Several ways exist to protect against the copying of games but, when CD-ROM games were first introduced, were there any measures taken by video game developers to prevent the copying of games?
When CD-ROM games were first introduced, game developers didn’t take any measures to prevent users from copying them, for two main reasons:
- CD-ROMs could contain more data than most hard drives at the time;
- CD writers were rare, and extremely expensive.
The Wikipedia page on CD-R gives some idea of the expense involved: in 1990, CD recording systems cost over $35,000; by 1992, that had dropped to around $10,000, and the $1,000 barrier was crossed in 1995.
In those days, a hard drive large enough to store a copy of a CD-ROM (assuming the latter was filled to capacity) cost a significant amount too ($2,000 in 1992) — and you’d need one, on a SCSI system, to have any chance of copying a CD correctly. Blank CDs were also quite expensive.
Another limiting factor initially was that CD writers wrote CDs at the nominal speed, so copying a full CD would take over an hour (not counting the time it took to read it).
Many CD games did however rely on some CD characteristics to make them harder to copy to another medium: they’d check that their CD was present, or rely on audio CD tracks.
(preface: While Stephen's answer already covers the basic points, I would like to put a different emphasis here - and merge in some private history :))
It was the game's size and the need to copy it to a CD again, combined with expensive and unreliable writers. Further, the CD itself was used as a copy protection. While games often got installed completely onto HD, they did check every now and then if the CD was still present in a drive connected, so people could not install and run it on multiple (friends') systems.
All of this is based around the fact, that copying CDs was expensive and unreliable - at least until the early 2000s.
In general, it's a bit like it was with floppies in the very beginning (my fist 10-pack did cost me 80 Marks), where copying to tape was prohibitive, just to be repeated in the mid 2000s when DVDs where rolled out when CD writing became cheap and reliable.
The Long Read
To start with, CD production was, at that time mostly confined to 'real' CDs and in a factory setup, as writeables not only where prohibitively expensive, but also did not always produce a result readable on (cheap) CD-ROM drives. When looking at prices it may be helpful to see that CDs weren't a thing for computer users until 1991's Soundblaster Pro got sold with a special interface for the Mitsumi CD Drive (and often packaged with one). Keep in mind this is about games. Here only mass market counts, not some high power users like many of us may have been.
CD as a game medium only started after that. At this time their sheer size was the most relevant copy protection. Game companies where often accused of adding unnecessary content to explicitly bloat the size of data used - including lots of clips and cut scenes together with purposely non-compressed data, to make it hard to copy games onto disks. In fact, numerous game reviews did test games that where delivered as a 'basic version' on floppies as well as their 'enhanced' CD counterpart and found that the CD-version didn't add anything useful.
While the cost for a CD writer in the mid-90s dropped close to 1000 USD, that's still above the usual juvenile threshold. And writers still suffered from compatibility issues with regular (cheap) players. Even if daddy did invest in the son's future by buying a PC with a well-fitted SCSI based machine including a writer, media prices were comparably high. When bought as a 25 pack, a single writeable CD was close to 10 USD. Thus copying did still bear high cost.
The time argument doesn't matter so much on the hobbyist side, as waiting an hour for the CD to complete (*1) could be filled with whatever teenagers do anyway :))
It doesn't matter on a 'professional' scale as well. Just remember all the copying that was going on during early 80s with pirated video tapes. People had a dozen VCRs in their garage, hooked up to a 'Master' and copied in real time. More or less the same way with CDs in the mid to late 90s. Machines did run multiple drives at once, just this time the master was a hard disk. 8-12 drives per machine and 2-3 machines did make it a full time job. Heck, they even developed robotic handling (*2). Again, the media price was more of a limiting factor (and readability) than time.
At a 'professional' level the danger for game companies was rather on the side of real CD manufacturing, as a standard CD manufacturing machine could be bought for less than some high-end PCs. The only costly part was acquiring the glass master.
*1 - Whoever could at that time cash out for a writer, would always also buy a simple reader that goes with it, so transfer can be done in a single run.
