Chipwave Synthesis.

Phill Phelps has written a useful 4 channel chiptune emulator using PureData:

A very complex sequence of sound texture changes can be programmed very easily, to create sounds similar to Nintendo Gameboy, NES, Commodore C64 and Atari.
FEATURES:
* All waveforms have MIDI controlled pitch, they are not velocity sensitive.
* The amplitude envelopes for each channel are ADSR type, and can be edited on a per-channel basis.
* The square waveform has an adjustable pulse width using the MOD WHEEL MIDI CONTROLLER.
* The noise is a sample-and-hold variety, with a variable sample rate, to generate apparent rising and falling of noise "pitch".

As well as this patch, he has also written a paper titled "A modern implementation of chiptune synthesis". The following is an excerpt from the summary.

"A chiptune can be broadly categorised as a piece of music produced by sound chips in home computer/ games systems popular in the 1980s into the 1990s. The synthesis techniques often employed in chiptunes involve the careful control of the hardware features available in the sound chips. Such chips generally featured a limited number of simple oscillators with simple waveforms such as square, triangle, pseudorandom noise, and so on. In order to create more interesting sounds, chiptune composers rely on software synthesis structures that are used to configure the sound chip to alternate between waveforms, and to alter the pitch of the oscillators...

...The report presents the detailed research into the sound hardware, and software used for composing chiptunes, exposing the differences in user interface design between modern composition software tools such as MIDI sequencers and historic composition tools such as trackers. Several pieces of historic software are examined; with elements of each user interface being assessed for inclusion in a modern software synthesiser user interface. The report also explains in detail the internal hardware structure of several popular sound chips (such as the General Instruments AY-3-8910) and examines several examples of software structures used to control the hardware synthesis structures inside the chips."