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⌂ ∞ ▽ Elite on the 6502

Drawing lines: NLIN2

[Commodore 64 version]

       Name: NLIN2                                                   [Show more]
       Type: Subroutine
   Category: Drawing lines
    Summary: Draw a screen-wide horizontal line at the pixel row in A

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * NLIN4 calls NLIN2
             * TT22 calls NLIN2




 This draws a line from (0, A) to (255, A), which runs across the whole screen.



 Arguments:

   A                    The pixel row on which to draw the horizontal line



.NLIN2

 STA Y1                 ; Set Y1 = A

 STA Y2                 ; Set Y2 = A

;LDA #YELLOW            ; These instructions are commented out in the original
;JSR DOCOL              ; source

 LDX #0                 ; Set X1 = 0, so (X1, Y1) = (0, A)
 STX X1

 DEX                    ; Set X2 = 255, so (X2, Y2) = (255, A)
 STX X2

 JMP LL30               ; Call LL30 to draw a line from (0, A) to (255, A),
                        ; returning from the subroutine using a tail call

;LDA #CYAN              ; These instructions are commented out in the original
;JMP DOCOL              ; source
;RTS