HelpPC 2.10 Quick Reference Utility Copyright 1991 David Jurgens

           CMOS RTC - Real Time Clock and Memory (ports 70h & 71h)

       Reg#                      Description

        00  RTC seconds
        01  RTC seconds alarm
        02  RTC minutes
        03  RTC minutes alarm
        04  RTC hours
        05  RTC hours alarm
        06  RTC day of week
        07  RTC day of month
        08  RTC month
        09  RTC year
        0A  RTC Status register A:

           │7│6│5│4│3│2│1│0│  RTC Status Register A
            │ │ │ │ └─┴─┴─┴──── rate selection Bits for divider output
            │ │ │ │             frequency (set to 0110 = 1.024kHz, 976.562μs)
            │ └─┴─┴────────── 22 stage divider, time base being used;
            │                    (initialized to 010 = 32.768kHz)
            └────────────── 1=time update in progress, 0=time/date available
       
        0B  RTC Status register B:

           │7│6│5│4│3│2│1│0│  RTC Status Register B
            │ │ │ │ │ │ │ └──── 1=enable daylight savings, 0=disable (default)
            │ │ │ │ │ │ └───── 1=24 hour mode, 0=12 hour mode (24 default)
            │ │ │ │ │ └────── 1=time/date in binary, 0=BCD (BCD default)
            │ │ │ │ └─────── 1=enable square wave frequency, 0=disable
            │ │ │ └──────── 1=enable update ended interrupt, 0=disable
            │ │ └───────── 1=enable alarm interrupt, 0=disable
            │ └────────── 1=enable periodic interrupt, 0=disable
            └─────────── 1=disable clock update, 0=update count normally

        0C  RTC Status register C (read only):

           │7│6│5│4│3│2│1│0│  RTC Status Register C  (read only)
            │ │ │ │ └─┴─┴─┴──── reserved (set to 0)
            │ │ │ └────────── update ended interrupt enabled
            │ │ └────────── alarm interrupt enabled
            │ └────────── periodic interrupt enabled
            └────────── IRQF flag

        0D  RTC Status register D (read only):

           │7│6-0│  RTC Status Register D  (read only)
            │  └───── reserved (set to 0)
            └────── 1=CMOS RAM has power, 0=CMOS RAM has lost power

        0E  Diagnostic status byte:

           │7│6│5│4│3│2│1│0│  Diagnostic Status Byte
            │ │ │ │ │ │ └─┴──── reserved
            │ │ │ │ │ └─────── 1=time is invalid, 0=ok (POST validity check)
            │ │ │ │ └──────── 1=fixed disk 0 failed initialization, 0=ok
            │ │ │ └───────── 1=memory size doesn't match config info, 0=ok
            │ │ └────────── 1=invalid config info found, 0=ok (see below)
            │ └─────────── 1=config record checksum is bad, 0=ok
            └──────────── 1=RTC lost power, 0=power state stable
           
        0F  Shutdown status byte:

              0  soft reset or unexpected shutdown
              1  shut down after memory size determination
              2  shut down after memory test
              3  shut down with memory error
              4  shut down with boot loader request
              5  JMP DWORD request with INT init
              6  protected mode test 7 passed
              7  protected mode test 7 failed
              8  protected mode test1 failed
              9  block move shutdown request
              A  JMP DWORD request without INT init

        10  Diskette drive type for A: and B:
           
           │7│6│5│4│3│2│1│0│  Diskette drive type for A: and B:
            │ │ │ │ └─┴─┴─┴──── second diskette type
            └─┴─┴─┴─────────── first diskette type

               0000  no drive installed
               0001  DSDD 48 TPI drive
               0010  DSQD 96 TPI drive
               other values are reserved

        11  Reserved
        12  Fixed disk drive type for drive 0 and drive 1

           │7│6│5│4│3│2│1│0│  Diskette drive type for A: and B:
            │ │ │ │ └─┴─┴─┴──── second hard disk drive code (0000=no disk)
            └─┴─┴─┴─────────── first hard disk drive code (0000=no disk)

        13  Reserved
        14  Equipment byte

           │7│6│5│4│3│2│1│0│  Equipment byte
            │ │ │ │ │ │ │ └──── 1=diskette drives installed, 0=none
            │ │ │ │ │ │ └───── 1=math coprocessor installed, 0=none
            │ │ │ │ └─┴────── unused
            │ │ └─┴─────── primary display
            └─┴────────── number of diskette drives installed

           Bits                       Bits
            54  Primary Display        76  Number of Drives
            00  reserved               00  1 diskette drive
            01  40 column color        01  2 diskette drives
            10  80 column color        10  reserved
            11  monochrome             11  reserved

        15  LSB of system base memory in 1k blocks
        16  MSB of system base memory in 1k blocks
        17  LSB of total extended memory in 1k blocks
        18  MSB of total extended memory in 1k blocks
        19  Drive C extension byte (reserved AT)
        1A  Drive D extension byte (reserved AT)
        1B  13 bytes reserved
        2E  CMOS checksum of bytes 10h-20h (MSB)
        2F  CMOS checksum of bytes 10h-20h (LSB)
        30  LSB of extended memory size found above 1 megabyte during POST
        31  MSB of extended memory size found above 1 megabyte during POST
        32  Date century byte in BCD ( BIOS interface to read and set)
        33  Information flags (set during power-on)

           │7│6│5-0│  Information Flags
            │ │  └───── reserved
            │ └─────── initial setup message flag
            └──────── 1=IBM 128k expansion installed, 0=none

        34  12 bytes reserved


       Programming Considerations:

       Write CMOS address to read or write to port 70h
       Read/write port 71h to get/set data


       - the information here is only applicable to AT and PS/2 systems
       - INT 1A is used to read/set the Time of Day and Alarm.  To use the
         alarm, INT 4A must be a valid interrupt service routine.
       - configuration settings are maintained using the Motorola MC146818
         Real Time Clock.  Each of this chips 64 memory registers is used
         for storage (0-3F).
       - Bit 5 of the diagnostic (0Eh) status byte is set during a power
         on test.  This Bit is set if no floppy disks are found or the
         display doesn't match the system display switch setting.
       - all addresses sent to port 70h have Bits 7&6 clear since Bit 7
         of port 70h is used to enable/disable NMI.  Setting this Bit 7
         enables NMI, clearing this Bit disables NMI.
       - when masking the NMI through using port 70H, port 71H should be
         read immediately after or the RTC may be left in an unknown state.
         This wont affect the PS/2 watchdog timer or system channel timeout.

       - see   INT 1A

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