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specs/freevga/vga/vgatext.htm
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specs/freevga/vga/vgatext.htm
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<HTML>
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<HEAD>
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<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">
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<META NAME="Author" CONTENT="Joshua Neal">
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<META NAME="Description" CONTENT="Pure VGA/SVGA hardware programming (registers, identification, and other low-level stuff.)">
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<META NAME="KeyWords" CONTENT="VGA SVGA hardware video programming">
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<TITLE>VGA/SVGA Video Programming--VGA Text Mode Operation</TITLE>
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</HEAD>
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<BODY>
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<CENTER><A HREF="../home.htm">Home</A> <A HREF="#intro">Intro</A> <A HREF="#memory">Memory</A>
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<A HREF="#attributes">Attributes</A> <A HREF="#fonts">Fonts</A> <A HREF="#cursor">Cursor</A>
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<A HREF="vga.htm#general">Back</A>
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<HR WIDTH="100%"><B>Hardware Level VGA and SVGA Video Programming Information
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Page</B></CENTER>
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<CENTER>VGA Text Mode Operation
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<HR WIDTH="100%"></CENTER>
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<UL>
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<LI>
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<A HREF="#intro">Introduction</A> -- gives scope of this page.</LI>
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<LI>
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<A HREF="#memory">Display Memory Organization</A> -- details how the VGA's
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planes are utilized when in text mode.</LI>
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<LI>
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<A HREF="#attributes">Attributes</A> -- details the fields of the attribute
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byte.</LI>
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<LI>
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<A HREF="#fonts">Fonts</A> -- details the operation of the character generation
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hardware.</LI>
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<LI>
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<A HREF="#cursor">Cursor</A> -- details on manipulating the text-mode cursor.</LI>
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</UL>
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<A NAME="intro"></A><B>Introduction</B>
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<BR> This section is intended
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to document the VGA's operation when it is in the text modes, including
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attributes and fonts. While it would seem that the text modes are adequately
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supported by the VGA BIOS, there is actually much that can be done with
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the VGA text modes that can only be accomplished by going directly to the
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hardware. Furthermore, I have found no good reference on the VGA text modes;
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most VGA references take them for granted without delving into their operation.
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<P><A NAME="memory"></A><B>Display Memory Organization</B>
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<BR> The four display memory
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planes are used for different purposes when the VGA is in text mode. Each
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byte in plane 0 is used to store an index into the character font map.
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The corresponding byte in plane 1 is used to specify the attributes of
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the character possibly including color, font select, blink, underline and
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reverse. For more details on attribute operation see the Attributes section
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below. Display plane 2 is used to store the bitmaps for the characters
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themselves. This is discussed in the Fonts section below. Normally, the
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odd/even read and write addressing mode is used to make planes 0 and 1
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accessible at interleaved host memory addresses.
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<P><A NAME="attributes"></A><B>Attributes</B>
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<BR> The attribute byte is divided
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into two four bit fields. The field from 7-4 is used as an index into the
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palette registers for the background color which used when a font bit is
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0. The field from 3-0 is used as an index into the palette registers for
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the foreground which is used when a font bit is 1. Also the attribute can
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control several other aspects which may modify the way the character is
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displayed.
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<BR> If the <A HREF="attrreg.htm#10">Blink
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Enable</A> field is set to 1, character blinking is enabled. When blinking
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is enabled, bit 3 of the background color is forced to 0 for attribute
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generation purposes, and if bit 7 of the attribute byte for a character
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is set to 1, the foreground color alternates between the foreground and
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background, causing the character to blink. The blink rate is determined
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by the vertical sync rate divided by 32.
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<BR> If the bits 2-0 of the attribute
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byte is equal to 001b and bits 6-4 of the attribute byte is equal to 000b,
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then the line of the character specified by the <A HREF="crtcreg.htm#14">Underline
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Location</A> field is replaced with the foreground color. Note if the line
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specified by the <A HREF="crtcreg.htm#14">Underline Location</A> field
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is not normally displayed because it is greater than the maximum scan line
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of the characters displayed, then the underline capability is effectively
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disabled.
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<BR> Bit 3 of the attribute byte,
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as well as selecting the foreground color for its corresponding character,
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also is used to select between the two possible character sets (see <A HREF="#fonts">Fonts</A>
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below.) If both character sets are the same, then the bit effectively functions
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only to select the foreground color.
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<P><A NAME="fonts"></A><B>Fonts</B>
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<BR> The VGA's text-mode hardware
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provides for a very fast text mode. While this mode is not used as often
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these days, it used to be the predominant mode of operation for applications.
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The reason that the text mode was fast, much faster than a graphics mode
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at the same resolution was that in text mode, the screen is partitioned
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into characters. A single character/attribute pair is written to screen,
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and the hardware uses a font table in video memory to map those character
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and attribute pairs into video output, as opposed to having to write all
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of the bits in a character, which could take over 16 operations to write
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to screen. As CPU display memory bandwidth is somewhat limited (particularly
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on on older cards), this made text mode the mode of choice for applications
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which did not require graphics.
