You run runtime.exe in <path-to-game>
This access the CD and loads clcd32.dll, clcd16.dll.. and almighty secdrv.sys.
And at this point, some very neat magic comes into play.
Of course it's an old as the hills 32 bit driver.
But even if compiled in October 1999, it was ahead of its times back then, thanks to being WDM compliant.
The super secure and finicky nature of copy protection is probably what undermined forward compatibility, though Microsoft worked around this issue pretty well.
So well, it does even work out of the box with 64 bit OSs.
When the game tries to install/load the driver, drvmgt.dll transparently pass the request to the system built-in one (the very thing removed in W10 btw)
You can notice a "Security Driver" is always running, after you have enabled "Show hidden devices" in device manager under "Non Plug and Play driver" node
Stopping this service prevents the game from running (the specific reason can be read in "Event viewer" logs).
After all this tinkering, for no reason HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\C07ft5Y registry key is created.
Then clokspl.exe starts and checks 00000410.256. I don't know why this, and not the other similar files in the CD root, but I'd bet the registry key does.
Last, the same runtime.exe process we ran at the beginning executes runtime.icd (from hdd), which is the actual executable that all this scrambling allowed to decode.
for some reason %WINDIR%\win.ini is read many and many times
And this above was the first CD check to pass DRM check.
The second one being after the loading screen.
It has nothing to do with SafeDisc, it's present even in unpacked exes and it doesn't even technically require CD2.
It only seems some kind of game logic that controls if there's any disk in drives with either CD1, CD2 or CD3 placeholders.
Then, here game menu finally appears.
Take note that contrarily to digital releases, retail one won't accept "file not found" as an answer. it has not patch.dll redirect?
For example, if intro clips are removed, it will check if the files are available on the mounted disks (as in a minimal or typical installations)
If even this fails, an error screen is returned.
