The DES example is taken from the book "Applied Cryptography" 
by Bruce Schneier.  It is adapted for the Keil C51-Compiler.
The example code is not optimized for code or data space and
shows the processing from long value arithmetics (56 bit) on
an 8051 microcontroller.

The DES program is available in different targets:
  Simulator:  DES example in TINY model for Simulation