Offset
Description
0x00
Current second (Real Time Clock) 1
0x01
Alarm second1
0x02
Current minute (Real Time Clock) 1
0x03
Alarm minute1
0x04
Current hour (Real Time Clock) 1
0x05
Alarm hour1
0x06
Current day of week (Real Time Clock) 1
0x07
Current date of month (Real Time Clock) 1
0x08
Current month (Real Time Clock) 1
0x09
Current year (Real Time Clock) 1
0x0a
RTC Status Register A
0x0b
RTC Status Register B
0x0c
RTC Status Register C
0x0d
RTC Status Register D
0x0e
POST diagnostics status byte
0x0f
Shutdown status byte
0x10
Diskette drive types
0x11
Reserved
0x12
Hard Disk drive types
0x13
Reserved
0x14
Equipment byte
0x15
Base memory (low byte)
0x16
Base memory (high byte)
0x17
Extended memory (low byte)
0x18
Extended memory (high byte)
0x19-0x2d
Reserved
0x2e
Check sum (low byte)
0x2f
Check sum (high byte)
0x30
Extended memory above 1 Mb (low byte)
0x31
Extended memory above 1 Mb (high byte)
0x32
Current century1
0x33
Information Flags
0x34-0x3f
Reserved
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.
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.
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