====== Opening borders and other things ======

Opening the upper and lower borders is a very common VIC trick that I'm sure coders still use today.

The basic idea of this is to change the rows-bit of $d011 on the last row of a frame. This makes the VIC think that the borders have already started being drawn- when in reality they haven't. The VIC will place graphics from $3fff/$7fff/$bfff/$ffff depending on your selected VIC bank in the opened area. Make sure to set $3fff to 0 so this can't be seen. This doesn't need to be done with stable timing.

So, here's an example piece of code (TASS syntax):

<code>;Opening the upper and lower borders
;Coding by Karmic/HVSC

  sei ;disable interrupts
  lda #$35 ;bank off Kernal and BASIC roms
  sta $01
  lda #$7f ;disable timer and CIA IRQs/NMIs
  sta $dc0d
  sta $dd0d
  lda $dc0d ;acknowledge any pending interrupt
  lda $dd0d
  lda #<irq ;set the IRQ vectors
  sta $fffe
  lda #>irq
  sta $ffff
  lda #$f8 ;set rasterline- can be done on lines $f8-$fa
  sta $d012
  lda #$01 ;enable raster irqs
  sta $d01a
  lda #$00 ;make it so $3fff area displays nothing
  sta $3fff
  sta $d010 ;all MSBs of X positions off
  lda #$ff ;make a block sprite at $3000
  ldx #63
  sta $3000,x
  dex
  bpl *-4
  ldx #$07
loop
  lda #$01 ;set sprite colors to white
  sta $d027,x
  lda #$c0 ;set sprite pointers
  sta $07f8,x
  dex
  bpl loop
  inx ;set sprite positions
  clc
  lda #$40
loop2
  sta $d000,x
  sta $d001,x
  adc #$18
  inx
  inx
  cpx #$10
  bcc loop2
  lda #$ff ;enable sprites now
  sta $d015
  inc $d019 ;acknowledge any pending VIC IRQs
  cli ;enable interrupts again
main
  ldy #8 ;delay
  dey
  bpl *-1
  nop
  inc $d020 ;something to look at
  inc $d021
  jmp main
  
irq
  lda #$13 ;make it 24 rows
  sta $d011
  lda #$fc ;wait for line $fc- anything not in the last row will work
  cmp $d012
  bne *-3
  lda #$1b ;set back to 25 rows
  sta $d011
  clc
  ldx #1 ;move the sprites around
loop3
  inc $d000,x
  dec $d002,x
  txa
  adc #4
  tax
  cpx #$10
  bcc loop3
  inc $d019 ;acknowledge IRQ
  rti ;end</code>

Wait, but that's not all. You can also disable the main screen and show nothing but sprites. $3fff will be displayed on the whole screen. This has the advantage of not having to worry about badlines. This idea is very similar to the previous, except with a few differences:

You must write $1b to $d011 first, and then write $00.
The first write can be done on lines $31-$f6, and the second write can be done on lines $f8-$fa.