System information

6.9 Reserved Locations in Page Zero
Main memory page zero, between locations 0H and 0FFH, contains several segments of code and
data that are used during CP/M processing. The code and data areas are given in the following
table.
Table 6-6. Reserved Locations in Page Zero
Locations Contents
0000H-0002H Contains a jump instruction to the warm start entry location 4A03H+b.
This allows a simple programmed restart (JMP 0000H) or manual restart
from the front panel.
0003H-0003H Contains the Intel standard IOBYTE is optionally included in the user's
CBIOS (refer to Section 6.6).
0004H-0004H Current default drive number (0=A,...,15=P).
0005H-0007H Contains a jump instruction to the BDOS and serves two purposes: JMP
0005H provides the primary entry point to the BDOS, as described in
Section 5, and LHLD 0006H brings the address field of the instruction to
the HL register pair. This value is the lowest address in memory used by
CP/M, assuming the CCP is being overlaid. The DDT program changes
the address field to reflect the reduced memory size in debug mode.
0008H-0027H Interrupt locations I through 5 not used.
0030H-0037H Interrupt location 6 (not currently used) is reserved.
0038H-003AH Restart 7; contains a jump instruction into the DDT or SID program when
running in debug mode for programmed breakpoints, but is not otherwise
used by CP/M.
003BH-003FH Not currently used; reserved.
6.7 A Sample BIOS CP/M Operating System Manual
6-26