Drawing the screen: WSCAN

       Name: WSCAN                                                   [Show more]
       Type: Subroutine
   Category: Drawing the screen
    Summary: Wait for the vertical sync
    Context: See this subroutine in context in the source code
 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) on PAL systems, or 60 times a second (60Hz) on NTSC systems.

 We do this by monitoring the value of RASTCT, which is updated by the
 interrupt routine at COMIRQ1 as it draws the two different parts of the screen
 (the upper part containing the space view, and the lower part containing the
 dashboard).


.WSCAN

 PHA                    ; Store A on the stack so we can preserve it

.WSC1

 LDA RASTCT             ; Wait until RASTCT is non-zero, which indicates that
 BEQ WSC1               ; the VIC-II is now drawing the dashboard

.WSC2

 LDA RASTCT             ; Wait until RASTCT is zero, which indicates that the
 BNE WSC2               ; VIC-II is now drawing the top line of the visible
                        ; screen

 PLA                    ; Restore A from the stack so it is unchanged

 RTS                    ; Return from the subroutine