Applesoft BASIC Emulator

A Windows command-line emulator for Apple II Applesoft BASIC, written in C#. Create, edit, and run simple Applesoft BASIC programs just like on a real Apple ][.

                APPLESOFT BASIC EMULATOR
          Based on Apple ][ Applesoft BASIC

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

]

Getting Started

Prerequisites

• .NET 8 SDK or later

Build & Run

dotnet build
dotnet run

The emulator launches with the classic ] prompt. Type BASIC commands directly or enter line-numbered programs. Type QUIT or EXIT to leave.

Usage

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 20,40

Direct-Mode Commands

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 the disk folder
LOAD "MYPROG"	Load a program from the disk folder
CATALOG	List all programs in the disk folder
DEL 10,50	Delete lines 10 through 50
QUIT / EXIT	Exit the emulator

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)
SGN(X)	Sign (-1, 0, or 1)
SIN(X)	Sine
COS(X)	Cosine
TAN(X)	Tangent
ATN(X)	Arctangent
LOG(X)	Natural logarithm
EXP(X)	e raised to power X
PEEK(X)	Read simulated memory
POS(X)	Current cursor column

String

Function	Description
LEN(A$)	Length of string
LEFT$(A$,N)	First N characters
RIGHT$(A$,N)	Last N characters
MID$(A$,S,N)	Substring from position S, length N
STR$(X)	Convert number to string
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) — REPL loop with the classic ] prompt

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.