Maniac Mansion was first released for the C64 and Apple II on October 5th, 1987. The Commodore 64 was the platform I had, so I will strictly limit my focus to it.
Home computers of that era used removable physical storage for games. There were three main kinds: cassettes, diskettes and cartridges.
Cartridges are a form of solid-state devices. They contain a set of ROM chips, enclosed in a plastic package. They expose a metal edge connector to interface with the computer. As solid state components, they offer instantaneous read access. Loading times were inexistent, as the computer could read data from ROM as fast as if it were in RAM.
Cartridges are reliable and durable. Even after 40 years of use, they work fine. But they were expensive to manufacture. The C64 had a very limited number of releases in this format due to the cost.
Tape, instead, was cheap. There were several physical formats for magnetic tape. Among them, the de-facto standard called cassette tape. Everyone had them and they were easy to find. They were the most affordable way to record and play music. They were the humbler relative of the vinyl record. The audio quality was, at best, decent.
For storing data, cassettes are not completely reliable. They are vulnerable to two problems:
Loss of data, if placed near a strong magnetic field, like the shield on a speaker.
Read errors on a healthy cassette. Misalignment between the reading head and the magnetic track produced errors. Which meant corrupted graphics, sound or, more likely, an unusable program. The problem was frequent enough in those times, even using new cassettes.
C64 cassettes were abysmally slow: by default, the C64 reads data from tapes at 50 bytes per second. It would take it 2 seconds to read the previous sentence of this paragraph. And a minute and a half to read this whole article.
50 bytes per second is 400 bits per second or 0.4 kilobits per second. That's 10,000 times slower than a 4 Megabits/s transfer rate and 10 million times slower than a 4 Gigabit/s rate. A modern cell phone compatible with Wi-Fi 7 can transfer up to 23 Gigabits/s. Current consumer technology is roughly 60 million times faster.
The cassette was designed for audio playback. It's intended to play uninterrupted from the beginning. If you wanted to skip to a different song or position, you had to move the tape yourself. With no visual indications, you had to guess where to stop, in a trial and error process.
Storing data on a cassette meant that you could only perform a sequential read. This worked well if the game fit completely inside the C64's memory, as you can read everything from tape in one go. The C64 had exactly 64K of RAM available. A significant number of small games did fit and thus could be stored on tape.
If the game was large enough that it did not fit completely in memory, then the options narrowed. The author could split the game into smaller, independent stages. For instance, the design could force the player to advance only to the next stage. He could never return to a previous one. We would call that a linear progression game. In that case, a cassette version was feasible.
A notable exception are some of the multi-event Epyx games. Classics like Summer Games, Winter Games and World Games. These allow loading any specific event, but you had to position the tape yourself.
All these games have an independent piece of code called the loader. Its purpose is to load different parts of the game into memory. It's also responsible for finding and identifying the desired part. Finally, it verifies the loaded data.
When the player completed a stage, the game discarded the data from memory. The loader took control and put in motion the cassette again, reading the next level's data. The most well-known example of a linear cassette game is The Last Ninja.
But some games needed to constantly read data from arbitrary locations. Using a tape for them was impossible. There wasn't even a way for a program to rewind the tape, the player had to do it.
Maniac Mansion is definitely a big game. The large number of rooms and game assets would not fit in the computer's memory, not all at once. The possibility of visiting locations back and forth made it a non-linear game. This also required reading data from arbitrary locations. Or, as we call it, performing random-access reads.
Enter the diskette.
