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⌂ ∞ ▽ Elite on the 6502

Drawing lines: LOIN

[BBC Master version]

       Name: LOIN                                                    [Show more]
       Type: Subroutine
   Category: Drawing lines
    Summary: Draw a one-segment line

    Context: See this subroutine in context in the source code
 Variations: See code variations for this subroutine in the different versions
 References: This subroutine is called as follows:
             * BLINE calls LOIN
             * BOMBOFF calls LOIN
             * DVLOIN calls LOIN
             * LASLI calls LOIN
             * LL9 (Part 12 of 12) calls LOIN
             * LSPUT calls LOIN
             * TT15 calls LOIN
             * WPLS2 calls LOIN




 Arguments:

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

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

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

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



.LOIN

 STY YSAV               \ Store Y in YSAV so we can retrieve it below

 LDA #%00001111         \ Set bits 1 and 2 of the Access Control Register at
 STA VIA+&34            \ SHEILA &34 to switch screen memory into &3000-&7FFF

 JSR LOINQ              \ Draw a line from (X1, Y1) to (X2, Y2)

 LDA #%00001001         \ Clear bits 1 and 2 of the Access Control Register at
 STA VIA+&34            \ SHEILA &34 to switch main memory back into &3000-&7FFF

 LDY YSAV               \ Retrieve the value of Y we stored above

 RTS                    \ Return from the subroutine