CMOS Registers
The CMOS RAM data area consists of two banks of 128 bytes.
The first 64 bytes in both bank are the same and contain clock and calendar
information. The second 64 bytes in bank 1 control security and system
start-up features.
Note: Access to the two banks in
CMOS RAM is controlled by the bank-control bit in Status Register A (see
Status Register A (Hex 00A)).
The following figure shows the addresses and definition
for the first 64 bytes of RT/CMOS RAM.
RAM Address Map
RT/CMOS
ADDRESS
(HEX) RT/CMOS
RAM BYTES
000-00D Real-Time Clock bytes
00E
Diagnostic Status byte
00F
Shutdown Status byte
010
Diskette Drive Type byte
011
First Fixed Disk Drive Type byte
012, 013 Reserved
014
Equipment byte
015, 016 Low and high Base Memory
bytes
017, 018 Low and high Expansion Memory
bytes
019-031 Reserved
034
Reserved
035, 036 Low and high usable memory
above 1MB
037
Date Century byte
038-07F Reserved
Warning:
The operation that follows a write operation to address hex 0070 must
read from address hex 0071; otherwise intermittent malfunctions and unreliable
operation of the RT/CMOS RAM can occur.
CMOS RAM Bank 0
000-00D
Real-Time Clock Bytes
00A
Status Register A Timer Related
00B
Status Register B Date and Alarm Related
00C
Status Register C Interrupt Related
00D
Status Register D CMOS Valid Byte
CMOS RAM Configuration
00E
Diagnostic Status Byte Configuration Related
00F
Shutdown Status Byte Set by power-on diagnostics
010
FD Type Byte Type of Floppy installed
011
1st Fixed Disk Drive Type Byte Type HD installed
011
2nd Fixed Disk Drive Type Byte Type HD installed
012,013
Reserved Bytes
014
Equipment Byte #FDs, Display, Co-pro, A: present
015,016
Low and High Base Memory Bytes Memory <640KB
017,018
Low and High Expansion Memory Bytes Memory >1MB
019-031
Reserved Bytes
032,033
Configuration CRC Bytes CRC data for RTC CMOS
034
Reserved Byte
035,036
Low and High Usable Memory Bytes Memory 1-16MB
037
Date-Century Byte BCD Value for century
038-04F
Reserved Bytes
CMOS RAM Bank 1
40-47F
System Number System Identifier
48
Century Byte BCD Value for Cenuty
49
Wake-up Alarm Date Date set for Power-On
4A
Extended Control Register A Security and power-on
4B
Extended Control Register B Security and power-on
4C-7F
Reserved
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