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          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, 976.562.s)
             | +-------------- 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