attached to the peripheral bus, such as printers or disk drives. In the case of a disk drive, the device number refers to the unit's controller, not the drive mechanism(s) within the unit. By convention, the first disk drive unit on a system has device number 8, the second drive, if present, 9, etc., up to a maximum number of 15 (when eight disk drives are attached). The device number scheme was derived from the IEEE-488 (or general purpose interface GPIB) bus that was used with the Commodore PET/CBM models. In assembly language programs, this value is often referred to as FA or PA (physical address), again from the mnemonic for the memory location where the device number is stored. 4 This parameter, the secondary address, which may range from 0 to 15 inclusive, refers to a specific communication channel established with the device's controller and is passed to the device when it is commanded to "talk" or "listen" on the peripheral bus. As with the file number, the secondary address is determined by the programmer and must be unique for the device in question. The range 0 to 14 inclusive is used for passing data to or from the device, whereas 15, referred to as the "command channel", is used to issue commands to the device's controller (such as to rename a disk file), if the device is able to support such an operation. In disk drives, secondary addresses 0 to 14 inclusive are mapped to buffers within the controller, hence establishing communication with a specific file on a specific disk; since as mentioned above the drive does not know about the file number, it can only use the secondary address to make a difference between several files that are open at the same time. On the other hand, the host operating system is agnostic about the secondary address; it is transmitted to the drive on every access to the file, but not otherwise used by the host. In assembly language programs, this value is often referred to as SA (secondary address). COMMAND STRING The "0:ADDRESSBOOK,S,W" parameter is officially referred to in Commodore documentation as the command string and is interpreted by the controller of the device being accessed. In the case of a disk drive unit, the formal command string structure consists of the drive mechanism number (0:, not to be confused with the device number), filename (ADDRESSBOOK),