@program	prog_code,prog_name

;%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
;% Grayscale Example for Newbies - by Jonas Minnberg (Sasq) %
;%                                                          %
;% Since so many people on the FargoList asks about this, I %
;% put together a small example that you may use freely.    %
;% It simply uses a pixelplot routine do draw horisontal    %
;% lines in different colors.                               %
;%                  /Jonas Minnberg (md94-jmi@nada.kth.se)  %
;%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

;************** Start of Fargo program ***************

POS:    dc.l    0       ;Holds X & Y position for next pixel to plot

prog_code:
        jsr     graylib[on]             ;Turn on Grayscale display
        move.l  a6,plane2               ;Save pointer to second bitplane

        move.w   #959,d7                ;Clear second bitplane
\cloop0:
        clr.l   (a6)+                   ;Slow clearing but who cares :)
        dbf     d7,\cloop0

        move.w   #959,d7                ;Clear first bitplane
        lea     $4440,a6
\cloop1:
        clr.l   (a6)+
        dbf     d7,\cloop1

        moveq   #0,d3                   ;D3 Holds color number
        lea     POS(pc),a2              ;A2 Holds offset to coordinates
        clr.l   (a2)                    ;Start at (0,0)
        moveq   #127,d6                 ;Loop index 1, 128 lines
\loop:
        move.w   #239,d7                ;Loop index 2, 240 rows
\loop2:
        movem.w  (a2),d0/d1             ;Put (X,Y) in DO & D1
        bsr     put_grayscale_pixel     ;Call our pixelplot routine
        add.w   #1,(a2)                 ;Increase X coordinate
        dbf     d7,\loop2               ;Do the (Inner) Loop

        clr.w   (a2)                    ;Clear X coordinate and...
        add.w   #1,2(a2)                ;increase Y coordinate
        addq    #1,d3                   ;Also increase colorvalue
        and.b   #$03,d3                 ;and make sure it's always 0-3
        dbf     d6,\loop                ;Do the (Outer) Loop

        jsr     flib[idle_loop]         ;Wait for a keypress
        jsr     graylib[off]            ;And turn off grayscale display
	rts

;D0.W = X position, D1.W = Y position, D3.B = Color (0-3)
put_grayscale_pixel:
        move.w  d0,d4
        move.w  d1,d5
        lea     $4440,a0                ;First bitplane
        btst    #1,d3                   ;Test bit 1 in colorvalue to see if
        beq.s   \cpix0                  ;we should set or clear this pixel
        bsr     set_pixel
        bra.s   \done0
\cpix0:
        bsr     clr_pixel
\done0:
        move.w  d4,d0
        move.w  d5,d1
        move.l  plane2(pc),a0           ;Second bitplane
        btst    #0,d3                   ;Test bit 0 in colorvalue to see if
        beq.s   \cpix1                  ;we should set or clear this pixel
        bsr     set_pixel
        bra.s   \done1
\cpix1:
        bsr     clr_pixel
\done1:
        rts

;Clear Pixel
;A0.L = bitplane, D0.W = X position, D1.W = Y position
;Destroys D0-D2
clr_pixel:
        bsr     calc_offset
        bclr.b  d2,0(a0,d1)
        rts

;Set Pixel
;A0.L = bitplane, D0.W = X position, D1.W = Y position
;Destroys D0-D2
set_pixel:
        bsr     calc_offset
        bset.b  d2,0(a0,d1)
        rts

calc_offset:
        add.w   d1,d1           ;Multiply Y coordinate by 30
        move.w  d1,d2           ;without MUL instruction
        lsl.w   #4,d1           ;(32*Y-2*Y)
        sub.w   d2,d1           ;Only shifts and adds

        move.b  d0,d2
        eor.b   #$7,d2
        lsr.b   #3,d0
        add.w   d0,d1           ;D1 Now holds offset to byte
        rts

plane2:
        dc.l    0

prog_name:
        dc.b    "Grayscale example for Newbies",0

;*************** End of Fargo program ****************

	reloc_open
        add_library     flib
	add_library	graylib
	reloc_close
	end