System information

All simple character I/O operations are assumed to be performed in ASCII, upper- and
lower-case, with high-order (parity bit) set to zero. An end-of-file condition for an input device is
given by an ASCII CTRL-Z (1AH). Peripheral devices are seen by CP/M as logical devices and
are assigned to physical devices within the BIOS.
To operate, the BDOS needs only the CONST, CONIN, and CONOUT subroutines. LIST,
PUNCH, and READER can be used by PIP, but not the BDOS. Further, the LISTST entry is
currently used only by DESPOOL, the print spooling utility. Thus, the initial version of CBIOS
can have empty subroutines for the remaining ASCII devices.
The following list describes the characteristics of each
device.
-CONSOLE is the principal interactive console that communicates with the operator and
it is accessed through CONST, CONIN, and CONOUT. Typically, the CONSOLE is a
device such as a CRT or teletype.
-LIST is the principal listing device. If it exists on the user's system, it is usually a
hard-copy device, such as a printer or teletype.
-PUNCH is the principal tape punching device. If it exists, it is normally a high-speed
paper tape punch or teletype.
-READER is the principal tape reading device, such as a simple optical reader or teletype.
A single peripheral can be assigned as the LIST, PUNCH, and READER device simultaneously.
If no peripheral device is assigned as the LIST, PUNCH, or READER device, the CBIOS gives
an appropriate error message so that the system does not hang if the device is accessed by PIP or
some other user program. Alternately, the PUNCH and LIST routines can )'ust simply return,
and the READER routine can return with a 1 AH (CTRL-Z) in register A to indicate immediate
end-of-file.
For added flexibility, you can optionally implement the IOBYTE function, which allows
reassignment of physical devices. The IOBYTE function creates a mapping of logical-to-physical
devices that can be altered during CP/M processing, see the STAT command in Section 1.6.1.
6.6 BIOS Entry Points CP/M Operating System Manual
6-17