by the drive's microprocessor. Very few programs ever made use of this file type. Some applications that use non-standard low-level disk structures save data in USR format, which came to be considered a sort of "leave me alone, don't try to copy or delete" indication to the user. Most notably, GEOS' "VLIR" files show up as USR files. DEL An undocumented internally used file type similar in structure to a sequential file. Creation of this file type must be accomplished by direct manipulation of the disk directory. The presence of an asterisk (*) prepended to the file type in a directory listing (for example, *SEQ) indicates that the file was not properly closed after writing. When the drive is commanded to close a file that has been opened for writing, the associated buffer is flushed to the disk and the block availability map (BAM) is updated to accurately reflect which blocks have been used. If a program crash or other problem (such as the user removing the disk while a file is open) results in an "orphan file", also referred to as a "poison" or "splat" file, buffers are not flushed and the BAM will not accurately reflect disk usage, putting the disk at risk of corruption. A poison file generally cannot be accessed (but can be opened in "modify" mode), and an attempt to use the DOS scratch command to delete the file may cause filesystem corruption, such as crosslinking. The only practical method of removing one of these files is by opening the file in "modify" mode (and fixing it), or by validating the disk (see the DOS validate command below), the latter which rebuilds the BAM and removes poison file references from the directory. The infamous save-with-replace bug could result in creation of splat files. *DEL is a special type written into the on-disk directory entry of files that have been deleted. Such files are not shown in a normal directory listing, and their data blocks and directory entries will be reused by files that are subsequently created. Some utility programs allow the "un-deletion" of such files if their data blocks and directory entries haven not yet been overwritten by other files. DEL files are commonly used to insert banners or comment sections into a directory listing. File types with < after them (for example, PRG<) are "locked", and cannot be deleted—they can be opened for reading,