Commodore 128 Programmer's Reference Manual page 497

n
n
CP/M 3.0 ON THE COMMODORE 128 487
The BDOS contains a set of functions that the CCP and applications programs call
to perform disk and character input and output operations.
The BIOS contains a Jump Table with a set of thirty-three entry points that the
BDOS calls to perform hardware-dependent primitive functions, such as peripheral
device I/O. For example, CONIN is an entry point of the BIOS called by the BDOS to
read the next console input character.
Similarities exist between the BDOS functions and BIOS functions, particularly
for simple device I/O. For example, when a transient program makes a console output
function call to the BDOS, the BDOS makes a console output call to the BIOS. In the
case of disk I/O, however, this relationship is more complex. The BDOS file may make
many BIOS function calls to perform a single BDOS file I/O function. BDOS disk I/O is
in terms of 128-byte logical records. BIOS disk I/O is in terms of physical sectors and
tracks.
The System Control Block (SCB) is a 100-byte (decimal) CP/M 3.0 data structure
that resides in the BDOS system component. The BDOS and the BIOS communicate
through fields in the SCB. The SCB contains BDOS flags and data, CCP flags and data,
and other system information, such as console characteristics and the current date and
time. You can access some of the System Control Block fields from the BIOS.
Note that the SCB contains critical system parameters that reflect the current state
of the operating system. If a program modifies these parameters, the operating system
can crash. See Section 3 of the DRI CPIM Plus System Guide and the description of
BDOS Function 49 in the DRI CPIM Plus Programmer's Guide for more information on
the System Control Block.
Page zero is a region of memory that acts as an interface between transient
programs and the operating system. Page zero contains critical system parameters, includ
ing the entry to the BDOS and entry to the BIOS Warm BOOT routine. At system
start-up, the BIOS initializes these two entry points in page zero. All linkage between
transient programs and the BDOS is restricted to the indirect linkage through page zero.
1 CP/M 3.0 BIOS OVERVIEW
This section describes the organization of the CP/M 3.0 BIOS and the BIOS jump
vector. It provides an overview of the System Control Block, followed by a discussion
of system initialization procedures, character I/O, clock support, disk I/O, and memory
selects and moves.
ORGANIZATION OF THE BIOS
The BIOS is the CP/M 3.0 module that contains all hardware-dependent input and
output routines. To configure CP/M 3.0 for a particular hardware environment, the
sample BIOS supplied with the CPIM Plus System Guide must be adapted to the specific
hardware of the target system (the Commodore 128).
The BIOS essentially is a set of routines that performs system initialization,