The data stored in a content-addressable memory (CAM) are accessed bas
ed on their contents, rather than their location, and this functionali
ty is useful in many applications, including databases, table look-up,
and associative computing. The central building blocks of these memor
ies are the core cells, and this paper surveys 14 such cells, primaril
y from a qualitative perspective. The cells date from 1970, and many i
nnovations have been introduced since then, both in circuit design and
fabrication technology. Among the cells described, 11 employ static s
torage and four use dynamic storage; 14 can be implemented in CMOS, wh
ile one requires BICMOS technology. A discussion follows, highlighting
significant design improvements, advocating particular cells for gene
ral-purpose use, and suggesting avenues for future research.