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
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

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. 

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

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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