Turbo Debugger Dosbox: Cracking Copy Protection in Descent II Setup Program

When it comes to cracking copy protection in old games, one of the most challenging tasks is finding where the code resides in the game executable that performs the check. In this case, I'm targeting the CD-based copy protection in the Descent II setup program.

To defeat such a protection without making any changes to the rest of the program, you need to find the place where it happens and modify the code so that regardless of the outcome of the check, the same 'Continue execution' code path is taken. This involves understanding how conditional jumps are translated into assembly or machine code.

In the case of Descent II, the CD check serves as a hassle for legitimate owners by forcing them to keep the CD in the drive at all times to play the game or run the setup program. However, some games like Descent II allow a full installation, where all the game content is copied onto the hard drive to speed up gameplay.

In the age of DOS gaming, CD-based copy protection was often simple – the program would try to access the CD drive during its startup and abort if it didn't find what it expected. This can be frustrating for gamers who want to play their favorite games without having to search for their CDs every time.

To crack the Descent II setup program and remove the CD check, I decided to use a turbo debugger like Dosbox to analyze the code and identify where the protection is implemented. With this approach, you can 'cut off' one of the branches by replacing a conditional jump with an unconditional one or removing it altogether using no-operation (NOP) CPU instructions.

By modifying the code in this way, you can bypass the CD check without affecting the rest of the program. This technique is effective in defeating copy protection mechanisms that rely on CD access during startup.

Overall, cracking copy protection in old games like Descent II requires a combination of understanding how conditional jumps are translated into assembly or machine code and using tools like turbo debuggers to analyze the code. With patience and persistence, you can remove these annoying checks and enjoy your favorite games without any hassle.

Turbo debugger dosbox