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VGA TEXT ATTRIBUTES AND SPECIAL EFFECTS
by Chris Dunford
While working on a project recently, I realized that I didn't know very
much about how VGA text attributes work. I knew that there wasn't a
simple one-to-one relationship between attribute numbers and colors, but
that's about it. So, I did a little research and fooling around, and
found that lots of interesting effects are possible. You can draw
people's attention to important screen information in ways more (and
less!) subtle than simply flashing it on and off or using intensified
colors. For example, you can have colors that gradually shade from
normal to high intensity and back, "pulsing" on the screen; you can
display text that changes from one color to another (gradually shading
from blue to red and back, for example); you can soften blinking by
having the color "fade" rather than just disappear. And, there are some
things you can do under software control that can't be done with the
usual hardware attributes; for example, you can use flashing
backgrounds, or you can using flashing foregrounds without sacrificing
high intensity backgrounds. These can be done very efficiently, without
the need to change the attributes of individual characters.
This document is an attempt to share some of what I learned; it
explains how VGA text attributes work, and how these effects are
possible. The archive that contains this document should also contain
a demo program and a linkable object module (with source) that allow
you to use the effects in your own programs. You might want to do a
quick run of DEMO1 to see some of the possible effects in action.
What's different with the VGA?
------------------------------
The CGA implemented a direct relationship between a 4-bit attribute and
a color. Each attribute mapped to one color, and the color could not be
changed.
That changed with the EGA (which had palette registers to play with),
and changed again, radically, with the VGA. The attribute number
specifies the displayed color very indirectly; it is combined with bits
and pieces of 5 different internal VGA registers before it appears on
the screen as one of the 262,144 possible colors. There are therefore
several ways to change a character's color other than by changing its
attribute. And, of course, if you change the way an attribute is
displayed, all characters with that attribute are affected. Thus, it is
possible to "design" attributes with interesting effects, and to alter
the way entire screen fields are displayed very efficiently.
Mapping attributes to colors
----------------------------
Before we start, let me note that we are working here with 4-bit
attributes. The 8-bit attribute that's associated with a character is,
of course, two 4-bit attributes: one for the foreground, and one for the
background. This document discusses how these 4-bit attributes become
colors; everything here applies equally to foreground and background
attributes (thus it is possible to use flashing backgrounds, etc.)
Screen colors in text modes are controlled by the VGA's attribute
controller. In one of its two modes (the one that the PS/2 BIOS sets
by default), here is how a color is selected by an attribute:
AND
attribute |-------| color plane enable register
|
| selects
|
palette register 0-15 color select register
| |
| |
| bits 0-5 bits 6-7 |
+------------+------------+
|
| AND
+-------+--------video DAC mask register
|
| selects
|
video DAC color register 0-255
This appears complex, but it can be simplified. First, notice the two
logical AND operations. Since both the color plane enable register and
the video DAC mask register normally contain all ones, they tend to drop
out of the picture, resulting in:
attribute 0-15
|
| selects
|
palette register 0-15 color select register
| |
| |
| bits 0-5 bits 6-7 |
+------------+------------+
|
| selects
|
video DAC color register 0-255
The 4-bit attribute (0-15) selects one of the 16 palette registers. The
palette register contains a number. This number is combined with the
contents of the color select register to form another number. For
example, suppose:
attribute = 13
palreg[13] = 1Bh
CSR = 3
Attribute 13 selects palette register 13, which contains 1BH. The color
select register contains 3. We put the contents of the color select
register in bits 6-7 of the final number, and the contents of palreg[13]
in bits 0-5:
3 1B
- --
11 011011
The resulting number is 11011011 binary, 219 decimal. This number
selects one of the 256 video DAC color registers.
A C-like formula for the video DAC color resgister selection is:
r = (p[a] & 0x3F) | ((c << 6) & 0xC0)
where r is the DAC color register number, p[] is the array of 16 palette
registers, a is the attribute number (0..15), and c is the current
contents of the color select register.
The color that will be displayed is the color defined by the selected
video DAC color register (DAC stands for Digital-to-Analog Converter).
