Drawing lines: DrawVerticalLine (Part 1 of 3)

       Name: DrawVerticalLine (Part 1 of 3)                          [Show more]
       Type: Subroutine
   Category: Drawing lines
    Summary: Draw a vertical line from (X1, Y1) to (X1, Y2)
  Deep dive: Drawing lines in the NES version
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * DrawLaunchBox calls DrawVerticalLine


 Arguments:

   X1                   The screen x-coordinate of the line

   Y1                   The screen y-coordinate of the start of the line

   Y2                   The screen y-coordinate of the end of the line


 Returns:

   Y                    Y is preserved


.DrawVerticalLine

 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

 STY YSAV               ; Store Y into YSAV, so we can preserve it across the
                        ; call to this subroutine

 LDY Y1                 ; Set Y = Y1

 CPY Y2                 ; If Y1 = Y2 then the start and end points are the same,
 BEQ vlin3              ; so return from the subroutine (as vlin3 contains
                        ; an RTS)

 BCC vlin1              ; If Y1 < Y2, jump to vlin1 to skip the following code,
                        ; as (X1, Y1) is already the top point

 LDA Y2                 ; Swap the values of Y1 and Y2, so we know that (X1, Y1)
 STA Y1                 ; is at the top and (X1, Y2) is at the bottom
 STY Y2

 TAY                    ; Set Y = Y1 once again

.vlin1

 LDA X1                 ; Set SC2(1 0) = (nameBufferHi 0) + yLookup(Y) + X1 / 8
 LSR A                  ;
 LSR A                  ; where yLookup(Y) uses the (yLookupHi yLookupLo) table
 LSR A                  ; to convert the pixel y-coordinate in Y into the number
 CLC                    ; of the first tile on the row containing the pixel
 ADC yLookupLo,Y        ;
 STA SC2                ; Adding nameBufferHi and X1 / 8 therefore sets SC2(1 0)
 LDA nameBufferHi       ; to the address of the entry in the nametable buffer
 ADC yLookupHi,Y        ; that contains the tile number for the tile containing
 STA SC2+1              ; the pixel at (X1, Y), i.e. the line we are drawing

 LDA X1                 ; Set S = X1 mod 8, which is the pixel column within the
 AND #7                 ; character block at which we want to draw the start of
 STA S                  ; our line (as each character block has 8 columns)
                        ;
                        ; As we are drawing a vertical line, we do not need to
                        ; vary the value of S, as we will always want to draw on
                        ; the same pixel column within each character block

 LDA Y2                 ; Set R = Y2 - Y1
 SEC                    ;
 SBC Y1                 ; So R is the height of the line we want to draw, which
 STA R                  ; we will use as a counter as we work our way along the
                        ; line from top to bottom - in other words, R will the
                        ; height remaining that we have to draw

 TYA                    ; Set Y = Y1 mod 8, which is the pixel row within the
 AND #7                 ; character block at which we want to draw the start of
 TAY                    ; our line (as each character block has 8 rows)

 BNE vlin4              ; If Y is non-zero then our vertical line is starting
                        ; inside a character block rather than from the very
                        ; top, so jump to vlin4 to draw the top end of the line

 JMP vlin13             ; Otherwise jump to vlin13 to draw the middle part of
                        ; the line from full-height line segments, as we don't
                        ; need to draw a separate block for the top end