Display Memory Organization
The four display memory
planes are used for different purposes when the VGA is in text mode. Each
byte in plane 0 is used to store an index into the character font map.
The corresponding byte in plane 1 is used to specify the attributes of
the character possibly including color, font select, blink, underline and
reverse. For more details on attribute operation see the Attributes section
below. Display plane 2 is used to store the bitmaps for the characters
themselves. This is discussed in the Fonts section below. Normally, the
odd/even read and write addressing mode is used to make planes 0 and 1
accessible at interleaved host memory addresses.
Attributes
The attribute byte is divided
into two four bit fields. The field from 7-4 is used as an index into the
palette registers for the background color which used when a font bit is
0. The field from 3-0 is used as an index into the palette registers for
the foreground which is used when a font bit is 1. Also the attribute can
control several other aspects which may modify the way the character is
displayed.
If the Blink
Enable field is set to 1, character blinking is enabled. When blinking
is enabled, bit 3 of the background color is forced to 0 for attribute
generation purposes, and if bit 7 of the attribute byte for a character
is set to 1, the foreground color alternates between the foreground and
background, causing the character to blink. The blink rate is determined
by the vertical sync rate divided by 32.
If the bits 2-0 of the attribute
byte is equal to 001b and bits 6-4 of the attribute byte is equal to 000b,
then the line of the character specified by the Underline
Location field is replaced with the foreground color. Note if the line
specified by the Underline Location field
is not normally displayed because it is greater than the maximum scan line
of the characters displayed, then the underline capability is effectively
disabled.
Bit 3 of the attribute byte,
as well as selecting the foreground color for its corresponding character,
also is used to select between the two possible character sets (see Fonts
below.) If both character sets are the same, then the bit effectively functions
only to select the foreground color.
Fonts
The VGA's text-mode hardware
provides for a very fast text mode. While this mode is not used as often
these days, it used to be the predominant mode of operation for applications.
The reason that the text mode was fast, much faster than a graphics mode
at the same resolution was that in text mode, the screen is partitioned
into characters. A single character/attribute pair is written to screen,
and the hardware uses a font table in video memory to map those character
and attribute pairs into video output, as opposed to having to write all
of the bits in a character, which could take over 16 operations to write
to screen. As CPU display memory bandwidth is somewhat limited (particularly
on on older cards), this made text mode the mode of choice for applications
which did not require graphics.
For each character
position, bit 3 of the attribute byte selects which character set is used,
and the character byte selects which of the 256 characters in that font
are used. Up to eight sets of font bitmaps can be stored simultaneously
in display memory plane 2. The VGA's hardware provides for two banks of
256 character bitmaps to displayed simultaneously. Two fields, Character
Set A Select and Character Set B Select
field are used to determine which of the eight font bitmaps are currently
displayed. If bit 3 of a character's attribute byte is set to 1, then the
character set selected by Character Set A Select
field, otherwise the character set specified by Character
Set B Select field is used. Ordinarily, both character sets use the
same map in memory, as utilizing 2 different character sets causes character
set A to be limited to colors 0-7, and character set B to be limited to
colors 8-15.
Fonts are either 8 or 9
pixels wide and can be from 1 to 32 pixels high. The width is determined
by the 9/8 Dot Mode field. Characters normally
have a line of blank pixels to the right and bottom of the character to
separate the character from its neighbor. Normally this is included in
the character's bitmap, leaving only 7 bit columns for the character. Characters
such as the capital M have to be squished to fit this, and would look better
if all 8 pixels in the bitmap could be used, as in 9 Dot mode where the
characters have an extra ninth bit in width, which is displayed in the
text background color, However, this causes the line drawing characters
to be discontinuous due to the blank column. Fortunately, the Line
Graphics Enable field can be set to allow character codes C0h-DFh to
have their ninth column be identical to their eighth column, providing
for continuity between line drawing characters. The height is determined
by the Maximum Scan Line field which is set
to one less than the number of scan lines in the character.
Display memory plane 2 is
divided up into eight 8K banks of characters, each of which holds 256 character
bitmaps. Each character is on a 32 byte boundary and is 32 bytes long.
The offset in plane 2 of a character within a bank is determined by taking
the character's value and multiplying it by 32. The first byte at this
offset contains the 8 pixels of the top scan line of the characters. Each
successive byte contains another scan line's worth of pixels. The best
way to read and write fonts to display memory, assuming familiarity with
the information from the Accessing the Display Memory
page, is to use standard (not Odd/Even) addressing and Read Mode 0 and
Write Mode 0 with plane 2 selected for read or write.
The following example shows
three possible bitmap representations of text characters. In the left example
an 8x8 character box is used. In this case, the Maximum
Scan Line field is programmed to 7 and the 9/8
Dot Mode field is programmed to 0. Note that the bottom row and right-most
column is blank. This is used to provide inter-character spacing. The middle
example shows an 8x16 character. In this case the Maximum
Scan Line field is programmed to 15 and the 9/8
Dot Mode field is programmed to 0. Note that the character has extra
space at the bottom below the baseline of the character. This is used by
characters with parts descending below the baseline, such as the lowercase
letter "g". The right example shows a 9x16 character. In this case the
Maximum Scan Line field is programmed to 15
and the 9/8 Dot Mode field is programmed to
1. Note that the rightmost column is used by the character, as the ninth
column for 9-bit wide characters is assumed blank (excepting for the behavior
of the the Line Graphics Enable field.) allowing
all eight bits of width to be used to specify the character, instead of
having to devote an entire column for inter-character spacing.
Cursor
The VGA has the hardware capability
to display a cursor in the text modes. Further details on the text-mode
cursor's operation can be found in the following section: