On a cassette-only machine, 0x0E00 is the start of user program memory. With OS extension ROMs fitted such as the a filing system ROM, more memory is allocated above this point; DFS ROMs generally use another 2.75 KiB to cache the disc catalogue and manage random access buffers. A network filing system ROM (for Econet) allocates another 0.5 KiB on top of this. This is a serious problem because MOS does not support relocation of machine code, which must be run from the address at which it was assembled, so some programs which assumed a fixed start of user program memory could overwrite MOS workspace. The problem was alleviated in versions 3 to 5 by allowing ROMs to allocate workspace in an alternative RAM bank at 0xC000 to 0xDFFF which was present in Master series computers, though old ROMs could continue to allocate blocks of main memory.