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<P> For each character
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position, bit 3 of the attribute byte selects which character set is used,
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and the character byte selects which of the 256 characters in that font
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are used. Up to eight sets of font bitmaps can be stored simultaneously
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in display memory plane 2. The VGA's hardware provides for two banks of
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256 character bitmaps to displayed simultaneously. Two fields, <A HREF="seqreg.htm#03">Character
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Set A Select</A> and <A HREF="seqreg.htm#03">Character Set B Select</A>
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field are used to determine which of the eight font bitmaps are currently
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displayed. If bit 3 of a character's attribute byte is set to 1, then the
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character set selected by <A HREF="seqreg.htm#03">Character Set A Select</A>
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field, otherwise the character set specified by <A HREF="seqreg.htm#03">Character
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Set B Select</A> field is used. Ordinarily, both character sets use the
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same map in memory, as utilizing 2 different character sets causes character
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set A to be limited to colors 0-7, and character set B to be limited to
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colors 8-15.
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<BR> Fonts are either 8 or 9
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pixels wide and can be from 1 to 32 pixels high. The width is determined
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by the <A HREF="seqreg.htm#01">9/8 Dot Mode</A> field. Characters normally
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have a line of blank pixels to the right and bottom of the character to
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separate the character from its neighbor. Normally this is included in
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the character's bitmap, leaving only 7 bit columns for the character. Characters
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such as the capital M have to be squished to fit this, and would look better
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if all 8 pixels in the bitmap could be used, as in 9 Dot mode where the
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characters have an extra ninth bit in width, which is displayed in the
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text background color, However, this causes the line drawing characters
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to be discontinuous due to the blank column. Fortunately, the <A HREF="attrreg.htm#10">Line
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Graphics Enable</A> field can be set to allow character codes C0h-DFh to
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have their ninth column be identical to their eighth column, providing
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for continuity between line drawing characters. The height is determined
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by the <A HREF="crtcreg.htm#09">Maximum Scan Line</A> field which is set
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to one less than the number of scan lines in the character.
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<BR> Display memory plane 2 is
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divided up into eight 8K banks of characters, each of which holds 256 character
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bitmaps. Each character is on a 32 byte boundary and is 32 bytes long.
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The offset in plane 2 of a character within a bank is determined by taking
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the character's value and multiplying it by 32. The first byte at this
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offset contains the 8 pixels of the top scan line of the characters. Each
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successive byte contains another scan line's worth of pixels. The best
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way to read and write fonts to display memory, assuming familiarity with
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the information from the <A HREF="vgamem.htm">Accessing the Display Memory</A>
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page, is to use standard (not Odd/Even) addressing and Read Mode 0 and
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Write Mode 0 with plane 2 selected for read or write.
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<BR> The following example shows
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three possible bitmap representations of text characters. In the left example
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an 8x8 character box is used. In this case, the <A HREF="crtcreg.htm#09">Maximum
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Scan Line</A> field is programmed to 7 and the <A HREF="seqreg.htm#01">9/8
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Dot Mode</A> field is programmed to 0. Note that the bottom row and right-most
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column is blank. This is used to provide inter-character spacing. The middle
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example shows an 8x16 character. In this case the <A HREF="crtcreg.htm#09">Maximum
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Scan Line</A> field is programmed to 15 and the <A HREF="seqreg.htm#01">9/8
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Dot Mode</A> field is programmed to 0. Note that the character has extra
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space at the bottom below the baseline of the character. This is used by
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characters with parts descending below the baseline, such as the lowercase
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letter "g". The right example shows a 9x16 character. In this case the
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<A HREF="crtcreg.htm#09">Maximum Scan Line</A> field is programmed to 15
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and the <A HREF="seqreg.htm#01">9/8 Dot Mode</A> field is programmed to
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1. Note that the rightmost column is used by the character, as the ninth
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column for 9-bit wide characters is assumed blank (excepting for the behavior
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of the the <A HREF="attrreg.htm#10">Line Graphics Enable</A> field.) allowing
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all eight bits of width to be used to specify the character, instead of
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having to devote an entire column for inter-character spacing.
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<CENTER><A HREF="char.txt"><IMG SRC="Char.gif" ALT="Click for Textified Examples of Text Mode Bitmap Characters" BORDER=0 HEIGHT=256 WIDTH=376></A></CENTER>
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<P>
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<BR><A NAME="cursor"></A><B>Cursor</B>
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<BR> The VGA has the hardware capability
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to display a cursor in the text modes. Further details on the text-mode
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cursor's operation can be found in the following section:
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<UL>
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<LI>
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<A HREF="textcur.htm">Manipulating the Text-mode Cursor</A> -- details
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controlling the appearance and location of the cursor.</LI>
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</UL>
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Notice: All trademarks used or referred to on this page are the property
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of their respective owners.
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<BR>All pages are Copyright © 1997, 1998, J. D. Neal, except where
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noted. Permission for utilization and distribution is subject to the terms
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of the <A HREF="license.htm">FreeVGA Project Copyright License</A>.
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<BR>
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<BR>
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</BODY>
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</HTML>
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