DIR, ERA, REN, SAVE, TYPE, and USER as built-in commands. Transient commands in CP/M include ASM, DDT, DUMP, ED, LOAD, , PIP, STAT, SUBMIT, and SYSGEN. CP/M Plus (CP/M Version 3) includes DIR (display list of files from a directory except those marked with the SYS attribute), DIRSYS / DIRS (list files marked with the SYS attribute in the directory), ERASE / ERA (delete a file), RENAME / REN (rename a file), TYPE / TYP (display contents of an ASCII character file), and USER / USE (change user number) as built-in commands: CP/M 3 allows the user to abbreviate the built-in commands. Transient commands in CP/M 3 include COPYSYS, DATE, DEVICE, DUMP, ED, GET, HELP, HEXCOM, INITDIR, LINK, MAC, PIP, PUT, RMAC, SET, SETDEF, SHOW, SID, SUBMIT, and XREF. Basic Disk Operating System The Basic Disk Operating System, or BDOS, provided access to such operations as opening a file, output to the console, or printing. Application programs would load processor registers with a function code for the operation, and addresses for parameters or memory buffers, and call a fixed address in memory. Since the address was the same independent of the amount of memory in the system, application programs would run the same way for any type or configuration of hardware. Basic Input Output System The Basic Input Output System or BIOS, provided the lowest level functions required by the operating system. These included reading or writing single characters to the system console and reading or writing a sector of data from the disk. The BDOS handled some of the buffering of data from the diskette, but before CP/M 3.0 it assumed a disk sector size fixed at 128 bytes, as used on single-density 8-inch floppy disks. Since most 5.25-inch disk formats used larger sectors, the blocking and deblocking and the management of a disk buffer area was handled by model-specific code in the BIOS. Customization was required because hardware choices were not constrained by compatibility with any one popular standard. For example, some manufacturers used a separate computer terminal, while others designed a built-in integrated video display system. Serial ports for printers and modems could use different types of UART chips, and port addresses were not fixed. Some machines used memory-mapped I/O instead of the 8080 I/O address space. All of these variations in the hardware were concealed from other modules of the system by use of the BIOS,