Prog8 documentation - 1.7

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What is Prog8?

This is an experimental compiled programming language targeting the 8-bit 6502 / 6510 microprocessor. This CPU is from the late 1970’s and early 1980’s and was used in many home computers from that era, such as the Commodore-64. The language aims to provide many conveniences over raw assembly code (even when using a macro assembler), while still being low level enough to create high performance programs.

Prog8 is copyright © Irmen de Jong ( | The project is on github:

This software is licensed under the GNU GPL 3.0, see

3d rotating sprites Simple wizzine sprite effect Fully playable tetris clone

Code example

This code calculates prime numbers using the Sieve of Eratosthenes algorithm:

%import c64utils
%zeropage basicsafe

~ main {

    ubyte[256] sieve
    ubyte candidate_prime = 2

    sub start() {
        memset(sieve, 256, false)

        c64scr.print("prime numbers up to 255:\n\n")
        ubyte amount=0
        while true {
            ubyte prime = find_next_prime()
            if prime==0
            c64scr.print(", ")
        c64scr.print("number of primes (expected 54): ")

    sub find_next_prime() -> ubyte {

        while sieve[candidate_prime] {
            if candidate_prime==0
                return 0

        sieve[candidate_prime] = true
        uword multiple = candidate_prime
        while multiple < len(sieve) {
            sieve[lsb(multiple)] = true
            multiple += candidate_prime
        return candidate_prime

when compiled an ran on a C-64 you’ll get:

result when run on C-64

Design principles and features

  • It is a cross-compiler running on modern machines (Linux, MacOS, Windows, …) The generated output is a machine code program runnable on actual 8-bit 6502 hardware.
  • Usable on most operating systems.
  • Based on simple and familiar imperative structured programming paradigm.
  • ‘One statement per line’ code style, resulting in clear readable programs.
  • Modular programming and scoping via modules, code blocks, and subroutines.
  • Provide high level programming constructs but stay close to the metal; still able to directly use memory addresses, CPU registers and ROM subroutines
  • Arbitrary number of subroutine parameters (constrained only by available memory)
  • Complex nested expressions are possible
  • Values are typed. Types supported include signed and unsigned bytes and words, arrays, strings and floats.
  • No dynamic memory allocation or sizing! All variables stay fixed size as determined at compile time.
  • Provide various quality of life language features and library subroutines specifically for the target platform.
  • Provide a very convenient edit/compile/run cycle by being able to directly launch the compiled program in an emulator and provide debugging information to the emulator.
  • The compiler outputs a regular 6502 assembly source code file, but doesn’t assemble this itself. The (separate) ‘64tass’ cross-assembler tool is used for that.
  • Goto is usually considered harmful, but not here: arbitrary control flow jumps and branches are possible, and will usually translate directly into the appropriate single 6502 jump/branch instruction.
  • There are no complicated built-in error handling or overflow checks, you’ll have to take care of this yourself if required. This keeps the language and code simple and efficient.
  • The compiler tries to optimize the program and generated code, but hand-tuning of the performance or space-critical parts will likely still be required. This is supported by the ability to easily write embedded assembly code directly in the program source code.

Required tools

64tass - cross assembler. Install this on your shell path. A recent .exe version of this tool for Windows can be obtained from my clone of this project. For other platforms it is very easy to compile it yourself (make ; make install).

A Java runtime (jre or jdk), version 8 or newer is required to run the packaged compiler. If you’re scared of Oracle’s licensing terms, most Linux distributions ship OpenJDK instead and for Windows it’s possible to get that as well: for instance, Azul’s Zulu is a certified OpenJDK implementation available for various platforms.

Finally: a C-64 emulator (or a real C-64 ofcourse) to run the programs on. The compiler assumes the presence of the Vice emulator.


The compiler is almost completely written in Kotlin, but the packaged release version only requires a Java runtime. All other needed libraries and files are embedded in the packaged jar file.


Building the compiler itself:

(re)building the compiler itself requires a Kotlin SDK version 1.3. The compiler is developed using the IntelliJ IDEA IDE from Jetbrains, with the Kotlin plugin (free community edition of this IDE is available). But a bare Kotlin SDK installation should work just as well. A shell script ( is provided to build and package the compiler from the command line. You can also use the Gradle build system to build the compiler (it will take care of downloading all required libraries for you) by typing gradle installDist for instance. The output of this gradle build will appear in the “./compiler/build/install/p8compile/” directory.


Development and testing is done on Linux, but the compiler should run on most operating systems. If you do have trouble building or running the compiler on another operating system, please let me know!