Combat demo: DrawScrollText

       Name: DrawScrollText                                          [Show more]
       Type: Subroutine
   Category: Combat demo
    Summary: Display a Star Wars scroll text
  Deep dive: The NES combat demo
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * ShowScrollText calls DrawScrollText


 Arguments:

   A                    The number of lines in the middle part of the scroll
                        text, which is the total number of text lines minus 4:

                          * 2 for scrollText1 (6 lines)

                          * 6 for scrollText2 and creditsText1 (10 lines)

                          * 5 for creditsText2 (9 lines)

                          * 3 for creditsText3 (7 lines)


.DrawScrollText

 PHA                    ; Store the number of lines in the scroll text on the
                        ; stack so we can retrieve it later

 JSR CalculateGridLines ; Reset the line coordinate tables and populate them
                        ; with the characters for the scroll text at (Y X),
                        ; setting INF(1 0) to the scroll text in the process

 LDA #$28               ; Set the visible colour to orange ($28) so the scroll
 STA visibleColour      ; text appears in this colour

 LDA #0                 ; Clear bits 6 and 7 of allowInSystemJump to allow
 STA allowInSystemJump  ; in-system jumps, so the call to UpdateIconBar displays
                        ; the fast-forward icon (though choosing this in the
                        ; demo doesn't do an in-system jump, but skips the rest
                        ; of the demo instead)

 LDA #2                 ; Set the scroll text speed to 2 (normal speed)
 STA scrollTextSpeed

 JSR UpdateIconBar_b3   ; Update the icon bar to show the correct buttons for
                        ; the scroll text

 LDA #40                ; Tell the NMI handler to send nametable entries from
 STA firstNameTile      ; tile 40 * 8 = 320 onwards (i.e. from the start of tile
                        ; row 10)

                        ; We now draw the scroll text and move it up the screen,
                        ; which we do in three stages
                        ;
                        ;   * Stage 1 moves the first few lines of the scroll
                        ;     text up the screen until the first line reaches
                        ;     the middle of the screen (i.e. just before it will
                        ;     start to disappear into the distance); stage 1 is
                        ;     always 81 frames long at normal speed
                        ;
                        ;  *  Stage 2 then draws the rest of the scroll text
                        ;     on-screen while moving everything up the screen,
                        ;     reusing lines in the line coordinate tables as
                        ;     they disappear into the distance; stage 2 is
                        ;     longer with longer scroll texts
                        ;
                        ;   * Stage 3 takes over when everything has been drawn,
                        ;     and just concentrates on moving the scroll text
                        ;     into the distance without drawing anything new;
                        ;     stage 3 is always 48 frames long at normal speed
                        ;
                        ; We start with stage 1

 LDA #160               ; Set the size of the scroll text to 160 to pass to
 STA scrollProgress     ; DrawScrollFrames
                        ;
                        ; This equates to 81 frames at normal speed, with each
                        ; frame taking scrollTextSpeed off the value of
                        ; scrollProgress (i.e. subtracting 2), and only
                        ; stopping when the subtraction goes past zero

 JSR DrawScrollFrames   ; Draw the frames for stage 1, so the scroll text gets
                        ; drawn and moves up the screen

                        ; We now move on to stage 2
                        ;
                        ; Stage 2 takes longer for longer scroll texts, and its
                        ; length is based on the value of A passed to the
                        ; routine (which contains the total number of text lines
                        ; minus 4)
                        ;
                        ; Specifically, stage 2 loop around A times, with each
                        ; loop taking a scrollProgress of 23 (which is 12 frames
                        ; at normal speed)
                        ;
                        ; Each loop draws an extra line of text in the scroll
                        ; text, and scrolls up by one line of text

 PLA                    ; Set LASCT to the value that we stored on the stack, so
 STA LASCT              ; LASCT contains the number of lines in the scroll text

.dscr1

 LDA #23                ; Set the size of the scroll text to 23 to pass to
 STA scrollProgress     ; DrawScrollFrames

 JSR ScrollTextUpScreen ; Scroll the scroll text up the screen by one full line
                        ; of text

 JSR GRIDSET            ; Call GRIDSET to populate the line coordinate tables at
                        ; X1TB, Y1TB and X2TB (the TB tables) with the lines for
                        ; the scroll text in INF(1 0) at offset XC

 JSR DrawScrollFrames   ; Draw the frames for stage 2, so the scroll text gets
                        ; drawn and moves up the screen by one text line

 DEC LASCT              ; Loop back until we have done LASCT loops around the
 BNE dscr1              ; above

                        ; We now move on to stage 3
                        ;
                        ; Stage 3 loops around four times, with each loop taking
                        ; a scrollProgress of 23 (which is 12 frames at normal
                        ; speed), so that's a grand total of 48 frames at normal
                        ; speed

 LDA #4                 ; Set LASCT = 4 so we do the following loop four times
 STA LASCT

.dscr2

 LDA #23                ; Set the size of the scroll text to 23 to pass to
 STA scrollProgress     ; DrawScrollFrames

 JSR ScrollTextUpScreen ; Scroll the scroll text up the screen by one full line
                        ; of text

 JSR DrawScrollFrames   ; Draw the frames for stage 3, so the scroll text moves
                        ; off-screen one text line at a time

 DEC LASCT              ; Loop back until we have done LASCT loops around the
 BNE dscr2              ; above

                        ; The scroll text is now done and is no longer on-screen

 LDA #0                 ; Reset the scroll speed to zero (though this isn't read
 STA scrollTextSpeed    ; again, so this has no effect)

 LDA #$2C               ; Set the visible colour back to cyan ($2C)
 STA visibleColour

 RTS                    ; Return from the subroutine