Introducing the C compiler

A project log for Kobold K2 - RISC TTL Computer

A 16 bit RISC computer with video display, from just a few TTL and memory chips. Online C compiler and simulator available.

roelhroelh 03/13/2020 at 21:000 Comments

The Kobold K2 processor is quite suitable to run a language like C. 

Fortunately I had started a compiler project several years ago, but I never finished it. So now I had the opportunity to get some results from all the hours that already went into that project. 

Another possibility would be to use an existing compiler and change the code generation part. Several C compilers generate pseudo-instructions, that only have to be translated to instructions for your own CPU. But since the Kobold is such an odd beast, it is needed to have control over the code generation at a high level, to get an acceptable code output. And I had the unfinished project as a start point, so that was the way to go.

My compiler project started in the period that I discovered that you could program a HTML server to generate webpages using the PHP language, and that PHP was actually a full featured programming language. To make a long story short, the compiler was programmed in PHP and ran on the webserver. 

It generated some intermediate code. Many of my fellow HAD'ers could face a similar simulation (wanting to generate code for your homebuilt processor, but having difficulty in interfacing to an existing front-end compiler). How much easier would it be, if the intermediate code was standardized ? 

I here propose a standard interface between the parser and code generator. If this interface is standardized, a codegenerator for a certain CPU can be used with all C compilers that comply to this standard. It is described HERE (but is not fully complete yet). Its syntax is JSON.

In the last months, I added (to my PHP compiler) a section that translated the old intermediate code to the new JSON standard. I then started with the code generation section, programmed in Javascript. The code generator does several optimizations. The resulting assembly code is reasonable compact, and also fast because native code is generated. Finally, the K2 assembler produces the binary code.

The C compiler is now integrated into the Kobold K2 online Javascript assembler/simulator. The generated assembler code is displayed side-by-side to the C source code. The C code can be written in the included online editor that has syntax highlighting, matching parenthesis indication and several other features (it is the ACE editor that was integrated here). The C compiler is not yet full-featured. Its capabilities are in the Language Manual (also reachable from within the online editor)

On a single browser page you can now edit, compile, assemble and simulate the C code, and download a file to flash it into the Kobold. With the 'assembler' checkbox, it is still possible to program directly in assembler.

See it live at !

The page loads a prime-number generator as demo program. You only have to press the RUN button to start it. However, the simulator is quite slow because it simulates almost at the gate level (see comments in the C code of the demo).

On the hardware side, the bug in the NOR instruction was found (a short on the pcb), and the RAM is now also working.