Combat demo: CalculateGridLines

       Name: CalculateGridLines                                      [Show more]
       Type: Subroutine
   Category: Combat demo
    Summary: Reset the line coordinate tables and populate them with the
             characters for a specified scroll text
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * DrawScrollText calls CalculateGridLines


 Arguments:

   (Y X)                The content of the scroll text to display


 Returns:

   INF(1 0)             The content of the scroll text to display


.CalculateGridLines

 STX INF                ; Set INF(1 0) = (Y X)
 STY INF+1

 SETUP_PPU_FOR_ICON_BAR ; If the PPU has started drawing the icon bar, configure
                        ; the PPU to use nametable 0 and pattern table 0

                        ; We start by clearing out the buffer at Y1TB

 LDY #240               ; The buffer contains 240 bytes, so set a byte counter
                        ; in Y

 LDA #0                 ; Set A = 0 so we can zero the buffer

.resg1

 STA Y1TB-1,Y           ; Zero the entry Y - 1 in Y1TB

 DEY                    ; Decrement the byte counter

 BNE resg1              ; Loop back until we have reset the whole Y1TB buffer

                        ; We now populate the grid line buffer with the lines
                        ; for the scroll text at INF(1 0)

 LDX #0                 ; Set XP = 0, so the scroll text starts at x-coordinate
 STX XP                 ; 0, on the left of the screen

 LDA #5*W2Y             ; Set YP so the scroll text starts five lines of scroll
 STA YP                 ; text down the screen (as W2Y is the height of each
                        ; line in scroll text coordinates)

 LDY #0                 ; Set XC = 0, so we start from the first character of
 STY XC                 ; INF(1 0)

 LDA #4                 ; Set LASCT = 4, so we process four lines of text in the
 STA LASCT              ; following loop

.resg2

 JSR GRIDSET+5          ; Populate the line coordinate tables with the pixel
                        ; lines for one 21-character line of scroll text,
                        ; drawing the line at (0, YP)

 LDA YP                 ; Set YP = YP - W2Y
 SEC                    ;
 SBC #W2Y               ; So YP moves down the screen by one line (as W2Y is the
 STA YP                 ; height of each line in scroll text coordinates)

 DEC LASCT              ; Decrement the loop counter in LASCT

 BNE resg2              ; Loop back until we have processed LASCT lines of
                        ; scroll text

 RTS                    ; Return from the subroutine