Drawing lines: LOIN

       Name: LOIN                                                    [Show more]
       Type: Subroutine
   Category: Drawing lines
    Summary: Add a line segment to the multi-segment line buffer
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * SLIDE calls LOIN


 Arguments:

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

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

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

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


 Returns:

   Y                    Y is preserved


.LOIN

 STY T1                 \ Store Y in T1 so we can preserve it through the call
                        \ to LOIN

 LDY LBUP               \ Set Y to the size of the line buffer

 LDA X1                 \ Store X1, Y1, X2 and Y2 in the Y-th to Y+3-th bytes of
 STA LBUF,Y             \ the line buffer at LBUF
 LDA Y1
 STA LBUF+1,Y
 LDA X2
 STA LBUF+2,Y
 LDA Y2
 STA LBUF+3,Y

 TYA                    \ Set A = Y + 4
 CLC                    \       = LBUP + 4
 ADC #4

 STA LBUP               \ Update LBUP with the value in A, to grow the line
                        \ buffer by the four bytes we just added

 CMP #250               \ If A >= 250, jump to LBFL to draw the line in the
 BCS LBFL               \ line buffer

 LDY T1                 \ Restore the value of Y from T1, so it is preserved

 RTS                    \ Return from the subroutine