The AddTimeToTimer subroutine - Revs on the BBC Micro

       Name: AddTimeToTimer                                          [Show more]
       Type: Subroutine
   Category: Drivers
    Summary: Add time to the specified timer
  Deep dive: Scheduling tasks in the main loop
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * ProcessTime calls AddTimeToTimer
             * UpdateLapTimers calls AddTimeToTimer


 Arguments:

   X                    The timer to increment:

                          * 0 = the clock timer
                                (clockMinutes clockSeconds clockTenths)

                          * 1 = the lap timer
                                (lapMinutes lapSeconds lapTenths)


 Returns:

   C flag               Denotes whether the number of seconds has changed:

                          * Set if the time just ticked on to the next second

                          * Clear if the time is unchanged


.AddTimeToTimer

 SED                    \ Set the D flag to switch arithmetic to Binary Coded
                        \ Decimal (BCD)

 LDA #&09               \ Set A = &09, so we add 9/100 of a second below

 LDY timerAdjust        \ If timerAdjust <> trackTimerAdjust (which is 24 for
 CPY trackTimerAdjust   \ the Silverstone track), jump to time1 to skip the
 BNE time1              \ following

                        \ If we get here then timerAdjust = trackTimerAdjust, so
                        \ we need to apply the clock adjustment, as this gets
                        \ applied every trackTimerAdjust iterations around the
                        \ main driving loop
                        \
                        \ The clock adjustment speeds the clock up by advancing
                        \ the clock twice as fast as usual, for this tick only

 LDA #&18               \ Set A = &18, so we add 18/100 of a second below

.time1

 CLC                    \ Add A to the tenths for the timer
 ADC clockTenths,X      \
 STA clockTenths,X      \ starting with the tenths of a second

 PHP                    \ Store the C flag on the stack, so we can return it
                        \ from the subroutine below (the C flag will be set
                        \ if the lap time just ticked on to the next second)

 LDA clockSeconds,X     \ Then we add the seconds into A
 ADC #0

 CMP #&60               \ If A < &60, then the number of seconds is still valid,
 BCC time2              \ so jump to time2 to skip the following instruction

 LDA #0                 \ Otherwise set A = 0, so 60 seconds on the timer
                        \ increments back round to 0 seconds

.time2

 STA clockSeconds,X     \ Update the seconds value for the timer

 LDA clockMinutes,X     \ Finally, we add the minutes
 ADC #0
 STA clockMinutes,X

 BPL time3              \ If the updates minutes value for the timer is
                        \ positive, jump to time3 to skip the following

 JSR ZeroTimer          \ Otherwise the timer just reached the maximum possible
                        \ value, so wrap it back round to zero

 LDY currentPlayer      \ Set the total race time for the current player to &80,
 LDA #&80               \ or 80 minutes (as totalRaceMinutes is BCD), otherwise
 STA totalRaceMinutes,Y \ the driver might finish with a very low time just
                        \ because the race timer looped back to zero

.time3

 PLP                    \ Retrieve the C flag from the stack, which will be set
                        \ if the addition of tenths overflowed (in other words,
                        \ if the lap time just ticked on to the next second)

 CLD                    \ Clear the D flag to switch arithmetic to normal

 RTS                    \ Return from the subroutine