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									The PC‟s Real-Time Clock

 An introduction to the capabilities
  and programming of the Real-
 Time Clock and CMOS memory
       Non-Volatile Memory
• The original IBM-PC had no internal clock
• Users had to run a utility program to reset
  the date and time after any system reboot
• This defect was eliminated in the IBM-AT
• A special battery-powered peripheral was
  added to keep track of the time and date
• It also provided a small amount of memory
  which could retain configuration settings
      Motorola‟s MC146818A
• PC-AT‟s Real-Time Clock plus RAM was
  manufactured by Motorola Corporation
• Other companies have „cloned‟ this chip
• Its capabilities are described online in an
  official datasheet by Dallas Semiconductor
  (see „Maxim‟ integrated circuit: DS12887)
• You can also get the Motorola datasheet
  (by writing to its corporate headquarters)
       Features of DS12887
• Can operate over ten years without power
• Counts seconds, minutes, hours, days,
  day-of-the-week, date, month, and year
  (with leap-year compensation), valid up
  until the year 2100 AD, with options for
  12/24-hour clock and Daylight Savings
• Can use binary or BCD representation
• Provides 114 bytes of nonvolative storage
      Programming Interface
• The RTC interfaces with system software
  as an array of 128 bytes, accessed via i/o
  ports 0x70 and 0x71 using a multiplexing
  scheme:
          port 0x70: address-port
          port 0x71: data-port
• A system quirk: The most significant bit at
  port 0x70 is used to control a gate that can
  „mask‟ the Non-Maskable Interrupt circuitry
      Ten clock/calendar bytes
0x0   Current seconds     Range is 0..59

0x1    Alarm seconds      Range is 0..59

0x2   Current minutes     Range is 0..59

0x3    Alarm minutes      Range is 0..59

0x4    Current hours      Range is 0..23 or 1..12

0x5     Alarm hours       Range is 0..23 or 1..12
      Day-of-the-Week
0x6                       Range is 1..7 (Sunday=1)
      Date of the Month
0x7                       Range is 1..31
       Current Month
0x8                       Range is 1..12 (January=1)
        Current Year
0x9                       Range is 0..99
       Operating Capabilities
• The RTC can be programmed to generate
  an interrupt under any combination of the
  following three conditions:
      1) time/date counters were updated
      2) current time equals the alarm time
      3) periodic frequency interval restarts
• The frequency of the periodic interrupt is a
  selectable rate (e.g., from 122 to 500ms)
       Four Status/Control bytes
0xA
      UIP      Divider bits        Rate-Select



0xB   SET    PIE   AIE   UIE SQWE DM 24/12 DSE



0xC   IRQF   PF    AF     UF   0    0     0      0


0xD   VRT    0      0     0    0    0     0      0
     Other NVRAM locations
• Besides these 14 dedicated RTC bytes,
  there are 114 byte locations which can
  serve as nonvolatile storage in whatever
  manner the system designer decides
• IBM has established some „standard‟ uses
  for many (but not all) of these locations
• A fairly complete CMOS Memory Map is
  accessible online (see course website)
      Example: Diagnostic Status
      Power Check POST    RAM     Fixed   CMOS
0xE   Status Sum Config Size      Disk     Time reserved reserved
      failure bad invalid wrong    bad    invalid




  During the Power-On Self-Test, the ROM-BIOS routines
  perform tests of the memory and peripheral devices, and
  record any failures/errors in this Diagnostic Status byte
     Note on the NMI circuitry
• The CPU has a special input-signal that is
  „non-maskable‟ (i.e., CLI / STI instructions
  have no effect on it), intended to be used
  for signaling urgent or catastrophic events
  (such as loss of power or memory failures)
• But sometimes, during a “critical section”
  of system code, it is necessary to prohibit
  even these urgent interrupts (e.g., when
  stack or Interrupt Descriptors are invalid)
 Non-Maskable Interrupt gate


                 Error-signals

                                             AND   NMI


                    Logic GATE                        CPU
                  (port 0x70, bit 7)



           PIC                         PIC         INTR   IF

IRQ 8-15                 IRQ 0-7
   Example: setting alarm time
; inhibit clock updates while reprogramming
      mov al, #0x8B        ; access register B
      out      #0x70, al   ; and disable NMI
      in       al, #0x71   ; read register B
      or       al, #0x80   ; set the SET bit
      out      #0x71, al   ; write register B
Set the alarm for 6:30:00 am
 mov   ax, #0x0685   ; hours
 out   #0x70, ax
 mov   ax, #0x3083   ; minutes
 out   #0x70, ax
 mov   ax, #0x0081   ; secons
 out   #0x70, ax
 Finish SET (and reenable NMI)
; clear the SET bit in register B
       mov al, #0x8B            ; select register B
       out    #0x70, al         ; for access w/o NMI
       in     al, #71           ; read register B
       and    al, #0x7F         ; clear its SET bit
       out     #71, al          ; write register B
; reenable the Non-Maskable Interrupts
       mov     al, #0x0D        ; select register D
       out    #0x70, al         ; and reenable NMI
     Our „rtcdemo.s‟ program
• We illustrate the Real-Time Clock chip‟s
  „update‟ interrupt by redisplaying the time
  whenever the clock-counters are updated
• Register B controls which interrupts occur
• But don‟t change bits 0, 1, 2 (they control
  the data-format (binary or BCD), select 12
  or 24 hour clock, and activate the Daylight
  Savings Time counter-compensation
              Event-Queue paradigm
HEAD



                            Event    Event    Event
              free   free                              free   free   free
                            record   record   record
       base                                                             edge


TAIL




The Interrupt Service Routine(s) insert new event-records in the queue,
while the main application-loop removes event-records from the queue.
  An important „design pattern‟
• Many modern user-centered applications use
  the „event-queue‟ paradigm:
      initialize_the_system();
      while ( !done )
              {
              dequeue_next_event( &event_record);
              process_that_event( &event_record );
              }
      cleanup_the_system();
      exit(0);
           In-class exercise
• Modify our „rtcdemo.s‟ program so that an
  „alarm‟ interrupt will get triggered after an
  agreed amount of time has elapsed (for
  example, 10 seconds) and use that event
  to exit the program‟s main loop (instead of
  using the built-in loop-counter variable)
    Suggested implementation
• Easiest and most elegant way to achieve the
  requested program modification is to adjust the
  Interrupt-Service Routine so it will save a 4-part
  event-record (instead of a 3-part event-record),
  the new item to be included is the RTC status-
  register value
• Then the main loop can test the status it finds
  within an event-record, and can either loop or
  exit, depending on which event-type it found

								
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