AmigaExamples/014.lace_mode

interlaced playfield

For interlace mode, we can enable it by simply setting the LACE bit in BPLCON0:

  move.w	#(SCREEN_BIT_DEPTH<<12)|COLOR_ON|HOMOD|LACE,BPLCON0(a6)

Now each alternating frame will be offset vertically by half a scan line.

So for this to work, we need to make some other changes. Firstly we need twice the number of rows in our bitplane data. Then on each alternating frame we need to set up the bitplane pointers such that they point at the correct set of row data. For this we set up two copper lists, then poke offset bitplane pointers into one of them:

 ;; poke the bitplane pointers for the two copper lists.
 move.l	#SCREEN_WIDTH_BYTES*SCREEN_BIT_DEPTH,d0
 lea 	copper(pc),a0
 bsr.s	pokeBitplanePointers
 
 moveq.l	#0,d0
 lea 	copperLOF(pc),a0
 bsr.s	pokeBitplanePointers	

Then, during the vertical blank, depending on the LOF bit in the VPOSR register, we install the correct copper list:

 .mainLoop:
	bsr     waitVerticalBlank
	btst.w  #VPOSRLOFBIT,VPOSR(a6)
	beq.s   .lof
	lea     copper(pc),a0
	move.l  a0,COP1LC(a6)
	bra     .done
 .lof:
	lea     copperLOF(pc),a0
	move.l  a0,COP1LC(a6)
 .done
	bra     .mainLoop

This example allows your to enable/disable INTERLACE mode in the Makefile:

INTERLACE=1

try it

• Download disk image
• Run in SAE

interlaced

non-interlaced