Each video DAC register is an 18-bit register that contains three 6-bit
values: one for each of the red, green, and blue primary colors. Each
value specifies the intensity of that primary color on the screen. A
zero for a primary means that it's not included in the color; 63 is
maximum intensity.
Suppose, in our example, that video DAC register 219 contains the
numbers 0, 3FH, and 3FH (it's convenient to think of the register as
three 6-bit values rather than one 18-bit value). This means that red
will be off, and green and blue will be at maximum intensity. The color
displayed by attribute 13 will be bright cyan.
Notice that you could change the color of a character with FG attribute
N in four different ways:
1. Change the attribute
2. Change the contents of palreg[N]
3. Change the contents of the video DAC register specified by
palreg[N] and the color select register
4. Change the contents of the color select register
These all have different effects on the total screen display. Changing
an attribute affects a single character; changing a palette register
affects all characters with that attribute; changing a video DAC
register affects any character whose attribute, when combined with color
select, selects that register--this could be several attributes;
changing the color select register could very well affect everything on
the screen.
Another way to visualize color selection
----------------------------------------
There is a simpler way to look at the process. The two bits used from
the color select register always end up in the high two bits of the
video DAC color register number. Thus, the selected DAC color register
will always be one of:
CSR=0: 00xx xxxx (register range 0-63)
CSR=1: 01xx xxxx (64-127)
CSR=2: 10xx xxxx (128-191)
CSR=3: 11xx xxxx (192-255)
The remaining 6 bits come directly from the selected palette register.
If we break the 256 video DAC color registers into four palettes of 64
colors each, as follows:
DAC registers Palette
0-63 0
64-127 1
128-191 2
192-255 3
then the color select register selects one of the four 64-color
palettes, and the palette register (which is selected by the attribute)
selects one of the 64 colors of the CSR-selected palette.
Thus, it's simply stated: the attribute selects one of the 64 colors
available from the palette selected by the color select register.
Mode 1
------
I mentioned that the above represents one of the two modes supported by
the attribute controller; call it "mode 0". The other mode (mode 1) is
very similar; the only difference is in which bits are combined from the
palette register and the color select register. The simplified diagram
is as follows:
attribute 0-15
|
| selects
|
palette register 0-15 color select register
| |
| |
| bits 0-3 bits 4-7 |
+------------+------------+
|
| selects
|
video DAC color register 0-255
The difference is that there are four bits each from the palette
register and the color select register (instead of 6 and 2 under the
mode 0).
A C-like formula for the video DAC color resgister selection in mode 1:
r = (p[a] & 0x0F) | ((c << 4) & 0xF0)
The effect of mode 1 is that the 256 video DAC color registers are
broken down into sixteen 16-color palettes instead of four 64-color
palettes:
DAC registers Palette
0-15 0
16-31 1
32-47 2
...
240-255 15
The palette register (still selected by the attribute) selects one of
the 16 colors available in the palette selected by the current color
select register.
The more complex effects (graded color changes, pulsing, etc.) of the
demo program DEMO1 are generated under mode 1. This is because all 256
colors defined by the video DAC color registers can be accessed by
simply changing the contents of the color select register, which is very
efficient (it takes about 1/100th of a second).
Under mode 0, only 75% of the available colors can be accessed by just
changing the color select register (this is because there are 64 colors
per palette, but only 16 attributes). To get to the remaining colors,
you have to change the palette registers. But there are 16 palette
registers; if you need to alter more than one displayed color, this is
less efficient than using the color select register.
A mode 1 technique
------------------
As mentioned, there are a number of ways to alter displayed colors. I
will concentrate on one technique: creating a series of palettes and
then switching palettes by systematically changing the contents of the
color select register. This document concentrates on mode 1; some of
the same effects (such as periodic intensification and simulated
blinking) can be obtained in mode 0.
