VGA Text Mode
An introduction to font selection
and to reprogramming of the
Character Generator ram
alphanumeric information
• Most early PC programs weren’t graphical
• Screen was similar to a typewriter’s output
• Used only a limited set of character-images:
– letters, numerals, and punctuation symbols
• Hardware can efficiently render such glyphs
• VGA can emulate the MDA/EGA text modes:
– mode 1: emulates EGA 40x25 text (320x200 pixels)
– mode 3: emulates EGA 80x25 text (640x400 pixels)
– mode 7: emulates MDA 80x25 text (720x350 pixels)
Font images in ROM
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VGA has built-in firmware (ROM-BIOS)
Both code and data are provided in ROM
VGA’s ROM is normally at 0x000C0000
Can be addressed by CPU in ‘real-mode’
VGA ROM includes character glyph tables
– 8x8 character-set is used with mode 1
– 8x16 character-set is used with mode 3
– 8x14 character-set is used with mode 7
A character-glyph example
• Here’s a sample 8x8 character glyph (‘A’):
This glyph can be represented as an array of 8 bytes:
0x00, 0x38, 0x6C, 0xC6, 0xFE, 0xC6, 0x00, 0x00
VGA Timer-Sequencer
• A ‘character table’ is copied into VRAM
• Glyphs organized as an array of bitmaps
• Array holds 256 images (32 bytes/image)
– Table-size: 256x32 = 8K bytes (= 0x2000)
• The ascii-codes serve as array-indexes:
– Example: ‘A’ has ascii-code 0x41 (=65)
• Sequencer hardware accesses these images
• Dedicated area of VRAM is used (plane 2)
• Planes 0 and 1 are used as a text frame-buffer
Arrangement of VRAM planes
plane 3
plane 2
plane 1
plane 0
Not used (masked)
Holds the character glyph table(s)
Stores color attribute-bytes for displayed text
Stores ASCII- codes for the currently display text
CPU can rea/write only to planes 0 and 1 (using odd/even addressing mode)
multiple fonts supported
• Plane 2 is the Character Generator ram
• Enough room for eight separate tables:
– 8 x 8K = 64K
• Two tables can be in use simultaneously
• Tables must start at prescribed offsets:
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Table 0: 0x0000
Table 1: 0x4000
Table 2: 0x8000
Table 3: 0xC000
Table 4: 0x2000
Table 5: 0x6000
Table 6: 0xA000
Table 7: 0xE000
Text color ‘attributes’
• In text mode, the picture-elements consist
of character images, shown in two colors:
– Foreground color, and Background color
• Character colors individually programmed
• A byte-pair in VRAM selects the bit-image
(ascii code) and color-pair (attribute byte)
• All colors come from a palette of 16
• But the color palette is ‘programmable’
Layout for an ‘attribute’ byte
Background Color
R
G
B
Bit function is Programmable:
Default is “EnableBlinking”
Foreground Color
R
G
B
Bit function is programmable
Default is “EnableIntensity”
VGA Sequencer Registers
• Five registers comprise VGA Sequencer:
– index 0:
– index 1:
– index 2:
– index 3:
– index 4:
Reset register
Clocking Mode register
Map Mask register
Character Map Select register
Memory Mode register
• All accessed via i/o ports 0x3C4-0x3C5
using “multiplexing” scheme (index/data)
Access to character ram
• In text modes, CPU can’t access Plane 2
• Plane 2 gets accessed by the Sequencer
• Sequencer performs a ‘glyph-lookup’ for
each ascii code stored by CPU in Plane 0
• Both Sequencer and Graphics Controller
must be suitably reprogrammed in order
for the CPU to a read or write to Plane 2
