HP Vectra Technical Reference Manual page 381

CMOS Address Application
OFH
*Shut down status byte
10H
Flexible disc drive type (A and B)
11H
Reserved
12H
Fixed disc type (C and D)
13H
Reserved
14H
Equipment byte
15H
Low base memory
16H
High base memory
17H
Extended memory size (low byte)
18H
Extended memory size (high byte)
19-20H
Reserved
21-27H
* Reserved
28H
*HP checksum for bytes 29, 2A, 28, 2C
29-2BH
0/0
* Reserved
2CH
0/0
* Reserved
2DH
* Reserved
2E-2FH
*2-byte industry standard CMOS checksum for bytes 1OH to 20H
30H
*Extended memory size (low byte, defined by POST)
31H
*Extended memory size (high byte)
32H
*Date century byte
33H
*Information flags
34-3FH
* Reserved
Notes:
*These bytes are not included in the industry standard CMOS checksum
%
These bytes are included in HP's checksum
C.1 Real-Time Clock/CMOS Access
Port 70H and port 71 H provide the interface to the real-time clock and CMOS memory controller.
~
Port 70H is used to specify the byte address to read or write. Port 71 H is used to pass the data.
For example, to read the equipment byte, the programmer would write 14H to port 70H, then
read the data byte from port 71 H. A read or write to port 71 H must always be preceeded by a
write to port 70H.
372 CMOS Memory Layout and Real-Time Clock