Oberon Compiler

Introduction

The current codebase ports a subset of Wirth's reference implementation for the Oberon07 language to C (i.e., concerning the Oberon compiler, not the OS). It was written primarily for self-educational purposes as a kind of intensive code reading exercise. Some notable omissions and deviations are outlined below, which may be remedied in future versions:

• No generation of object- and symbol files. Instead, the compiler combines with a RISC-0 emulator into a single executable, with an in-memory array serving as their interface. Compare this to the implementation of PL/0 found in prior editions of Algorithms + Data Structures = Programs.
• By extension, separate compilation and imports are unsupported. For I/O, the built-in functions are extended with primitives Read, Write and WriteLn. The first one takes an INTEGER or CHAR variable as argument, while the latter two may take a STRING argument as well. Finally, WriteLn 's argument is optional, outputting only a newline if absent. See test/io.mod for some examples.
• No support for floating-point numbers. In particular, the built-in functions operating thereon specified in the Oberon language report have been removed.
• No support for unsigned arithmetic. I.e., the built-in functions ADC, SBC and UML have been removed, with the RISC-0 emulator similarly ignoring the Carry bit.
• In general, type descriptors have not been implemented. As a result, dynamic memory allocation (via NEW) and case statements are not supported.
• Despite the absence of NEW, POINTERs can still be used, albeit only through recource to the unsafe SYSTEM.ADR and SYSTEM.VAL functions. See test/pointer.mod for an example.

Requirements

• GCC or Clang (C99-compatible)
• GNU Make
• [Optional] Valgrind (Remove from Makefile if not present on your system)

Installation

Clone or download the sources. E.g,

wget https://github.com/arnobastenhof/oberon/archive/master.zip
unzip master.zip

To compile, type the following command from the project root,

make build

This creates an executable oc in the directory build. To run the test suite, type

make test

The above two commands are combined into

make all

Module overview

Largely following Wirth's reference implementation, the codebase is divided into the following modules.

• Pool (pool.h, pool.c) exposes an arena-based memory allocator, for the most part constituting a simplified variant of the one used in LCC as described in Fraser and Hanson's "A retargetable C compiler". Rather than singalling an allocation failure through a special return value like NULL, instead a rudimentary implementation of exceptions is used (see except.h) to return control to a handler that may, e.g., clean up resources prior to exiting. This considerably simplifies application code. Note this is the only module that has no counterpart in Wirth's code, as Oberon features a garbage collector.
• ORS (ors.h, ors.c) contains the scanner.
• ORB (orb.h, orb.c) defines the symbol table.
• ORG (org.h, org.c) contains the code generator for Wirth's RISC-0 architecture, described in his book "Compiler Construction".
• ORP (orp.h, orp.c) contains the parser.
• RISC (risc.h, risc.c) contains a RISC-0 emulator.

Finally, unit tests are implemented using a modest extension of Jera Design's Minunit test framework (see minunit.h and minunit.c).

Some notes on code style

• Oberon makes the export of global variables more safe by making them read-only for other modules, and the reference implementation for its compiler certainly makes use of this. The closest analogy for C would probably be to expose Java-style getters instead as part of a header, while keeping the variables themselves private to the implementation (i.e., declared static). Though normally I would feel hestitant to use global variables with external linkage in C, which differs from Oberon in this respect, I have nonetheless chosen to stay close to the source materials in this particular case.
• In his book 'C interfaces and implementations,' Hanson advocates the use of opaque pointers in header files to hide the implementation details of one's data structures. Though certainly the better programming practice, especially when programming in the large, I have not followed this advice mainly to allow for stack allocation whenever possible (where you need to know a struct's size at compile time), as well as to stay more close to the source materials for this particular case.
• One aspect where I deviated from the source materials is in the ordering of function definitions. Oberon, like C, prohibits forward declarations for its procedure calls, but, unlike C, does not support prototypes. A typical Oberon source file will thus typically read very much like a linear narrative, with the definition of support functions preceding their use. For the current port, I have instead chosen to follow what I perceive as more standard practice in C by: (a) prototyping all static functions, (b) first defining all functions with external linkage, and (c) only then providing definitions for the static functions. (As an aside, when it comes to mutual recursion, Oberon works around its prohibition of forward references, albeit at a small runtime cost, by declaring procedure pointer variables and setting them in the module initializer.)
• Method names have been written in UpperCamelCase, type names and enum constants in lowerCamelCase, variable names in lower_snake_case, and #defined names in UPPER_SNAKE_CASE. In addition, typedefd names and union- and struct tags have been postfixed with _t, _u and _s, resp., while global variables similarly bear a g_ prefix. Finally, enum constants begin with k (e.g., kCondU).
• Methods declared extern bear a prefix identifying their module. E.g., ORS_Init.
• Statement blocks following keywords that signal conditional- and loop constructs are delimited by braces, with the sole exception of empty blocks, where I resort to a single properly indented ;.

Roadmap

I tend to work on private projects for one or two months at a time, with the current codebase the result of such an iteration. The limitations mentioned in the introduction are, in particular, a consequence of this self-imposed time constraint. When I pick this codebase up again, my priorities will likely be as found below. All these, it should be noted, concern parts of the original source code that I hadn't ported yet.

• Separate the parser from the interpreter through the generation of object files.
• Generate symbol files.
• Support imports.
• Support type descriptors along with dynamic allocations and garbage collection.
• Support case statements.

Further reading

The primary influence for this work came from the following sources:

• The 2017 revision of Wirth's Compiler Construction, available at https://www.inf.ethz.ch/personal/wirth/CompilerConstruction/index.html
• The 2013 revision of Project Oberon (including the reference implementation for the Oberon07 compiler), available at https://www.inf.ethz.ch/personal/wirth/ProjectOberon/index.html
• Programming in Oberon, available at https://www.inf.ethz.ch/personal/wirth/Oberon/PIO.pdf

In addition, we have taken some inspiration from the following works, most notably for memory management:

• Fraser, C.W., & Hanson, D.R. (1995). A retargetable C compiler: design and implementation. Addison-Wesley Longman Publishing Co., Inc.
• Hanson, D.R. (1996). C interfaces and implementations: techniques for creating reusable software. Addison-Wesley Longman Publishing Co., Inc.