bintools

What is bintools?

This project provides a collection of code and tools for exploring and working with Object files. Specifically the a.out object file format used by early PDP-11 and UNIX operating systems. The a.out format was used in Linux up until the 1.2 kernel.

According to Wikipedia, the a.out file format name was an abbreviation of "assembler output" and was the default output filename from Ken Thompson's PDP-7 assembler.

The bintools project is built regularly on Windows using Visual Studio as well as Mac OSX using Clang.

Why bintools?

I am fascinated by the process of compiling, linking and executing programs. Linkers had always seemed complex to the point of being a bit magical. While I had previously worked on compilers and interpreters, I had not yet had the opportunity to work directly with object files and linkers.

It seemed clear that the best way to learn a lot more about linkers and the linking process was to create one. To keep the linker scope manageable the a.out format was chosen. While the a.out format may be considered somewhat outdated, it benefits from being well understood and documented. Once the basic linking process was understood it would always be an option to re-implement the linker using a more modern object file format like ELF or PE.

If you want to learn more about linkers and object file formats, I highly recommend the book 'Linkers & Loaders' by John R. Levine

In order to test the linker it was necessary to have a way to generate linkable object files. Certainly, it would have been possible to chose an existing assembler that produced a.out object files. However, to keep the linker scope more manageable it seemed important to avoid large and complex instruction sets. Which lead to the need to create a simple 8-bit CPU architecture and an assembler for it.

And finally, a way was needed to prove that the linker successfully produced a valid executable image. For this a CPU emulator was created that could load, run and debug executables.

The bintools tools

The bintools project consists of a number of tools. As mentioned above most of the tools were created in order to provide the input, and test the output of the linker.

The current set of tools are:

• asm - an assembler that generates a.out object files
• ln - a linker which combines a.out object files into an executable
• dumpbin - a utility to explore a.out object files and executables
• cisc - an 8-bit CPU simulator and debug monitor
• strip - utility to strip symbols and relocation data