Note that the use of mode 1 largely dictates that you must reinitialize
the palette registers. The VGA BIOS initializes some of these registers
with values greater than 15; these will be masked in mode 1 to the low
4 bits, yielding values in the range 0..15. These values will probably
not result in the colors you intend (two attributes will yield red
foreground, for example). The simplest solution is to just fill the
palette registers sequentially with numbers from 0..15; this largely
causes the palregs to drop out of the picture: attribute N will map
directly to color N within the palette selected by the color select
register.
Assuming that the palette registers are set as described, create a
"base" palette in palette 0. That is, fill palette 0 (video DAC color
registers 0-15) with 16 base color definitions. It makes sense to set
the standard color combinations for the 16 CGA-like colors that people
will expect to see (color intensity values in hex):
DAC regs Attr R G B Color
0-2 0 0 0 0 Black
3-5 1 0 0 2A Blue
6-8 2 0 2A 0 Green
9-11 3 0 2A 2A Cyan
12-14 4 2A 0 0 Red
15-17 5 2A 0 2A Magenta
18-20 6 2A 2A 0 Brown
21-23 7 2A 2A 2A White
24-26 8 0 0 15 "Gray" (not really)
27-29 9 0 0 3F Bright blue
30-32 10 0 3F 0 Bright green
33-35 11 0 3F 3F Bright cyan
36-38 12 3F 0 0 Bright red
39-41 13 3F 0 3F Bright magenta
42-44 14 3F 3F 0 Yellow
45-47 15 3F 3F 3F Intense white
(In practice, I use 33h in place of 3Fh. This leaves some room to flash
even the bright colors by intensification.)
The second step is to duplicate palette 0 into palettes 1-15, and then
vary the color definitions for attributes of interest in some systematic
way. For example, let's specify that attribute 1 is not going to be
"blue"; it's going to be a green that continuously cycles from normal
intensity to high intensity and back. The effect is similar to flashing
text, but more subtle. To do this, set the colors for attribute 1 in
each of the palettes as follows:
Palette 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
R 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
G 2A 2C 2E 31 33 35 36 37 38 39 3A 3B 3C 3D 3E 3F
B 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Note that the intensity of the green primary slowly increases from
normal (2A) in palette 0 to maximum intensity (3F) in palette 1. Also
note that attribute 1 no longer has anything to do with blue--it's
green. (See below for information on how to set a video DAC color
register definition.)
If these values are loaded into the video DAC color registers, and you
then sequence through the palettes in a cyclical pattern (0->15->0),
characters with attribute 1 will slowly "pulse" from green to bright
green. The effect is more subtle--and readable--than simply switching
the characters off and on.
The palette change is easily and efficiently accomplished by setting the
color select register--say, in a timer tick intercept routine. Using a
timer intercept is an ideal way to accomplish this sort of special
effect. Unlike hardware flashing, the special effects described in this
document must be accomplished by your software. Using an externally
triggered routine allows this to occur in background; you simply set up
the attributes the way you want them and then go about your work. The
special effects will continue to be generated even when you're in DOS or
BIOS, say, waiting for a keystroke. The skeleton for a typical timer
routine might be (assume that DELTA, COUNT, and RESET are all
initialized to 1):
count = count - 1
if count = 0 then
palette = palette+delta
if palette > 15 then
palette = 15
delta = -delta
else if palette < 0 then
palette = 0
delta = -delta
end
set color select register to palette
count = reset
end
This is both efficient and flexible: the blink rate can be adjusted by
altering the values of either DELTA and RESET (or both). Increasing
DELTA causes the blink rate to speed up by skipping palettes; increasing
RESET slows the blinking by skipping clock ticks. Be sure that PALETTE
and DELTA are treated as signed numbers. (Note that FLASH1 uses a
slughtly fancier technique.)
Effects
-------
Many "special effects" are possible using this technique. "Pulsing" has
already been discussed; here are some others:
PERIODIC INTENSIFICATION
Pulsing is a gradual change from a low intensity color to a high
intensity color. The effect can be hardened by using the normal color
for half of the palettes and an intnsified version for the other half.
