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⌂ ∞ ▽ Elite on the BBC Micro and NES

Version analysis of WSCAN

This code appears in the following versions (click to see it in the source code):

Code variations between these versions are shown below.

       Name: WSCAN
       Type: Subroutine
   Category: Drawing the screen

Code variation 1 of 3A variation in the comments only

Tap on a block to expand it, and tap it again to revert.

Cassette, Flight, Docked
6502SP, Master
    Summary: Wait for the vertical sync

    Summary: Implement the #wscn command (wait for the vertical sync)




 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

Code variation 2 of 3Minor and very low-impact

Tap on a block to expand it, and tap it again to revert.

Cassette, Flight, Docked, 6502SP
Master
 LDA #0                 \ Set DL to 0
 STA DL

 STZ DL                 \ Set DL to 0

Code variation 3 of 3A variation in the labels only

Tap on a block to expand it, and tap it again to revert.

Cassette, Flight, Docked
Master
6502SP
 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)

{
.DELL1                  \ This label is a duplicate of a label in the DELT
                        \ routine (which is why we need to surround it with
                        \ braces, as BeebAsm doesn't allow us to redefine
                        \ labels, unlike BBC BASIC)

 LDA DL                 \ Loop round these two instructions until DL is no
 BEQ DELL1              \ 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)

}

.WSCAN1

 LDA DL                 \ Loop round these two instructions until DL is no
 BEQ WSCAN1             \ 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