User`s guide

However, use caution when you access the SCB through Function 49 for two
reasons. First, the SCB is a CP/M 3 data structure. Digital Research's multi-user
operating system, MP/M, does not support BDOS Function 49. Programs that access
the SCB can run only on CP/M 3. Secondly, the SCB contains critical system param-
eters that reflect the current state of the operating system. If a program modifies these
parameters illegally, the operating system might crash. However, for application writ-
ers who are writing system-oriented applications, access to the SCB variables might
prove valuable.
For example, the CCP default drive and current user number are maintained in the
System Control Block. This information is displayed in the system prompt. If a
transient program changes the current disk or user number by making an explicit
BDOS call, the System Control Block values are not changed. They continue to reflect
the state of the system when the transient program was loaded. For compatibility
with CP/M Version 2, the current disk and user number are also maintained in
location 0004H of Page Zero. The high-order nibble contains the user number, and
the low-order nibble contains the drive.
Refer to the description of BDOS Function 49 in Section 2.5 for more information
on the System Control Block. The SCB fields are also discussed in Appendix A.
End of Section 1
1-28
1.7 System Control Block CP/M 3 Programmer's Guide