Utility routines: CheckTimePassed

       Name: CheckTimePassed                                         [Show more]
       Type: Subroutine
   Category: Utility routines
    Summary: Flag whether 9 centiseconds have passed since the last call
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * MainLoop (Part 12 of 15) calls CheckTimePassed


 Arguments:

   P                    The current time, incrementing 100 times a second


 Returns:

   C flag               Returns:

                          * Clear if fewer than 9 centiseconds have passed since
                            the last call to this routine that reset the counter

                          * Set if 9 centiseconds or more have passed since the
                            last call to this routine, in which case the counter
                            resets


.CheckTimePassed

 LDX P                  \ Set X = P

 TXA                    \ Set A = P - previousTime
 SEC                    \
 SBC previousTime       \ so A is the number of centiseconds that have passed
                        \ since the last call to this routine (unless the time
                        \ has wrapped, in which case it will be negative)

 BPL time1              \ If A is positive, skip the following three
                        \ instructions

 EOR #&FF               \ A is negative, so negate A using two's complement, so:
 CLC                    \
 ADC #1                 \   A = |P - previousTime|

.time1

 CMP #9                 \ If |A| < 9, skip the following instruction
 BCC time2

 STX previousTime       \ Update previousTime to the current timer in X, so the
                        \ count of 9 centiseconds since the last call can start
                        \ again

.time2

 RTS                    \ Return from the subroutine

 EQUB &3D, &41          \ These bytes appear to be unused
 EQUB &42, &43
 EQUB &77, &78
 EQUB &44, &45
 EQUB &46, &50