Back when CD-ROM was a main thing, I remember that sometimes on games you could insert the CD, launch the game, and then remove the CD, the game would still run (I believe that was the case for Age of Empires 2 for example).
How was this possible?
In: 3
The game data was transferred to the hard disk. The only reason to still check for the disk was as a copy protection measure.
It was just an early form of copy protection. You’d install the game from the CD to your PC, but it would not install some validation file of some description. Then, when you start the game, it would check for said validation. Once it’s done with that, it no longer checks, and you just run it from your hard drive. The point was, obviously, to require you to have the CD, such that you can’t use one CD to install the game on as many PCs as you want, and thus gain a free copy. Worked about as well as most copy protections, meaning very poorly.
In addition to the other answers, it’s possible that everything the game needed to run was already loaded into memory. Which means it would work until it needed something from the disk (such as going to a new level) and it would prompt you to put the CD back in.
Reading data from a CD Rom had a noticeable delay compared to reading from a hard drive (CDs stop spinning when they’re not being read). Some games would store large media files on CD but most games copied all the game files to the hard drive during the installation process (so the game would run faster).
If the CD was needed to play the game it was often only there for a single copy protection check during the game start up, and once completed the CD wasn’t needed any longer.
A few others here mention copying from CD to the hard drive, there was another way:
On the PlayStation 1, some games would continue to play after opening the disc tray… yet the PS1 didn’t have a hard drive.
Most game levels could load into RAM. A whopping 2MB main, 1MB for video, and 512KB for audio I believe.
Some games would use the disc to playback music during the game. If you put a PlayStation 1 game into an audio CD player, sometimes you’d get the games music tracks, as they were standard CD audio files.
As mentioned it was dependent on the game, and sometimes you had different options. I remember a Mech warrior game could either install the minimum like 80MB and then load from disc as you played, so you needed to leave it in. It did offer a Full install, closer to 650MB. Often you still needed the cd but just for antipiracy purposes. You could usually find a no-cd crack version of the exe to avoid that.
Some games like Quake1 installed fine and didnt need the cd, but the cd was required if you wanted the music in game.
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