A memory chip, also called an eidetic chip was a (rather sizeable) bit of bio-electronics that was implanted in a person's brain to aid their memory. The only kind of memory chip specifically described in the Vorkosigan Saga was the Illyrican-made chip in Simon Illyan's head; it didn't catch on as a technology due to the 90% probability that the recipient of such a chip would develop schizophrenia as a result.
Illyan's chip was described as producing a second, parallel set of memories for its owner; though only for sight and sound – smell, taste, touch, and any emotional overtones were omitted from its records. By the time of the events of Memory, Illyan was the one person in the Nexus with the longest baseline for having the chip in place, namely 35 years. It is unknown if there were other successful installations of these chips or of similar technology.
Most books of the Vorkosigan Saga mention Illyan's chip, but the one with the most useful information is: