User`s guide

The Transient Program Area, TPA, is the region of memory where transient pro-
grams execute. The CCP also executes in this region.
The Console Command Processor, CCP, is not an operating system module, but is
a system program that presents a human-oriented interface to CP/M 3 for the user.
The Page Zero region is not an operating system module either, but functions
primarily as an interface to the BDOS module from the CCP and transient programs.
It also contains critical system parameters.
1.3 System Component Interaction and Communication
This section describes interaction and communication between the modules and
regions defined in Section 1.2. The most significant channels of communication are
between the BDOS and the BIOS, transient programs and the BDOS, and transient
programs and RSXS.
The division of responsibility between the different modules and the way they
communicate with one another provide three important benefits. First, because the
operating system is divided into two modules-one that is configured for different
ardware environments, and one that remains constant on every computer-CP/M 3
software is hardware independent; you can port your programs unchanged to differ-
ent hardware configurations. Second, because all communication between transient
programs and the BDOS is channeled through Page Zero, CP/M 3 transient programs
execute, if sufficient memory is available, independent of configured memory size.
Third, the CP/M 3 RSX facility can customize the services of CP/M 3 on a selective
basis.
1.3.1 The BDOS and BIOS
CP/M 3 achieves hardware independence through the interface between the BDOS
and the BIOS modules of the operating system. This interface consists of a series of
entry points in the BIOS that the BDOS calls to perform hardware-dependent primi-
tive functions such as peripheral device VO. For example, the BDOS calls the CONIN
entry point of the BIOS to read the next console input character.
A system implementor can customize the BIOS to match a specific hardware envi-
ronment. However, even when the BIOS primitives are customized to match the host
computer's hardware environment, the BIOS entry points and the BDOS remain
constant. Therefore, the BDOS and the BIOS modules work together to give the CCP
and other transient programs hardware-independent access to CP/M 3's facilities.
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1.2 System Components CP/M 3 Programmer's Guide