The original, and most complete, description of µCode is a chapter in my
PhD thesis:
Gian Pietro Picco. Understanding, Evaluating, Formalizing, and Exploiting Code Mobility. Ph.D. Thesis, February 1998.
This chapter has been published, somehow shortened and adapted, as:
Gian Pietro Picco. "µCode: A Lightweight and Flexible Mobile Code Toolkit". In Mobile Agents, Proceedings of the 2ndInternational Workshop on Mobile Agents 98 (MA'98), Stuttgart (Germany), K. Rothermel and F. Hohl eds., September 1998, Springer, Lecture Notes on Computer Science vol. 1477, pp. 160-171.
Although the API and design of µCode have changed since then, the motivating criteria and approach are still those described in the aforementioned documents.
Also, prerelease versions of µCode have been used to develop Avican,
a mobile videoconference server described in:
Mario Baldi, Gian Pietro Picco, and Fulvio Risso. "Designing a Videconference System for Active Networks". In Mobile Agents, Proceedings of the 2nd International Workshop on Mobile Agents 98 (MA'98), Stuttgart (Germany), K. Rothermel and F. Hohl eds., September 1998, Springer, Lecture Notes on Computer Science vol. 1477, pp. 273-284.
A revised version of the paper appeared also in Journal of Personal Technologies, (1):4, December 1998, as part of a selection of the best papers from MA'98.
All the documents above are available at Gian Pietro Picco's home page.