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                                CHAPTER 10

                               _____________
                               SYSTEM TIMING



System timing is controlled by two interrupts:

     1.  The NMI interrupt handles the real-time clock and auto-switch  off
         delay.

     2.  The  KEYBOARD  INTERRUPT  provides  a  frame-counter  and  handles
         keyboard delays and display timing etc.




      _______________
10.1  REAL TIME CLOCK

     The clock time is stored in binary in 6 fields from $20C5 to $20CA:


        VARIABLE        ADDRESS         DESCRIPTION             RANGE
        ========        =======         ===========             =====
        TMB_YEAR        $20C5           YEAR                    0 - 99
        TMB_MONS        $20C6           MONTH                   0 - 11
        TMB_DAYS        $20C7           DATE OF MONTH           0 - 31
        TMB_HOUR        $20C8           HOUR                    0 - 23
        TMB_MINS        $20C9           MINUTES                 0 - 59
        TMB_SECS        $20CA           SECONDS                 0 - 59

Note that:

     1.  A month of 0 represents JANUARY and a date of 0 represents the 1st
         day of the month.

     2.  On a cold start only, the clock  is  initialised  to  1  JAN  1987
         00:00:00

     3.  The real-time  clock  should  not  be  read  directly  from  these
         variables  because  it  may  be  being updated by an NMI.  Instead
         system service TM$TGET  should  be  used  to  get  a  valid  time.
         Similarly  these  variables  should  not  be  written  to  without
         checking for an NMI.



        ________________________
10.1.1  KEEPING TIME WITH NMI ON

     An NMI interrupt is generated from the semi-custom chip  every  second
to  provide  an  accurate real-time clock.  When the machine is on, the NMI
interrupt updates the time by 1 second, see TM$UPDT.



        _________________________
10.1.2  KEEPING TIME WITH NMI OFF

     When NMIs are switched off the processor, (e.g.  when  the  machine  is
switched  off)  an internal counter in the semi-custom chip is connected to
the NMI line so that NMIs can still be counted, enabling  the  time  to  be
updated when restoring NMIs to the processor.

The counter has 11 bits, so the maximum time it can  store  is  2048  secs.
When  the counter reaches this value, the ACOUT bit of PORT 5 goes high and
the machine switches on.  Hence the machine can  be  forced  to  switch  on
automatically  at  any  time  up  to  34mins  8secs after switching off, be
pre-counting the counter.  This is done when an alarm is due.

Whenever the machine switches on it updates  the  real-time  clock  by  the
amount  in  the  counter (less any pre-counting).  If ACOUT is high, and no
alarm was due, the machine switches back off immediately after updating the
clock.   This can be seen as the screen flashes on for an instant, every 34
mins and 8 secs while the machine is off.

To disable NMIs and keep the time, system services BT$NOF and BT$NON should
be used, see sections 5.7.3 and 5.7.4.



        ________________________
10.1.3  AUTO-SWITCH-OFF TIME OUT

     The following three variables control the auto-switch-off:

        VARIABLE        ADDRESS         DESCRIPTION
        ========        =======         ===========
        TMB_SWOF        $007C           AUTO-SWITCH-OFF FLAG
        TMW_TOUT        $007D,$007E     TIME LEFT BEFORE SWITCH OFF
        TMW_TCNT        $20CD,$20CE     DEFAULT NUMBER OF SECONDS TO
TIME-OUT

The time before the machine switches off (in secs) is  stored  in  TMW_TCNT
and  is  set to $012C on cold start (5 mins).  The contents of TMW_TCNT are
copied into TMW_TOUT whenever a key is pressed or  KB$GETK  is  called  and
TMW_TOUT  is  decremented  until  zero by the NMI routine every second.  If
TMW_TOUT is found to be zero in KB$TEST, the machine will switch off.

The contents  of  TMW_TCNT  can  be  changed  at  any  time  to  alter  the
auto-switch-off  time,  up to a maximum of 65535 seconds (18 hours, 12 mins
and 15 secs).  If it is set to less than 15, the machine  will  still
stay on for 15 secs.

To disable the auto-switch-off completely, TMB_SWOF should be set to  zero.
This  will  inhibit  the NMI from decrementing TMW_TOUT and prevent KB$TEST
testing it.



      _________________________
10.2  KEYBOARD INTERRUPT TIMERS

     The timer 1 compare interrupt is used to scan the  keyboard  to  allow
keyboard  buffering  and  to  provide  a  timing service.  The time between
interrupts is controlled by the variable KBW_TDEL which is  initialised  on
cold  start  to  be $B3DD.  This value makes the KI interrupt occur exactly
every 50 milliseconds and is used extensively by the operating  system  for
timing purposes.



