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Drawing the screen: WSCAN

[BBC Micro disc version, Flight]

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 Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * DELAY calls WSCAN * DK4 calls WSCAN * Main flight loop (Part 13 of 16) 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 LDA #0 \ Set DL to 0 STA DL LDA DL \ Loop round these two instructions until DL is no BEQ P%-2 \ 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