Drawing the screen: WSCAN

       Name: WSCAN                                                   [Show more]
       Type: Subroutine
   Category: Drawing the screen
    Summary: Implement the #wscn command (wait for the vertical sync)
    Context: See this subroutine in context in the source code
 Variations: See code variations for this subroutine in the different versions
 References: This subroutine is called as follows:
             * DELAY calls WSCAN
             * DK4 calls WSCAN
             * TT16 calls WSCAN


 Wait for vertical sync to occur on the video system - in other words, wait
 for the screen to start its refresh cycle, which it does 50 times a second
 (50Hz).


.WSCAN

 STZ DL                 \ Set DL to 0

.DELL1K                 \ This label is a duplicate of a label in the DELT
                        \ routine
                        \
                        \ In the original source this label is DELL1, but
                        \ because BeebAsm doesn't allow us to redefine labels,
                        \ I have renamed it to DELL1K

 LDA DL                 \ Loop round these two instructions until DL is no
 BEQ DELL1K             \ longer 0 (DL gets set to 30 in the LINSCN routine,
                        \ which is run when vertical sync has occurred on the
                        \ video system, so DL will change to a non-zero value
                        \ at the start of each screen refresh)

 RTS                    \ Return from the subroutine