many disk commands it is customary to open a file to the device's command channel at the start of the program and not close it until the program has finished. Commodore BASIC versions 4.0 and later provide a pseudo-variable referred to as DS$ that may be used to retrieve drive status in lieu of the above code. This reserved variable is not available on earlier versions of BASIC, so the command channel must be manually read as demonstrated above. Note that immediately after power-on or reset, the DOS revision will be returned. For example, a 1541 will return . Error code 73 is common to all drive models and may be used to determine if the drive has been reset to its power-on state. DOS commands There are also a command for seeking in RELative type files (RECORD#), several block-level direct-access commands (BLOCK-READ, BLOCK-WRITE, BUFFER-POINTER), block management (BLOCK-ALLOCATE, BLOCK-FREE), drive memory manipulation and execution of program code on the drive's processor (MEMORY-WRITE, MEMORY-READ, MEMORY-EXECUTE, BLOCK-EXECUTE) and user-definable functions (USER and & commands). Some of the theoretically user-definable functions were rededicated for accessing new functionality in DOS versions after 1.0. References Notes Immers, Richard; Neufeld, Gerald G. (1984). Inside Commodore DOS. The Complete Guide to the 1541 Disk Operating System. DATAMOST, Inc & Reston Publishing Company, Inc. (Prentice-Hall). . Englisch, Lothar; Szczepanowski, Norbert (1984). The Anatomy of the 1541 Disk Drive. Grand Rapids, MI: Abacus Software (translated from the original 1983 German edition, Düsseldorf: Data Becker GmbH). . Lundahl, Reijo (1986). 1541-Levyasema. Amersoft. Commodore 64 software Commodore DOS VIC-20 software