Drawing lines: HLOIN (Part 1 of 5)

       Name: HLOIN (Part 1 of 5)                                     [Show more]
       Type: Subroutine
   Category: Drawing lines
    Summary: Draw a horizontal line from (X1, Y) to (X2, Y) using EOR logic
  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 HLOIN
             * DrawSunEdgeLeft calls HLOIN
             * DrawSunEdgeRight calls HLOIN
             * LL164 calls HLOIN
             * SUN (Part 2 of 2) calls HLOIN


 Arguments:

   X1                   The screen x-coordinate of the start of the line

   X2                   The screen x-coordinate of the end of the line

   Y                    The screen y-coordinate of the line


 Returns:

   Y                    Y is preserved

   X2                   X2 is decremented


.hlin1

 JMP hlin23             ; Jump to hlin23 to draw the line when it's all within
                        ; one character block

 LDY YSAV               ; Restore Y from YSAV, so that it's preserved

.hlin2

 RTS                    ; Return from the subroutine

.HLOIN

 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

 LDX X1                 ; Set X = X1

 CPX X2                 ; If X1 = X2 then the start and end points are the same,
 BEQ hlin2              ; so return from the subroutine (as hlin2 contains
                        ; an RTS)

 BCC hlin3              ; If X1 < X2, jump to hlin3 to skip the following code,
                        ; as (X1, Y) is already the left point

 LDA X2                 ; Swap the values of X1 and X2, so we know that (X1, Y)
 STA X1                 ; is on the left and (X2, Y) is on the right
 STX X2

 TAX                    ; Set X = X1 once again

.hlin3

 DEC X2                 ; Decrement X2 so we do not draw a pixel at the end
                        ; point

 TXA                    ; 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

 TYA                    ; Set Y = Y 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)
                        ;
                        ; As we are drawing a horizontal line, we do not need to
                        ; vary the value of Y, as we will always want to draw on
                        ; the same pixel row within each character block

 TXA                    ; Set T = X1 with bits 0-2 cleared
 AND #%11111000         ;
 STA T                  ; Each character block contains 8 pixel rows, so to get
                        ; the address of the first byte in the character block
                        ; that we need to draw into, as an offset from the start
                        ; of the row, we clear bits 0-2
                        ;
                        ; T is therefore the offset within the row of the start
                        ; of the line at x-coordinate X1

 LDA X2                 ; Set A = X2 with bits 0-2 cleared
 AND #%11111000         ;
 SEC                    ; A is therefore the offset within the row of the end
                        ; of the line at x-coordinate X2

 SBC T                  ; Set A = A - T
                        ;
                        ; So A contains the width of the line in terms of pixel
                        ; bytes (which is the same as the number of character
                        ; blocks that the line spans, less 1 and multiplied by
                        ; 8)

 BEQ hlin1              ; If the line starts and ends in the same character
                        ; block then A will be zero, so jump to hlin23 via hlin1
                        ; to draw the line when it's all within one character
                        ; block

 LSR A                  ; Otherwise set R = A / 8
 LSR A                  ;
 LSR A                  ; So R contains the number of character blocks that the
 STA R                  ; line spans, less 1 (so R = 0 means it spans one block,
                        ; R = 1 means it spans two blocks, and so on)