Statistics driver

Doom incorporates the ability to integrate with an external statistics driver: in this setup, the Doom engine is invoked by an external statistics program. At the end of each level, Doom passes statistics about the level back to the statistics program.

No known statistics drivers have ever been written.

Technical
The system works using the statcopy command line argument. The statistics program passes the address in memory of a structure in which to place statistics. For example, the following would instruct Doom to place statistics in the memory location "1234567":

doom -statcopy 1234567

At the start of each level, statistics are then copied into this buffer. There is no trigger activated to inform the statistics driver that this has occurred. The statistics driver would have to set up its own timer to peridically check the buffer for new information.

This system is only possible because of DOS's lack of memory protection: this kind of sharing of memory is not so easily possible in modern operating systems.