For example, the following palettes flash cyan to bright cyan:
Palette 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
R 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
G 2A 2A 2A 2A 2A 2A 2A 2A 3F 3F 3F 3F 3F 3F 3F 3F
B 2A 2A 2A 2A 2A 2A 2A 2A 3F 3F 3F 3F 3F 3F 3F 3F
The effect can be varied by altering the ratio of low intensity palettes
to high intensity palettes ("beacons", below, are simply extreme cases
of periodic intensification).
COLOR CHANGES
Another effect is to flash text from one color to another. This set of
palettes flashes bright green text to bright red. It's quite an eye
catcher:
Palette 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
R 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
G 3F 3F 3F 3F 3F 3F 3F 3F 0 0 0 0 0 0 0 0
B 0 0 0 0 0 0 0 0 3F 3F 3F 3F 3F 3F 3F 3F
GRADED COLOR CHANGES
An interesting effect is to "grade" one color into another. Rather than
simply flashing green to red, change it gradually by fading out the
green primary and fading in the red:
Palette 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
R 0 2 4 6 9 0C 0F 12 15 18 1B 1E 21 24 27 2A
G 2A 27 24 21 1E 1B 18 15 12 0F 0C 9 6 4 2 0
B 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
This attribute will still change from green to red, but the change is
not instantaneous; it slowly fades from one color to the other, passing
through various other unnamed colors (all combinations of red and green)
on the way.
SIMULATED FLASHING
Regular hardware flashing can be simulated via a variant of the above
scheme; simply use the background color for half of the palettes (or the
foreground color, if you want to flash the background):
Palette 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
R 3F 3F 3F 3F 3F 3F 3F 3F 0 0 0 0 0 0 0 0
G 3F 3F 3F 3F 3F 3F 3F 3F 0 0 0 0 0 0 0 0
B 0 0 0 0 0 0 0 0 2A 2A 2A 2A 2A 2A 2A 2A
The example flashes yellow text if displayed on a blue background.
SOFTENED FLASHING
By placing the background attribute in half of the palettes (as for
flashing) and fading out the foreground in the other half, a softer
version of flashing is possible:
Palette 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
R 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
G 2A 24 1E 19 14 0F 0A 5 0 0 0 0 0 0 0 0
B 2A 24 1E 19 14 0F 0A 5 0 0 0 0 0 0 0 0
The example shows softened flashing of cyan text on a black background.
If any of the background primaries are also components of the
foreground color, do not fade those:
Palette 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
R 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
G 2A 24 1E 19 14 0F 0A 5 0 0 0 0 0 0 0 0
B 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A
The example shows softened flashing of cyan on blue.
FADING
This is an even softer version of flashing text. It fades the foregound
over the full 16 palettes by grading the foregound color into the
background color.
Palette 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
R 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
G 2A 27 24 21 1E 1B 18 15 12 0F 0C 9 6 4 2 0
B 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
The example fades a green foreground into a black background. If the
background is a color other than black, just use a graded color change
from the FG color to the BG color.
STROBES AND BEACONS
These are two final sample effects. A strobe is a color that's
invisible most of the time (BG=FG) but flashes briefly to maximum
intensity. The follow palettes strobe yellow when used with a blue
background:
Palette 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
R 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 3F
G 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 3F
B 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 0
A beacon is a low intensity color that briefly flashes to maximum
intensity. The following palletes create a blue beacon:
Palette 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
R 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
G 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
B 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 3F
Note to programmers: if you use the routines provided in FLASH1.ASM,
note that only palettes that are a multiple of DELTA are ever used.
E.g., if DELTA is 2, palette 15 will not be used; put the strobe or
beacon color in palette 14 instead.
Advantages/disadvantages
------------------------
Creating special effects in this manner has several advantages over
using hardware effects (such as flashing), and some disdvantages. Among
the advantages are:
- MANY more effects are possible; I have mentioned only a few of the
possibilities.
- Transition rates are under your control; the hardware blink rate
is fixed.
- Effects can be mixed on one screen (one attribute could be a
strobe, another a graded color change).
- Any effect can be applied to background as well as foreground.
