There are numerous techniques, but the easiest to understand is the PLL synthesizer. Motorola made a series of PLL-based frequency synthesizers, MC14515x, which had a number of versions. These, like any frequency synthesizer, take a reference frequency from a crystal timebase, and a VCO, and divide them down, separately of course, to a common nominal frequency, and then phase-compare them in order to generate an error voltage with which to steer the VCO to the precise frequency required from the divider. By using a high enough crystal frequency and a sufficiently flexible divider chain one can wind up with a very good approximation of the desired frequency. That's probably as good an approach as any.

Output filtering to limit the harmonic content wouldn't be a bad thing either.

There's nothing anywhere that limits you to a 12.000 MHz oscillator frequency in an 8051, BTW. You can use 440 KHz or 8.8 MHz if you like.

RE