Swift Pascal

A Pascal interpreter written in Swift that runs Turbo Pascal-era (ca. 1987) programs in a simulated DOS text-mode terminal.

What is this?

Swift Pascal is a macOS app that interprets Pascal source code from the late 1980s. It provides an 80x25 (or 40x25) character terminal with CGA 16-color graphics, simulating the DOS experience of running a Pascal program on an IBM PC.

The interpreter was built specifically to run programs from the Turbo Pascal era (including a two-player PONG game that I wrote in Turbo Pascal 3 and published in Pascal International, February 1988).

Building

Requires macOS 14+ and Swift 5.10+.

swift build
.build/debug/SwiftPascal

Or open the project in Xcode:

open Package.swift

Usage

1. Launch the app
2. Click Open .PAS (or Cmd+O) to load a Pascal source file
3. The program runs immediately in the terminal view
4. Click Stop (Cmd+.) to halt execution, Run (Cmd+R) to restart

Keyboard input goes directly to the running program. Click on the terminal to ensure it has focus.

Language Features

Declarations

• PROGRAM, USES, CONST, VAR, TYPE
• PROCEDURE and FUNCTION with nesting and local declarations
• Parameter passing (by value)

Types

• INTEGER, REAL, CHAR, STRING[n], BOOLEAN
• Subrange types (1..100, -1..1)
• Multi-dimensional arrays (ARRAY [1..25, 1..40, 1..2] OF CHAR)
• ABSOLUTE memory mapping for direct video memory access

Control Flow

• IF / THEN / ELSE
• FOR / TO / DOWNTO / DO
• WHILE / DO
• REPEAT / UNTIL
• BEGIN / END compound statements

I/O

• Write, WriteLn with format specifiers (:width, :width:decimals)
• Read, ReadLn (including to array elements)
• Read(Kbd, c) for single-character input without echo

Built-in Functions

Chr, Ord, Pred, Succ, Random, Pos, Copy, Length, Concat, UpCase, Abs, Sqr, Sqrt, Round, Trunc, Odd

CRT Unit

ClrScr, GotoXY, TextColor, TextBackground, TextMode, KeyPressed, Delay, Sound, NoSound, GraphBackground

DOS Simulation

• Video memory: ABSOLUTE $B800:0 maps array writes directly to the terminal buffer, exactly like CGA text mode
• Mem[] access: Mem[segment:offset] for BIOS data area simulation
• PC Speaker: Sound(frequency) / NoSound generate square-wave tones via AVAudioEngine
• CP437 characters: Box-drawing and block elements (lines, corners, half-blocks, shading) are rendered as pixel-perfect geometric shapes
• CGA palette: All 16 colors including the blink attribute
• Text modes: 40-column (C40) and 80-column (C80)

Architecture

Sources/
  SwiftPascal/                    # macOS app (SwiftUI + AppKit)
    SwiftPascalApp.swift          # App entry point
    ContentView.swift             # Main window layout
    TerminalView.swift            # NSView-based terminal renderer
    TerminalStore.swift           # Bridges interpreter <-> UI
  SwiftPascalCore/                # Interpreter library
    Lexer/                        # Tokenizer (case-insensitive, hex, CP437)
    Parser/                       # Recursive descent parser -> AST
    Interpreter/                  # Tree-walking async interpreter
    Terminal/                     # 80x25 character+attribute buffer
    Units/                        # Sound engine (AVAudioEngine)

The interpreter runs asynchronously, allowing Delay calls and keyboard input without freezing the UI. The terminal view refreshes at 30fps for smooth animation.

Sample Programs

File	Description
PONG.PAS	Two-player Pong game (ca. 1987). Uses direct video memory, sound, 40-column mode.
HELLO.PAS	Hello World with colored text
COLORS.PAS	Displays all 16 CGA colors

Running Tests

swift test

Comparison

DOSBox (original)	Swift Pascal

License

MIT