Describe your circuit a bit better.

Is there a series resistor to the base of the Darlington and what value? Is it an ordinary Darlington or one with additional components integrated? Is there any resistance in the emitter circuit? Is there anything in the collector circuit to the +12 apart from the motor coil? Critically what is the voltage on the 89C52 output pin when it's trying to turn on the Darlington?

And what do you mean by "i'd try to use a emulator to drive the circuit"? Is the emulator emulating a stepper motor or are we talking about a real component?

Note that an ordinary Darlington pair represents two diode voltage drops before anything will start to flow through the base emitter junctions at all. That's about 1.1 - 1.4v without anything else in circuit.