The SCUMM Diary: Stories behind one of the greatest game engines ever made
July 12, 2013 Page 1 of 6
This article is being highlighted on Gamasutra as one of the best stories of 2013.
SCUMM might "just" be a video game engine -- but it's a video game engine that can elicit emotions nearly as strong as the games that it powers.
When you talk about of the heyday of LucasArts adventure games, you have to talk about SCUMM, the "Script Creation Utility for Maniac Mansion" that powers some of the most memorable games ever made, such as Full Throttle, Day of the Tentacle and Sam & Max Hit the Road, and, of course, Maniac Mansion.
Aric Wilmunder, along with famed game designer Ron Gilbert, built SCUMM, in effect providing a way for games like these to exist. Wilmunder and journalist Mike Bevan recently got together over email, discussing SCUMM and the stories around it. Here are choice pieces of that conversation, all in the words of Wilmunder.
We thought it was important to put Wilmunder's words on Gamasutra, where they can "live," because SCUMM is not just an engine or a piece of tech. For many developers, it was how they expressed and shared their artistic vision with so many people, during one of the most memorable periods in video game history.
On the evolution of the engine
One of the capabilities that distinguished LucasArts from most other developers was that many of us came from a mainframe background and so we developed tools and compilers on Sun workstations and then created hardware that allowed us to download the code and data to our target machines. This started with the Atari 800 and when Ron [Gilbert] was hired to do C64 work a similar system was developed for that platform as well.
The key benefit was that we could develop tools in C, or in the case of SCUMM using YACC, to process and then tokenize the code. Most developers were using the same machine to write and run their code, so their tools could only be as powerful as their target machine. After the first few years of PC product development, the PC itself became as capable as some of our earlier workstations so one by one we began to migrate our tools until we effectively retired the Sun workstations altogether.
The evolution of features started very early. I was the first internal developer to work on PCs and at the time the existing tools and compilers were very crude. For example, the C compilers that we had been using would give very detailed messages if an error was encountered. On the PC you would get a message like “File: walk.c Line:409 Error: 4004“ and your options at this point were to look at the line of code and try to figure out the problem or get out the manual to convert the error code into something meaningful. Often the error descriptions were pretty obscure as well. Since we didn't have very good editors for writing code on the PC I would use the editors on the Sun and transfer the files over. When there were errors on the PC, I would recompile and debug the code on the Sun workstation so this forced me to write cleaner code that would run on two vastly different computers from day one.
The PC presented a number of additional challenges since controls, sounds, and graphics were vastly different. For controls, mice weren't always standard equipment, so controls were designed to work for mice, joysticks or keyboard. This made moving the game engine to other platforms easier since the controls were modularized. For sounds, we supported the internal speaker; I think it was a sound card called CMS that was a precursor to the Adlib, as well as the sound system for the Tandy computers. Graphics were also modularized since we were supporting the Black & White Hercules display, 4-color CGA graphics, 16-color EGA, VGA in a 16 color mode, and Tandy Graphics in yet another video mode. To make this possible, all of our graphics were rendered into a buffer in memory and then very specialized routines would then copy the buffer out to the video card. We used what is called a “dirty rectangle” system that kept track of what areas had been updated, so we only copied the portions of the screen that were necessary.
Speaking in SCUMM
SCUMM was a language that was compiled into a tokenized form. For example, the command 'walk dr-fred to laboratory-door' would convert the Walk command into a single byte command. The next byte was which actor or character the command was for, and then the object 'laboratory-door' was converted into a 2-byte number so the entire command was reduced into 4 bytes of data. For those unfamiliar, a Byte is a location in the computer's memory that can hold a numeric value from 0-255. As you can see, the tokenized language was very efficient and the interpreter never had any understanding that one actor or another was “dr-fred” he was simply an actor-number, so we always tried to avoid hard-coding any specific information about a game directly into the interpreter.
There was one exception to that rule during Maniac and that was that the color used by an actor to display text...
When Maniac was being developed there were a couple of exceptions to the rule that the interpreter should not have any game values encoded and that had to do with the colors used for the actors and the color of the text displayed when they talked. The original code had those values built into the interpreter, and when Maniac was upgraded to the PC and before Zak McKracken, two new commands were added so those values could be controlled by the scripts instead. “set-actor-talk-color” and “set-actor-color” moved the last game-specific commands out of the interpreter, so now the scripts controlled everything.
Since I am talking about the interpreter, let me be specific about what it is. The interpreter was the program that the end-user ran that would initialize the graphics and sounds, read the files from the disk, and interpret the scripts and data in those files to run the game. When we would ship a game we would rename the interpreter to “Monkey.exe” or “Dig.exe”, but during development this tool was called SPUTM, which stood for “SCUMM Presentation Utility (tm)”. The name wasn't really trademarked, but we wanted to name it after another bodily fluid.
SCUMM, or Script Creation Utility for Maniac Mansion was the tool that tokenized the scripts and also merged all of the game assets together into the files that we shipped on the disk. The version of SCUMM that was used for Maniac probably shared 80 percent or more of the commands used in later games such as Full Throttle. Once the language was developed, most of the key commands did not require modifications. “walk bernard to clock” and “walk ben to motorcycle” were essentially unchanged.
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