CMOS Data Area

Late PS/2 Planars

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).

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

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.


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 Century
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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Louis F. Ohland, Peter H. Wendt, David L. Beem, William R. Walsh, Tatsuo Sunagawa, Tomáš Slavotínek, Jim Shorney, Tim N. Clarke, Kevin Bowling, and many others.

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Last update: 08 May 2024 - Changelog | About | Legal & Contact