apple ][ style programs. The entire interpreter runs in your web browser using JavaScript—no backend server required. Programs are saved to your browser's IndexedDB storage.

                APPLESOFT BASIC EMULATOR
          Based on Apple ][ Applesoft BASIC

]10 PRINT "HELLO, WORLD!"
]RUN
HELLO, WORLD!

]

Getting Started

Online (Recommended)

Open the live emulator at: ashy-wave-08690e81e.2.azurestaticapps.net

Your programs are saved automatically to your browser's local storage.

Local Development

To run the emulator locally:

cd web
python -m http.server 5500

Then open http://localhost:5500 in your browser.

The interpreter runs entirely in the browser—no backend API is required.

Browser-Native Architecture

• Interpreter Engine: Full Applesoft interpreter implemented in JavaScript (web/runtime/local-emulator.js)
• Persistence: Programs saved to IndexedDB with automatic seeding of bundled programs
• No Backend Required: Static web app hosted on Azure Static Web Apps; zero server-side execution
• Interactive I/O: INPUT and GET statements pause execution and resume on user submit without restarting
• Offline Ready: Bundled programs available offline; SAVE/LOAD work entirely in-browser

Storage & Data

Program Storage

Programs are saved to your browser's IndexedDB under the key programs with the program name as the index. IndexedDB provides unlimited local storage (up to your browser's quota, typically 50GB per site).

Clearing Local Data

To reset all programs and variables, clear your browser's site data:

1. Press F12 to open Developer Tools
2. Go to Application → Storage
3. Click Clear site data

Or use the in-emulator NEW command to clear the current program and variables (but keep saved programs intact).

Command Reference

Direct-Mode Commands

Type these commands at the prompt (preceded by ]) to control the emulator:

Command	Description
RUN	Run the current program
RUN 100	Run starting from line 100
LIST	List the entire program
LIST 10	List line 10
LIST 10,50	List lines 10 through 50
NEW	Clear the program and all variables
SAVE "MYPROG"	Save the program to browser storage
LOAD "MYPROG"	Load a program from browser storage
CATALOG	List all saved programs
DEL N or DEL A,B	Delete single line or range
HOME	Clear the screen
HELP	Show this command summary
QUIT / EXIT	Close the emulator session

Entering a Program

Type lines with line numbers to store them. Lines are automatically sorted by number.

]10 PRINT "HELLO, WORLD!"
]20 FOR I = 1 TO 5
]30 PRINT "COUNT: ";I
]40 NEXT I

Editing a Program

• Replace a line: Re-enter it with the same line number
• Delete a line: Type just the line number with nothing after it
• Delete a range: DEL 10 or DEL 10,40

Interactive Input/Output

INPUT: Pauses execution and prompts for a line of text. Execution resumes after you type your input and press Enter.

]10 INPUT "What is your name? ";NAME$
]20 PRINT "Hello, ";NAME$

GET: Pauses execution and waits for a single keypress. Useful for games like WUMPUS.

]10 GET C$
]20 PRINT "You pressed: ";C$

Both INPUT and GET work interactively in the browser—you can pause mid-program, provide input, and the program continues without restarting.

Supported BASIC Statements

Statement	Example
PRINT	PRINT "HELLO" or ? "HELLO"
INPUT	INPUT "NAME? ";N$
LET (optional)	LET X = 10 or X = 10
IF...THEN	IF X > 5 THEN PRINT "BIG"
IF...THEN (line #)	IF X = 0 THEN 100
GOTO	GOTO 200
GOSUB / RETURN	GOSUB 1000
FOR...TO...STEP	FOR I = 1 TO 10 STEP 2
NEXT	NEXT I
DIM	DIM A(20), B$(10)
DATA / READ	DATA 1,2,3 / READ X
RESTORE	Reset DATA pointer to beginning
DEF FN	DEF FN SQ(X) = X * X
ON...GOTO / GOSUB	ON X GOTO 100,200,300
REM	REM THIS IS A COMMENT
HOME	Clear the screen
HTAB / VTAB	HTAB 10 / VTAB 5
POKE	POKE 768,0
END / STOP	End program execution

Multiple Statements Per Line

Separate statements with a colon:

]10 X = 5 : Y = 10 : PRINT X + Y

Operators

Type	Operators
Arithmetic	+ - * / ^
Comparison	= <> < > <= >=
Logical	AND OR NOT
String	+ (concatenation)

Built-in Functions

Numeric

Function	Description
ABS(X)	Absolute value
INT(X)	Floor (integer part)
SQR(X)	Square root
RND(X)	Random number (0 to 1)

String

Function	Description
LEN(A$)	Length of string
LEFT$(A$,N)	First N characters
RIGHT$(A$,N)	Last N characters
MID$(A$,S,N)	Substring from position S, length N
VAL(A$)	Convert string to number
CHR$(X)	Character from ASCII code
ASC(A$)	ASCII code of first character

Variables

• Numeric: X, A1, SCORE — default value is 0
• String: N$, NAME$ — default value is ""
• Arrays: DIM A(10) creates indices 0-10; auto-dimensions to 0-10 if used without DIM

Example Programs

Fibonacci Sequence

10 A = 0 : B = 1
20 FOR I = 1 TO 15
30 PRINT A;
40 C = A + B
50 A = B : B = C
60 NEXT I
70 PRINT

Guess the Number

10 N = INT(RND(1) * 100) + 1
20 PRINT "I'M THINKING OF A NUMBER (1-100)"
30 INPUT "YOUR GUESS";G
40 IF G < N THEN PRINT "TOO LOW!" : GOTO 30
50 IF G > N THEN PRINT "TOO HIGH!" : GOTO 30
60 PRINT "YOU GOT IT!"

Multiplication Table

10 FOR I = 1 TO 10
20 FOR J = 1 TO 10
30 PRINT I * J;
40 NEXT J
50 PRINT
60 NEXT I

Architecture

The emulator is composed of three main components:

• Tokenizer (Tokenizer.cs) — Lexes input into tokens (keywords, numbers, strings, operators)
• Expression Evaluator (ExpressionEvaluator.cs) — Recursive-descent parser handling operator precedence, function calls, and variable access
• Interpreter (Interpreter.cs) — Manages program storage, executes statements, and handles flow control (GOTO, GOSUB, FOR/NEXT, IF/THEN)
• Program (Program.cs) — ASP.NET Core minimal API host with per-session interpreter state

Limitations

• No graphics modes (LORES/HIRES) — text mode only
• No sound support
• PEEK/POKE use simulated memory (no real hardware mapping)
• CALL accepts but ignores machine-language addresses
• No cassette or disk I/O beyond SAVE/LOAD/CATALOG to the Disk/ folder

License

This project is provided as-is for educational and hobbyist use.