Well, Atari built 2 graphic-custom-chips into this great 8-bit workstation ;), one is called ANTIC, the 2nd is called GTIA ("Graphic Television Interface Adaptor"). The GTIA is the chip between the ANTIC and your monitor/tv-set. The main task of the GTIA is to convert the data which the Antic produces into a complete picture for the monitor.

Another job is to produce the player/missle-grafic.

But I dont want to bore you with things that you already know. What do I mean if I am talking about "GTIA-modes"?. If you have read the Antic-capture carefully then you have recognized that there are modes missing which you can access via BASIC... the Basic-Modes 9,10,11.

Actually this modes are Gr.8-Modes! But you can tell the GTIA to take the bits of this hires-mode to interprate the bits in another way. The main structure of such modes is that 4 bits (aka nibbles) are now selecting their display-option. As we have 8 bits in 1 byte and now 4 bits are grouped together we can display 2 pixels with 1 byte. For a "normal" 320 pixel wide scanline in gr.8 (40 bytes a 8 bits = 320) we can display 80 pixels in the gtia-modes!

But why are all new demos using this modes? why are they so special?
You can use the GTIA to display more colors or shades then in the other modes possible! You just have to select the modes by moving following values into the GTIA-Ctrl-Register $26f,$d01b:

Bit7  Bit6
 
 0     0    normal mode...as usual
 0     1    so called "mode9". Same as basic-gr.9. 1 color in 16 shades
 1     0    so called "mode10". Basic-gr.10. 9 colors in different shades
 1     1    so called "mode11". Basic-gr.11. 16 colors in 1 shade

Mode9: you define a basic-color via the background-color-register $d01a,712
       by putting $00,$10,$xx,$f0 there. But be carefull! Don`t set bits 0-3 
       So $02,$18,... is wrong and will reduce the maximum amount of shades! 

Mode10:The Atari has 9 different color-registers, 704-712 (inclduing the
       PM-color-registers 704-707+711. So we can theoreticly select 16 color-
       registers but as we have just 9 the others are not possible! Keep that
       in mind! In this mode the nibbles are not allowed to be greater than
       9! ($00-$08). The GTIA takes the nibble value, adds 704 and gets the
       color-value stored there and puts the pixel with that color on screen!
       In this mode you can define 9 different colors! (you are not limited
       in your selectin, all colors are possible).

Mode11:Similar to Mode9 but instead of using the background-color for defining
       the basic-color, GTIA uses this information for the basic-brightness of
       all 16 basic-XL-Colors! (the so called rainbow-colors).

dlist dfb $4f ($40 = lms-command in mode $0f = gr.8)
      dfw scanline0
      dfb $4f
      dfw scanline0
      dfb $4f
      dfw scanline0
      dfb $4f
      dfw scanline0
      dfb $4f
      dfw scanline1
      dfb $4f
      dfw scanline1
      ...

dlinit lda #dlist  ; where to generate the custom-display-list?
       sta adr     ; zero-page-pointer f.e. $b0
       lda #>dlist ; hi-byte       
       sta adr+1
       lda #screen ; our screen-ram
       sta scradr
       lda #>screen
       sta scradr+1
       ldx #58     ; 80x58 is our new resolution...
dl0    lda #$4f    ; set commands into dlist
       ldy #0      ; 4 times... look above
       sta (adr),y
       ldy #3
       sta (adr),y
       ldy #6
       sta (adr),y
       ldy #9
       sta (adr),y
       lda scradr  ; set now low-byte
       ldy #1
       sta (adr),y
       ldy #4
       sta (adr),y
       ldy #7
       sta (adr),y
       ldy #10
       sta (adr),y
       lda scradr+1 ; and now hi-byte
       ldy #2
       sta (adr),y
       ldy #5
       sta (adr),y
       ldy #8
       sta (adr),y
       ldy #11
       sta (adr),y

; we have set now the 4 scanline-commands with the same scanline!
; but now we have to move the adr-counters forward

      clc
      lda adr
      adc #12  ; 4*3 bytes in the dlist! look into your debugger!
      sta adr
      lda adr+1
      adc #0
      sta adr+1
      clc
      lda scradr ; and the next scanline!
      adc #40    ; 40 bytes per line, depends on YOU...
      sta scradr
      lda scradr+1
      adc #0
      sta scradr+1
      dex        ; 58x
      bpl dl0

;STOP! we are not finished! we must now set the $41 command to stable the
;monitor-picture...

      ldy #0
      lda #$41
      sta (adr),y
      iny       ; saves 1 byte... same as ldy #1 ;)
      lda #dlist
      sta (adr),y
      iny
      lda #>dlist
      sta (adr),y
      rts

init  jsr dlinit ; initialize mode9
      lda #0     ; stop interrupts
      sta nmien  ; $d40e
      lda #dlist
      sta dlistv ; $230,$231
      lda #>dlist
      sta dlistv+1
      lda #$40   ; vbi enabled
      sta nmien
      lda #$40   ; hey... without this we have just a mode8+ (gr.8) screen ;)
      sta gtiamode ; $26f = pm-priors+gtia-modes
      ...

add MAC 2  ; the 16-bit macro with 2 variables
    clc
    lda ?1
    adc #?2
    sta ?1
    lda ?1+1
    adc #0
    sta ?1+1
    END

move MAC 2 ; 16-bit poke
    lda #?2
    sta ?1
    lda #>?2
    sta ?1+1
    END

set MAC 1 ; sets the LMS-command in the display-list 
    ldy #?1
    sta (adr),y
    END

; so the dlinit-code looks like this:

dlinit .move screen,scradr
       .move dlist,adr
       ldx #58
dl0    lda #$4f
       .set 0
       .set 3
       .set 6
       .set 9
       lda scradr
       .set 1
       .set 4
       .set 7
       .set 10
       lda scradr+1
       .set 2
       .set 5
       .set 8
       .set 12
       .add scradr,40
       .add adr,12
       dex
       bpl dl0
       ...

But the most advantages are:

   ; setting pixel with shade $09 on position 2,3
   
   lda screen+3*40+(2 div 2)   
   and #$f0     ; keep pixel on pos. 1,3
   ora #$09       
   sta screen+3*40+(2 div 2)

display_fx ldx $a0
           lda texturelo,x
           ldx $a1
           ora texturehi,x
           sta screen
           ldx $a2
           lda texturlo,x
           ldx $a3
           ora texturhi,x
           sta screen+1
           ...