To perform a lookup for a group of plural portions of data in a cache together, a first part of an identifier for a first one of the portions of data in the group is compared with corresponding first parts of the identifiers for cache lines in the cache, the first part of the identifier for the first one of the portions of data in the group is compared with the corresponding first parts of the identifiers for the remaining portions of data in the group of plural portions of data, and a remaining part of the identifier for each portion of data is compared with the corresponding remaining parts of identifiers for cache lines in the cache. It is then determined whether a cache line for any of the portions of data in the group is present in the cache, based on the results of the comparisons.