        ________
10.2.1  TMW_FRAM

     TMW_FRAM is incremented by 1 on each keyboard interrupt.   When  $ffff
is  reached,  it  wraps back to $0000.  It can be read at any time and used
for accurate timing.



        ________
10.2.2  DPW_REDY

     DPW_REDY is decremented by 1 on each keyboard interrupt until zero  is
reached.   It  can  be  used  to  provide  delays (e.g.  TM$WAIT stores D in
DPW_REDY and waits for it to reach zero - see below).



      _______________
10.3  SYSTEM SERVICES

     This section describes the operating system calls available for system
timing.

Note that TM$DAYV and  TM$UPDT  work  from  any  buffer  pointed  to  by  X
containing  a  6  byte time representation exactly like the real-time clock
(TMB_YEAR...).  These routine should not operate  on  the  real-time  clock
itself if an NMI is imminent, so NMIs must be checked for or TM$TGET should
be used to copy the time to another buffer.



10.3.1  TM$DAYV

VECTOR NUMBER:          105
INPUT PARAMETERS:       X register - Address of time buffer.
OUTPUT VALUES:          B register - numeric value of day.
                        X register - address of 3 byte day name.

DESCRIPTION


         Calculates the day of the week for a given date between 1 JAN 1900
    and  31  DEC  1999.   X  must  point  to the time buffer containing the
    particular date in the standard format.

    The numeric value of the day (0 to 6) is returned in the B register  (0
    represents   MONDAY,   1   TUESDAY,  etc)  and  X  will  point  to  the
    corresponding 3 byte day  name  in  ascii  (MON,TUE,WED,THU,FRI,SAT  or
    SUN).

EXAMPLE:

The following routine will calculate and print the day for 2 JAN 1963:

        LDX     #BUFFER ;POINT TO TIME BUFFER
        LDD     #$3F00          ;YEAR 1963, MONTH JANUARY
        STD     0,X
        LDA     A,#$01          ;DATE 2ND
        STA     A,2,X
        OS      TM$DAYV         ;CALCULATE DAY
        LDA     A, 3
        PSH     A               ;DAY NAME LENGTH
        PSHX                    ;ADDRESS OF DAY STRING
        OS      UT$DISP
        .BYTE   D_FF            ;CLEAR DISPLAY
        .ASCIZ  /%b/            ;PRINT DAY


ERRORS:                 None.



10.3.2  TM$TGET

VECTOR NUMBER:          106
INPUT PARAMETERS:       X register - Address of 6 byte buffer to store
time.
OUTPUT VALUES:          None.
REGISTERS PRESERVED:    X

DESCRIPTION

    Get a copy of the real-time clock into  a  buffer  at  x.   It  is  not
    possible  to  read the clock directly in case it is being updated by an
    NMI.

EXAMPLE:

        LDX     #BUFFER         ;X POINTS TO A 6 BYTE BUFFER
        OS      TM$TGET         ;COPY TIME INTO BUFFER AT X


ERRORS:                 None.



10.3.3  TM$UPDT

VECTOR NUMBER:          107
INPUT PARAMETERS:       A register - Number of minutes to update.
                        B register - Number of seconds to update.
                        X register - Address of 6 byte buffer containing
time.
OUTPUT VALUES:          None.

DESCRIPTION


         Updates the time pointed to by X by A minutes and  B  seconds.   A
    and  B  must be in the range 0 - 59.  X must point to the year field of
    the 6 byte time buffer in standard format.

EXAMPLE:

        LDX     #BUFFER ;POINT TO 6 BYTE BUFFER
        OS      TM$TGET         ;GET REALTIME. PRESERVES X
        LDD     #$003B          ;UPDATE BY 0 MINS 59 SECS.
        OS      TM$UPDT         ;ADD 59 SECS TO TIME IN BUFFER


ERRORS:                 None.



10.3.4  TM$WAIT

VECTOR NUMBER:          108
INPUT PARAMETERS:       D register - Time to pause in ticks.
OUTPUT VALUES:          None.

DESCRIPTION


         Waits for D  ticks  (1  tick  is  the  interval  between  keyboard
    interrupts,  controlled  by  KBW_TDEL  and set to 50ms by default).  If
    interrupts are disabled (I mask set) then  this  routine  waits  for  D
    50ms.

EXAMPLE:

        LDD     #20             ;20 x 50ms
        OS      TM$WAIT         ;PAUSE FOR 1 SEC

ERRORS:                 None.