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Oberon-07 compiler

oberonc is a single pass, self-hosting compiler for the Oberon-07 programming language. It targets the Java Virtual Machine (version >= 1.8).

This project was started to showcase Niklaus Wirth's approach to writing compilers (see "Compiler Construction - The Art of Niklaus Wirth" by Hanspeter Mössenböck for more details).

oberonc is inspired by Niklaus Wirth's compiler for a RISC processor available here.

The compiler is compact and does not depend on any third party libraries. It produces Java bytecode in one pass while parsing the source file. Although generating code for a stack machine is straightforward, this task is exacerbated by a complex class file format and the fact that the JVM was designed with the Java language in mind. In fact the JVM lacks many of the primitives required to support Oberon's features, specifically:

  • value types
  • pass by reference evaluation strategy
  • procedure variables (pointer to functions) and relative structural compatibility of types

Implementing those features with workarounds increased significantly the size of the compiler, totaling roughly 6000 lines of Oberon.

The source code is written following as much as possible Niklaus Wirth's coding style. oberonc compile itself in less than 300 ms on an old Intel i5 @ 2.80GHz (~ 100 ms with a hot VM).

How to build

You can build the compiler on Linux or Windows, you need a JDK >= 1.8 installed, with java and javac in the environment path.

Because you need an Oberon compiler to compile the sources in src, I have added to the repository the binaries of the compiler to perform the bootstrapping.

By typing make build on the shell, the compiler will compile itself and write the files in the out folder. The make bootstrap command is equivalent to make build, but it overwrites the files in the bin folder.

To run the compiler, you need to have the OBERON_BIN environmental variable set to the bin folder of the repository. This is taken care for you when using make.

How to run the tests

One typical test is to make sure that, by compiling the compiler, we get the same (bit by bit) class files originally included in the bin folder. To run this test simply type make bootstrapTest (available only on Linux). This will compile the sources into the bootstrapOut folder and compare these resulting class files with the ones in bin. If something goes wrong sha1sums will complain.

To run the tests included in the tests folder, type make test. The output should look like this:

TOTAL: 101

Using the compiler

To use the compiler, you need to have the OBERON_BIN environmental variable set to the bin folder of the repository, for example on Linux export OBERON_BIN=~/oberonc/bin or set OBERON_BIN=C:\oberonc\bin on Windows. The command line syntax of oberonc is simple. Let's compile examples/Hello.Mod:

  IMPORT Out; (* Import Out to print on the console *)
  Out.String("Hello 世界");
  Out.Ln (* print a new line *)
END Hello.

Assuming you are at the root of the repository, the following command will compile the Hello.Mod example and place the generated classes in the current folder:

java -cp $OBERON_BIN oberonc . examples/Hello.Mod

java -cp %OBERON_BIN% oberonc . examples/Hello.Mod

The first argument of oberonc is ., this is the existing folder where the generated class will be written, the next arguments specify module files to be compiled.

This will generate Hello.class and Hello.smb. The second file is a symbol file, it is used only during compilation and enables oberonc to perform separate compilation of modules that import Hello. In this simple case Hello.Mod does not export anything, but the other modules in the examples folder do.

To run Hello.class, you need the OberonRuntime.class and Out.class. These are present in the bin folder so they are already in the class path, we just need to include the current folder as well to locate Hello.class:

java -cp $OBERON_BIN:. Hello

java -cp %OBERON_BIN%;. Hello

If you want to compile and run automatically a simple example called fern, type make runFern. It should open a window like this one:


Lastly, make clean will delete the output folders generated by build, test, runFern and bootstrapTest.


The compiler is distributed under the MIT license found in the LICENSE.txt file.