*2 - A good friend of mine was into this. He did the garage business for video tapes in the early 80s, and after some ... err ... troubles, slipped again into it with CDs. He had an awesome setup with two small robot arms handling the CDs of two drives connected to a single machine. That way he could have it run 24/7. He said it was good business, but I still think if he had invested the time it took to develop the software and handling mechanics in a start-up for robotics, he'd be a wealthy man by now. I just wish I had secured that machinery for my collection.
In addition to Stephen Kitt's answer:
- Copy protection existed long before CDs; most of these techniques could be applied to CD-ROMs too. Many games that were shipped on floppies had copy protection - sometimes a question in the beginning that asked you to look at the booklet it came with, Monkey Island had a disk with 2 parts that you needed to set according to instructions to get the right answer.
- It was obviously much harder to distribute pirated copies then than it is now. The internet wasn't a thing so you were fairly limited in what you could acquire. Generally one of your friends had to actually buy the game. Sharing of games was fairly common, though.
A quick review of the games consoles suggests that the Saturn and PlayStation were the first to implement copy protection as a firmware-level feature; between the 3DO, Mega CD and PC Engine there are some measures to ensure games are from licensed developers but no built-in protection against copies. I was also unable to find any evidence of software for those platforms implementing its own protection schemes.
So the answer seems to be: during the first wave of dedicated games hardware, no substantial effort was put into copy protection; one can guess that it just wasn't considered to be likely to be feasible within the usual commercial lifetime of a console prior to about 1994.
Afaict in the early days of CD-Roms, copying of the CD itself was less of a concern than running the game without the CD. It was only at the tail end of the 1990s that CD burners really went mainstream.
So when looking for their data files, (or in at least one case, a marker file full of garbage), games would look specifically for CD drives. If you copied the file to the same location on a hard drive, the game would just ignore it.
I am not 100% sure if the original question was just about the first CD-ROM games (so, I guess we're talking about the Late 1980's/Early 1990's?), but if the late 1990's are still acceptable, some games utilized an Illegal TOC as protection scheme.
The Table of Contents (TOC) is basically the description of the CD layout, that is, the position of individual tracks on the CD. Normally, you have 1 data track and then between 0 and 98 audio tracks, but some games pretended that there was a second data track (which didn't actually exist on the CD and might have pointed at the first data track) or other things that are just not "valid" according to the standards, but worked in consumer drives. Games could then check for the expected layout, which wasn't easy to duplicate at the time.
Some other games manipulated the file system (ISO 9660) to make it look like there was a HUGE file (multiple Gigabytes) on the CD. That file of course didn't really exist, but you couldn't copy it or create a file that large with regular tools on a copied CD, so Games would check for the presence of such a file of an expected size as a way to check for the original CD.
All these methods tried to achieve the same goal: Create a CD-ROM that was technically not valid according to the standards, but still worked properly. In many ways, this is similar to older Floppy-Disk protection schemes that also deliberately used bad sectors or illegal directory contents.
Most CD Burning tools at the time were made to create "proper" CD-ROMs with a valid TOC/File system structure, so they couldn't replicate these "deliberately defective" discs. This led to a bit of an arms race for a bit as better burning tools came along to try to duplicate these discs. I believe that a lot of the early CD Burners couldn't burn these kinds of invalid structures either, even if the software was otherwise capable of doing it.
However, it didn't take too long until a plethora of tools (I think CloneCD and CDRWIN were the original tools for duplication - along with new imaging formats like bin/cue and mds because the common .iso format couldn't duplicate the structure of the disc, just the data track) and most newly sold CD Burners could burn "invalid" structures without an issue. It's also when we learned about the difference between burning TAO (Track-at-Once) and DAO (Disc-at-Once), the latter being the required mode to create exact duplicates.
(Note: An Illegal TOC/File system is not to be confused with a regular oversize CD. Initially, CD-Rs had a capacity of 650 MiB. Some games were larger than that - for example, Anno 1602 was 684 MiB, and AFAIK you couldn't just omit stuff from the CD to get it to fit on a 650 MiB disc because the game checked. This type of protection scheme didn't last long because 700 MiB CD-Rs came along pretty quickly.)