I don't read a lot of modern patents, for the obvious reasons: there are legal risks, and the patent texts are useless even if the invention isn't. But there used to be a time when a software patent wasn't necessarily an obfuscated hodgepodge of legalese with no apparent technical content.
Consider for example the (expired, so should be safe to read) mostly-copying GC patent: http://www.google.com/patents/US4907151
A system description doesn't get much better than that. If somebody couldn't implement a GC based on that patent, they're not a competent systems programmer. (It was also definitely novel. Non-obviousness is harder to argue 25 years after the fact).