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⌂ ∞ Elite on the BBC Micro and NES

Combat demo: DrawScrollFrames

[NES version, Bank 6]

       Name: DrawScrollFrames                                        [Show more]
       Type: Subroutine
   Category: Combat demo
    Summary: Draw a scroll text over multiple frames
  Deep dive: The NES combat demo

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




.DrawScrollFrames

 LDA controller1A       ; If the A button is being pressed on controller 1, jump
 BMI scfr1              ; to scfr1 to speed up the scroll text

 LDA iconBarChoice      ; If the fast-forward button has not been chosen on the
 CMP #12                ; icon bar, jump to scfr2 to leave the speed as it is
 BNE scfr2

 LDA #0                 ; Set iconBarChoice = 0 to clear the icon button choice
 STA iconBarChoice      ; so we don't process it again

.scfr1

                        ; If we get here then either the A button has been
                        ; pressed or the fast-forward button has been chosen on
                        ; the icon bar

 LDA #9                 ; Set the scroll text speed to 9 (fast)
 STA scrollTextSpeed

.scfr2

 JSR FlipDrawingPlane   ; Flip the drawing bitplane so we draw into the bitplane
                        ; that isn't visible on-screen

 JSR DrawScrollFrame    ; Draw one frame of the scroll text

 JSR DrawBitplaneInNMI  ; Configure the NMI to send the drawing bitplane to the
                        ; PPU after drawing the box edges and setting the next
                        ; free tile number

 LDA iconBarChoice      ; If no buttons have been pressed on the icon bar while
 BEQ scfr3              ; drawing the frame, jump to scfr3 to skip the following
                        ; instruction

 JSR CheckForPause_b0   ; If the Start button has been pressed then process the
                        ; pause menu and set the C flag, otherwise clear it

.scfr3

 LDA scrollProgress     ; Set scrollProgress = scrollProgress - scrollTextSpeed
 SEC                    ;
 SBC scrollTextSpeed    ; So we update the scroll text progress
 STA scrollProgress

 BCS DrawScrollFrames   ; If the subtraction didn't underflow then the value of
                        ; scrollProgress is still positive and there is more
                        ; scrolling to be done, so loop back to the start of
                        ; the routine to keep going

 RTS                    ; Return from the subroutine