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From:	pfenwick AT sbrsim DOT ed DOT dreo DOT dnd DOT ca (Paul Fenwick)
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: Mode X
Date:	21 Jun 1995 12:53:12 GMT
Organization:	Defense Research Establishment Ottawa (DREO)
Lines:	58
References:	<3s7m1a$85d AT newsbf02 DOT news DOT aol DOT com>
Nntp-Posting-Host:	sbrsim.ed.dreo.dnd.ca
To:	djgpp AT sun DOT soe DOT clarkson DOT edu
Dj-Gateway:	from newsgroup comp.os.msdos.djgpp

In article <3s7m1a$85d AT newsbf02 DOT news DOT aol DOT com>,
Count0Int <count0int AT aol DOT com> wrote:
>  Could someone help me out.  As far as I see, when switching pages in
>various graphic modes (Mode X in this case), it requires the passing of a
>segment and an offset.  Could someone explain to me what I would call to
>switch pages in this case, as I've tried to obvious ...

You need to output the low address 8-bits followed by the high 8-bits to
the CRTC Register (0x3d4).  This should be done during vertical retrace
to avoid flicker.  The words written are formatted like:

Low WORD =   0xYY0d
High WORD =  0xZZ0c

where the address of the page offset is 0xZZYY.  This is the offset
from the start of the 64k graphics page (0xa0000).

The function attached is a gas conversion of the show_page function
from the XSHARP graphics package.

Hope this helps,

Paul

/********************************************************************/
/* x_show_page(WORD PageOffset); - sets visible page, waits for VR    */
/*               %ebp+8                                             */
/********************************************************************/
_x_show_page:
        pushl   %ebp
        movl    %esp, %ebp

/* Pre-load for fastest flip after Display Enable */
        movb    $0x0d, %bl      /* 0x0d - start address low */
        movb    8(%ebp), %bh
        movb    $0x0c, %cl      /* 0x0c - start address high */
        movb    9(%ebp), %ch
        movw    $0x3da, %dx     /* Input Status 1 */
WaitDE:
        inb     %dx, %al
        test    $1, %al
        jnz     WaitDE
/* DE is now active low - set start offset in display memory */
        movw    $0x3d4, %dx     /* CRTC Index */
        movw    %bx, %ax
        outw    %ax, %dx
        movw    %cx, %ax
        outw    %ax, %dx
        movw    $0x3da, %dx     /* Input Status 1 */
/* Wait for VSync so other page will be invisible when we draw */
WaitVS:
        inb     %dx, %al
        test    $8, %al
        jz      WaitVS

        pop     %ebp
        ret

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