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⌂ ∞ Elite on the BBC Micro and NES

Utility routines: clr_scrn

[Elite-A, I/O processor]

       Name: clr_scrn                                                [Show more]
       Type: Subroutine
   Category: Utility routines
    Summary: Clear the top part of the screen (the space view)

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * tube_table calls clr_scrn




 This routine is run when the parasite sends a clr_scrn command. It clears the
 top part of the screen (the mode 4 space view).



.clr_scrn

 LDX #&60               \ Set X to the screen memory page for the top row of the
                        \ screen (as screen memory starts at &6000)

.BOL1

 JSR ZES1               \ Call ZES1 to zero-fill the page in X, which clears
                        \ that character row on the screen

 INX                    \ Increment X to point to the next page, i.e. the next
                        \ character row

 CPX #&78               \ Loop back to BOL1 until we have cleared page &7700,
 BNE BOL1               \ the last character row in the space view part of the
                        \ screen (the top part)

 RTS                    \ Return from the subroutine