uses a ``virtual memory'' feature. All file characters are kept in disk files, and buffers of the most recently used blocks are maintained in 's memory. The ``buffers'' option allows you to set the number of buffers uses. The more buffers, the less often has to read and write the disk. also uses a data structure called a ``piece table'' to keep track of the edits made in a file. That is, represents the file as a sequence of one or more ``pieces''. Each piece is a span of characters from a disk file. Combined with the virtual memory feature, this means that spans of file characters are contained in 's disk buffers.