Among the disadvantages are:
- Additional programming effort is required, along with increased
code and data storage.
- A background timer routine, if used, has some effect on system
efficiency (but it's small).
- Effects are associated with attribute numbers, not with bit
positions.
The last one bears some explanation. Hardware blinking is associated
with a single bit (bit 7) of the FG/BG attribute byte. Thus, you have
16 FG colors, all of which can be flashed. Software-controlled special
effects, however, are associated with attributes. If you set, say,
attribute 6 to be pulsed green, you no longer have a brown attribute
available. Thus, the number of effective "base" colors is reduced for
each special effect attribute you create.
Mode 0
------
I have covered attribute control mode 1 in some detail, but some of the
same effects can be accomplished in mode 0. The primary difference is
that you have only four palettes to work with. The simpler effects such
as color changes, periodic intensification, and simulated blink can
easily be accomplished in mode one by putting the second color in
palette 1, and then alternating between palettes 0 and 1.
More complex effects would require periodic alterations to the palette
registers and/or color plane enable register.
Programming
-----------
The programming required to accomplish these special effects is
relatively straightforward. The VGA BIOS provides all necessary
services; no register-level progamming is required (but see the next
section). Useful BIOS services are described below.
The only element that might need clarification is how to alter a color
definition in a video DAC color register. BIOS provides services to set
a single color (BIOS video function 10h, subfunction 10h) and to set a
whole block of colors (function 10h, subfunction 12h). If you are
altering a few colors only, setting them individually is efficient, but
it's more efficient to update larger numbers of colors by reloading the
entire set of registers.
Video function 10h, subfunction 17h reads a block of DAC color registers
into memory allocated by your program. Each 18-bit register is split
into three bytes, one for each of the red, green, and blue primaries (in
that order). If you read all 256 registers, 3*256 or 768 bytes of
storage will be required.
To obtain the video DAC register number for a particular color in a
particular palette, the formula is:
r = 64*p + pr[a] (mode 0)
r = 16*p + pr[a] (mode 1)
where r=register number, p=palette number, pr[] is the array of 16
palette registers, and a=attribute number.
The array of palette registers can be read and set via video function
10, subfunctions 9 and 2 respectively. Note that if you have set the
palette registers sequentially (pr[0]=0, pr[1]=1,...,pr[15]=15), then
they drop out of the picture for calculation purposes, and the formula
becomes:
r = 64*p + a (mode 0)
r = 16*p + a (mode 1)
To locate the offset of the 3-byte color definition for a specific
video DAC color register within the table described above:
o = r * 3
where r is the register number described above.
A combined formula that locates an absolute address of the definition
of a particular attribute/palette combination:
address = base + 64*p + pr[a] (mode 0)
address = base + 16*p + pr[a] (mode 1)
where base is the base address of the table. The function GET_DAC_PTR
in FLASH1.ASM performs this calculation.
To alter the displayed color for a palette/register combination,
calculate the address of its definition, alter the R/G/B bytes as
desired, and use video BIOS function 10h, subfunction 12h. Because
this sunfunction can reload the entire set of DAC color registers,
you can alter as many registers as you want in one shot.
To alter the definitions for a particular attribute in all 16 palettes,
just locate the first one (palette 0). The definitions for subsequent
palettes will follow at intervals of 16*3 or 48 (30h) bytes in mode 1,
or at intervals of 64*3 bytes in mode 0. For example, if the mode 1
definition for attribute 8 in palette 0 is at 1000h, attribute 8 in
palette 1 will be at 1030h, in palette 2 at 1060h, etc.
Changing the current palette (color select register)
----------------------------------------------------
A palette is selected by setting the contents of the color select
register to the desired palette number. This can be done by using a
BIOS video service as described below (int 10H, function 10H,
subfunction 13h, BL=1). This takes about 1/100th of a second and can
easily be done from within a timer intercept routine without any real
problems.
