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Here’s something worth noting: The animation is extremely smooth on a 20 MHz 386. It is somewhat more jerky on an 8 MHz 286, because only 30 frames a second can be processed. If animation looks jerky on your PC, try reducing the number of kites.

The kites draw perfectly into the background, with no interference or fringe, thanks to masked copying. In fact, the kites also cross with no interference (the last-drawn kite is always in front), although that’s not readily apparent because they all look the same anyway and are moving fast. Listing 49.5 isn’t inherently limited to kites; create your own images and initialize the object list to display a mix of those images and see the full power of Mode X animation.

The external functions called by Listing 49.5 can be found in Listings 49.1, 49.2, 49.3, and 49.6, and in the listings for the previous two chapters.

LISTING 49.6 L49-6.ASM

; Shows the page at the specified offset in the bitmap. Page is displayed when
; this routine returns.
; C near-callable as: void ShowPage(unsigned int StartOffset);
INPUT_STATUS_1     equ     03dah   ;Input Status 1 register
CRTC_INDEX         equ     03d4h   ;CRT Controller Index reg
START_ADDRESS_HIGH equ     0ch     ;bitmap start address high byte
START_ADDRESS_LOW  equ     0dh     ;bitmap start address low byte

ShowPageParms   struc
                 dw      2 dup (?) ;pushed BP and return address
StartOffset dw   ?                 ;offset in bitmap of page to display
ShowPageParms   ends
        .model  small
        public  _ShowPage
_ShowPage       proc    near
        push    bp                 ;preserve caller’s stack frame
        mov     bp,sp              ;point to local stack frame
; Wait for display enable to be active (status is active low), to be
; sure both halves of the start address will take in the same frame.
        mov     bl,START_ADDRESS_LOW        ;preload for fastest
        mov     bh,byte ptr StartOffset[bp] ; flipping once display
        mov     cl,START_ADDRESS_HIGH       ; enable is detected
        mov     ch,byte ptr StartOffset+1[bp]
        mov     dx,INPUT_STATUS_1
        in      al,dx
        test    al,01h
        jnz     WaitDE  ;display enable is active low (0 = active)
; Set the start offset in display memory of the page to display.
        mov     dx,CRTC_INDEX
        mov     ax,bx
        out     dx,ax   ;start address low
        mov     ax,cx
        out     dx,ax   ;start address high
; Now wait for vertical sync, so the other page will be invisible when
; we start drawing to it.
        mov     dx,INPUT_STATUS_1
        in      al,dx
        test    al,08h
        jz      WaitVS  ;vertical sync is active high (1 = active)
        pop     bp      ;restore caller’s stack frame
_ShowPage       endp

Works Fast, Looks Great

We now end our exploration of Mode X, although we’ll use it again shortly for 3-D animation. Mode X admittedly has its complexities; that’s why I’ve provided a broad and flexible primitive set. Still, so what if it is complex? Take a look at Listing 49.5 in action. That sort of colorful, high-performance animation is worth jumping through a few hoops for; drawing 20, or even 10, fair-sized objects at a rate of 60 Hz, with no flicker, interference, or fringe, is no mean accomplishment, even on a 386.

There’s much more we could do with animation in general and with Mode X in particular, but it’s time to move on to new challenges. In closing, I’d like to point out that all of the VGA’s hardware features, including the built-in AND, OR, and XOR functions, are available in Mode X, just as they are in the standard VGA modes. If you understand the VGA’s hardware in mode 12H, try applying that knowledge to Mode X; you might be surprised at what you find you can do.

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Graphics Programming Black Book © 2001 Michael Abrash