carry bit streams, encoded in a different form to allow the half-length one bits observed in commercial recordings to be represented. There are some modal variables affecting the interpretation of these chunks: the baud rate, 1200 baud for Acorn signals or 300 baud for KCS; the exact carrier frequency, which determines the playing time of the reconstructed tape; and the phase of the signal. The latter two may change within a published recording, and their absolute values depend on the tape player, amplifier and sound card used to digitise the signal. A UEF file can contain markers to separate the tapes of a multiple-tape distribution, and the sides of each tape; positions of interest within each side can also be marked. Discs are stored as raw sector dumps of each surface, along with their geometry and a byte identifying the file system. Previous versions of the specification had provisions to encode discs at the byte stream level, or the magnetic domain level. With SSD and DSD sector dumps serving standard BBC discs well, and the mature FDI format catering for copy-protected software, the disc image function of UEF is little used. Sideways ROMs are likewise stored as raw data, plus an indication of their purpose and a ROM slot recommendation. Again the user base prefers bare ROM dumps for archival. State snapshot UEF files include standardised chunks to store the major portions of an Acorn Electron or BBC Micro's state: main, shadow and expansion bus memory, the CPU and the WD1770 floppy drive controller; also the Electron ULA and the Slogger Master RAM Board, a common Electron add-on. A patch memory chunk rewrites a block of memory at any address, allowing the UEF format to package pokes. To store state elements not accommodated in the standard chunks, emulators can define their own chunks. A private use area of chunk IDs is reserved for this or any other purpose, although some emulators save state under invalid chunk IDs in the public space. Multiplexed data is an extension for emulators, used by ElectrEm but without a published specification: One salient application mentioned by Harte is to superimpose "new graphics on old games", and a single example, a 256-colour enhanced Daredevil Dennis, is available from StairwayToHell.com to run in ElectrEm. Multiplexed data chunks are intended to follow ordinary data chunks in any of the above classes, supplementing the data. Their contents are not