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
                                                              0 - 255 (LZ)
         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.
  4. With Models CM, XP and LA TMB_YEAR rolls over to 00 after 99. This behavior is known as the y2k-bug. It is possible to poke a higher value into TMB_YEAR but the date is nevertheless displayed incorrectly and TMB_YEAR will again roll over to 00 at the end of the year.


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 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 x 50ms.

EXAMPLE:

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

ERRORS: None.