Editing CP/M (section)

=== Basic Input Output System ===
[[File:CP⁄M Ad, Dec 11, 1978.png|thumb|CP/M advertisement in the 11 December 1978 issue of ''[[InfoWorld]]'' magazine]]

The Basic Input Output System or BIOS,<ref name="Kildall_1975_BDOS"/><ref name="Kildall_1980_CPM"/> provides the lowest level functions required by the operating system.

These include reading or writing single characters to the system console and reading or writing a sector of data from the disk. The BDOS handles some of the buffering of data from the diskette, but before CP/M&nbsp;3.0 it assumes a disk sector size fixed at 128 bytes, as used on [[single-density]] 8-inch floppy disks. Since most 5.25-inch disk formats use larger sectors, the blocking and deblocking and the management of a disk buffer area is handled by model-specific code in the BIOS.

Customization is required because hardware choices are not constrained by compatibility with any one popular standard. For example, some manufacturers designed built-in integrated video display systems, while others relied on separate computer terminals. Serial ports for printers and modems can use different types of [[UART]] chips, and port addresses are not fixed. Some machines use memory-mapped I/O instead of the 8080 I/O address space. All of these variations in the hardware are concealed from other modules of the system by use of the BIOS, which uses standard entry points for the services required to run CP/M such as character I/O or accessing a disk block. Since support for serial communication to a modem is very rudimentary in the BIOS or may be absent altogether, it is common practice for CP/M programs that use modems to have a user-installed overlay containing all the code required to access a particular machine's serial port.