Rebuilding the Z-machine

Hi,

  I've created this blog to share my experiences in building a Z-machine interpreter. My name is Mike Greger and I am a C# developer and a gamer. I got started with computers in the early 1980's when I became fascinated by the worlds found within so-called 'text adventures'. Some of my favorites were written by a company called Infocom.

What is a Z-machine?
  Infocom created a clever way of making their games available for the many types of computers on the market at that time by designing a virtual machine now known as the Z-machine. Each game only needed to be written once, for the Z-machine. A Z-machine interpreter was then written for each platform to run the games.
  Fast forward to 2009: Infocom is long gone. The Z-machine format has been reverse engineered by talented people and the specifications made available. Armed with this information developers have created Z-machine interpreters for platforms big and small.

If it's already been done, then why do it?
  The short answer is because of the challenge and learning opportunities it presents. I also like to think I can improve on some things. Some time ago I set out to write my own interpreter and succeeded, sort of. My interpreter played all the infocom games just as I remembered them. However, when I first started my project I did not have a lot of programming experience. Looking back later at what I had written I found the code amateurish and not suitable for easy reuse. I decided to rebuild it and came up with a set of design criteria for a class library written in C# 3.0 which implements the Z-machine:

• Complete. Supports all features and Z-machine versions
• Easy to use and extend
• Decoupled from any particular interface (console, windows forms, WPF, etc.)
• CLS compliant
• Well documented
• Thread-safe
• Memory efficient for multiple instances
• Not overly dependent on framework types

The last requirement is to help ensure portability to platforms like Mono or others which may not implement the entire .Net framework. Thread safety and multiple instance support allows the library to be useful in a web server or other multi-user environment.

Why write a blog about it?
  In the hope that someone will find something useful, maybe for their own interpreter or other project. While the published specifications contain a lot of useful information, Z-machine behavior can be complex and seeing a working example can be beneficial. Additionally I hope to make clear why I chose certain designs, rather than just publishing source code. Code comments tend to explain how something works, not why it was written that way in the first place.

Next time: Z-machine architecture.