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⌂ ∞ Elite on the BBC Micro and NES

Drawing the screen: WSCAN

[Elite-A, Parasite]

       Name: WSCAN                                                   [Show more]
       Type: Subroutine
   Category: Drawing the screen
    Summary: Wait for the vertical sync by sending a sync_in command to the I/O
             processor

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * check_keys calls WSCAN
             * DELAY calls WSCAN
             * DK4 calls WSCAN
             * DK4_FLIGHT calls WSCAN
             * TT102 calls WSCAN




.WSCAN

 LDA #&85               \ Send command &86 to the I/O processor:
 JSR tube_write         \
                        \   =sync_in()
                        \
                        \ which will wait until the vertical sync before
                        \ returning control to the parasite

 JMP tube_read          \ Set A to the response from the I/O processor, which
                        \ will be sent when the vertical sync occurs (it doesn't
                        \ matter what the value is), and return from the
                        \ subroutine using a tail call