User`s guide

Appenix A
System Control Block
The System Control Block (SCB) is a CP/M 3 data structure located in the BDOS.
CP/M 3 uses this region primarily for communication between the BDOS and the
BIOS. However, it is also available for communication between application pro-
grams, RSXS, and the BDOS. Note that programs that access the System Control
Block are not version independent. They can run only on CP/M 3.
The following list describes the fields of the SCB that are available for access by
application programs and RSXS. The location of each field is described as the offset
from the start address of the SCB (see BDOS Function 49). The RW/RO column
indicates if the SCB field is Read-Write or Read-Only.
Table A-1. SCB Fields and Definitions
Offset RW/RO Definition
00 - 04 RO Reserved for system use.
05 RO BDOS Version Number.
06 - 09 RW Reserved for user use. Use these four bytes for your
own flags or data.
0A - 0F RO Reserved for system use.
10 - 11 RW Program Error Return Code. This 2-byte field can be
used by a program to pass an error code or value to a
chained program. CP/M 3's conditional command facil-
ity also uses this field to determine if a program exe-
cutes successfully. The BDOS Function 108 (Get/Set
Program Return Code) is used to get/set this value.
12 - 19 RO Reserved for system use.
A-1