• Six VGA registers are involved in that step
Reset (index 0)
7
6
5
4
3
2
1
0
Synchronous Reset bit (bit 1)
1 = normal sequencer operation
0 = initiate a synchronous reset
A synchronous reset is used in
advance of reprogramming the
Clocking Mode register (or the
Clock Select field in the VGA’s
Miscellaneous Output register)
Asynchronous Reset bit (bit 0)
1 = normal sequencer operation
0 = initiate an asynchronous reset
Halts VRAM refresh cycles
and clears VRAM contents
Map Mask (index 2)
7
6
5
4
3
2
1
Plane 3
Plane 2
Plane 1
Enables or disables CPU’s ability
to access specific memory planes
1 = write enable, 0 = write disable
Plane 0
0
Memory Mode (index 4)
7
6
5
“Chain-4” Addressing
1 = enabled
0 = disabled
i.e., each plane holds
every fourth byte
“Odd/Even” Addressing
1 = disabled
0 = enabled
It’s used for text modes
4
3
2
1
0
(For EGA only)
1= text
0 = graphics
Extended Memory (>64K)
1 = present, 0 = absent
GC: Miscellaneous (index 6)
7
6
5
4
3
Memory Map
Memory Map options:
00 = 0xA0000 (128K)
01 = 0xA0000 (64K)
10 = 0xB0000 (64K)
11 = 0xB8000 (32K)
2
1
Odd/
Even
enable
0
G/A
1 = use Odd/Even Addressing
0 = use Sequential Addressing
1 = Disable the character generator( graphics mode)
0 = Enable the character generator (use “text mode”)
Graphics Controller registers are accessed via i/o ports 0x3CE/0x3CF
GC: Mode (index 5)
7
6
256
colors
5
SHIFT
4
ODD/
EVEN
3
Read
Mode
2
1
Write Mode
(0, 1, 2, 3)
Should be 0 for text mode
This affects the VGA
Attribute Controller’s
operation (text color:
foreground color and
background color)
Should be 1 for text
1 = cpu data at odd addresses is
mapped to odd-numbered planes,
cpu data at even addresses gets
mapped to even-numbered planes
0 = cpu addressing is sequential
NOT: This affects only the Graphics
Controller. The Sequencer needs to
be programmed separately to match
0
How to modify character ram
• Algorithm:
– Reset the VGA for accessing Plane 2
– Then CPU reads or modifies Plane 2
– Reset the VGA for accessing Planes 0, 1
• Acknowledgement:
– Author Richard Wilton described this process
in his classic book “Programmer’s Guide to
PC Video Systems (2nd Edition)”
Details for this ‘prolog’
outw( 0x0100, 0x3C4 ); // do a synch. reset
outw( 0x0402, 0x3C4); // write Plane 2 only
outw( 0x0704, 0x3C4 ); // sequential access
outw( 0x0300, 0x3C4 ); // end the reset
outw( 0x0204, 0x3CE ); // read Plane 2 only
outw( 0x0005, 0x3CE ); // disable odd/even
outw( 0x0006, 0x3CE ); // VRAM at 0xA0000
Details for the ‘epilog’
outw( 0x0100, 0x3C4 ); // do a synch. reset
outw( 0x0302, 0x3C4); // write Planes 0 & 1
outw( 0x0304, 0x3C4 ); // odd/even access
outw( 0x0300, 0x3C4 ); // end the reset
outw( 0x0004, 0x3CE ); // restore to ‘default’
outw( 0x1005, 0x3CE ); // resume odd/even
outw( 0x0E06, 0x3CE );// VRAM at 0xB8000
Some Class Demos
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‘newzero.cpp’: installs new glyph for ‘0’
‘romfonts.cpp’: finds ROM glyph-tables
‘backward.cpp’: flips character images!
‘vm86blue.cpp’: changes the text’s color
Algorithm for ‘backward.cpp’
for (int i = 0; i < 8192; i++)
{
int
orig = vram[ i ], revs = 0;
for (int j = 0; j < 8; j++)
if ( orig & (1<<j) ) revs |= (1 << (7-j) );
vram[ i ] = revs;
}
In-Class Exercises
• Exercise #1: Design a new image for ‘A’
• Exercise #2: Draw all text upside-down
• Exercise #3: Draw ‘yellow-on-green’ text