Drawing the screen: DODIALS

       Name: DODIALS                                                 [Show more]
       Type: Subroutine
   Category: Drawing the screen
    Summary: Implement the #DODIALS <rows> command (show or hide the dashboard)
    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:
             * JMPTAB calls DODIALS


 This routine sets the screen to show the number of text rows given in X.

 It is used when we are killed, as reducing the number of rows from the usual
 31 to 24 has the effect of hiding the dashboard, leaving a monochrome image
 of ship debris and explosion clouds. Increasing the rows back up to 31 makes
 the dashboard reappear, as the dashboard's screen memory doesn't get touched
 by this process.


 Arguments:

   A                    The number of text rows to display on the screen (24
                        will hide the dashboard, 31 will make it reappear)


 Returns:

   A                    A is set to 6


.DODIALS

 TAX                    \ Copy the number of rows to display into X

 LDA #6                 \ Set A to 6 so we can update 6845 register R6 below

 SEI                    \ Disable interrupts so we can update the 6845

 STA VIA+&00            \ Set 6845 register R6 to the value in X. Register R6
 STX VIA+&01            \ is the "vertical displayed" register, which sets the
                        \ number of rows shown on the screen

 CLI                    \ Re-enable interrupts

 JMP PUTBACK            \ Jump to PUTBACK to restore the USOSWRCH handler and
                        \ return from the subroutine using a tail call