However, you may want to set the color select register using register
level programming, for one reason: programs such as screen blankers may
be watching for BIOS video activity. If you use BIOS services to change
the palette 18 times a second, your user's screen will never blank. The
technique is shown in the timer routine in FLASH1.ASM. It is assembled
only if the equate USE_BIOS is zero during assembly. If USE_BIOS is
nonzero, FLASH1 is assembled to use the BIOS service for palette
switching. There is no significant difference in efficiency between
using BIOS and programming the registers yourself; most of the time is
spent in waiting for a video retrace (and you thought you were through
with that when you got rid of your CGA, didn't you).
The demo (DEMO1.ASM) was assembled with USE_BIOS off.
The linkable object module
--------------------------
FLASH1.OBJ(ASM) provides a linkable object module that can be used to
generate all of the special effects described above, and others.
The module is organized for COM files; adjusting for other models should
be relatively painless. The primary assumption is that DS=ES; there is
no assumption that CS=DS (except in the timer intercept), or any
explicit use of the stack.
The module prologue describes the services available. Be sure to call
FLASH_INIT when you start and FLASH_TERM when you exit.
The demo program (DEMO1.ASM) demonstrates use of the module.
BIOS services
--------------
The VGA BIOS supports all necessary manipulation of VGA registers,
including the ability to read register contents (for state restoration
when you are done). All of the following are accessed through the
standard video BIOS service call (int 10h), function 10h. To use video
BIOS:
; Set reg AL and others as needed
mov ah,10h
int 10h
Most of these functions are reasonably efficient. An operation such as
setting the color select register can easily be accomplished within an
interrupt service routine.
PALETTE REGISTERS
Read one palette register
Entry:
AL=7
BL=palette register number (0-15)
Return:
BH=palette register value
Set one palette register
Entry:
AL=0
BH=color value
BL=palette register number (0-15)
Return:
none
Read all palette registers
Entry:
AL=9
ES:DX -> 17-byte buffer
Return:
ES:DX[0..15] = current palette regs
ES:DX[16] = current overscan (border) color value
Set all palette registers
Entry:
AL=2
ES:DX -> 17-byte buffer (formatted as for function 9)
Return:
None
VIDEO DAC REGISTERS
Read one video DAC register
Entry:
AL=15h
BX=register number (0..255)
Return:
CH=green intensity (0..63)
CL=blue intensity (0..63)
DH=red intensity (0..63)
Set one video DAC register
Entry:
AL=10h
BX=register number (0..255)
CH=green intensity (0..63)
CL=blue intensity (0..63)
DH=red intensity (0..63)
Return:
none
Read a block of video DAC registers
Entry:
AL=17H
BX=first register (0..255)
CX=number of registers
ES:DX -> buffer
Return:
On return, the buffer is filled with video DAC color values.
The first 3 bytes contain the R/G/B values for the first
specified register, etc. The buffer should be of size 3*CX
or more. Ensure that BX+CX <= 256.
Set a block of video DAC color registers
Entry:
AL=12H
BX=first register (0..255)
CX=number of registers
ES:DX -> buffer
The buffer is formatted as for function 12H. Ensure
that BX+CX <= 256.
Return:
None
ATTRIBUTE CONTROL MODE/COLOR SELECT REGISTER
Read attribute control mode and color select register
Entry:
AL=1AH
Return
BL=current attribute control mode (0/1)
BH=color select register contents
Set attribute control mode
Entry:
AL=13H
BL=0
BH=attribute control mode (0/1)
Return:
None
Set color select register
Entry:
AL=13H
BL=1
BH=value for color select register (0..3 for attribute
control mode 0, 0..255 for mode 1).
Return:
None
Author and reference
--------------------
This document and the DEMO1 and FLASH1 modules are by:
Chris Dunford
The Cove Software Group
PO Box 1072
Columbia MD 21044
301/992-9371
CompuServe 76703,2002
The document is copyright (C) 1989 by the author. Permission is granted
to distribute freely by electronic or other means, but it may not be
republished without permission.
Reference (and a darn good one, too): Programmer's Guide to IBM PC and
PS/2 Video Systems; Richard Wilton, Microsoft Press 1987.
10/09/89
