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@ -8,49 +8,49 @@ Chapter 33\
### The Basics of VGA Color Generation {#Heading2}
Kevin Mangis wants to know about the VGA’s 4-bit to 8-bit to 18-bit
Kevin Mangis wants to know about the VGA's 4-bit to 8-bit to 18-bit
color translation. Mansur Loloyan would like to find out how to generate
a look-up table containing 256 colors and how to change the default
color palette. And surely they are only the tip of the iceberg; hordes
of screaming programmers from every corner of the planet are no doubt
tearing the place up looking for a discussion of VGA color, and venting
their frustration at my mailbox. *Let’s have it*, they’ve said, clearly
their frustration at my mailbox. *Let's have it*, they've said, clearly
and in considerable numbers. As Eurythmics might say, who is this humble
writer to disagree?
On the other hand, I hope you all know what you’re getting into. To
On the other hand, I hope you all know what you're getting into. To
paraphrase Yogi, the VGA is smarter (and more confusing) than the
average board. There’s the basic 8-bit to 18-bit translation, there’s
the EGA-compatible 4-bit to 6-bit translation, there’s the 2- or 4-bit
color paging register that’s used to pad 6- or 4-bit pixel values out to
8 bits, and then there’s 256-color mode. Fear not, it will all make
average board. There's the basic 8-bit to 18-bit translation, there's
the EGA-compatible 4-bit to 6-bit translation, there's the 2- or 4-bit
color paging register that's used to pad 6- or 4-bit pixel values out to
8 bits, and then there's 256-color mode. Fear not, it will all make
sense in the end, but it may take us a couple of additional chapters to
get there—so let’s get started.
get there—so let's get started.
Before we begin, though, I must refer you to Michael Covington’s
excellent article, “Color Vision and the VGA,” in the June/July 1990
Before we begin, though, I must refer you to Michael Covington's
excellent article, "Color Vision and the VGA," in the June/July 1990
issue of *PC TECHNIQUES*. Michael, one of the most brilliant people it
has ever been my pleasure to meet, is an expert in many areas I know
nothing about, including linguistics and artificial intelligence. Add to
that list the topic of color perception, for his article superbly
describes the mechanisms by which we perceive color and ties that
information to the VGA’s capabilities. After reading Michael’s article,
you’ll understand what colors the VGA is capable of generating, and why.
information to the VGA's capabilities. After reading Michael's article,
you'll understand what colors the VGA is capable of generating, and why.
Our topic in this chapter complements Michael’s article nicely. Where he
focused on color perception, we’ll focus on color generation; that is,
Our topic in this chapter complements Michael's article nicely. Where he
focused on color perception, we'll focus on color generation; that is,
the ways in which the VGA can be programmed to generate those colors
that lie within its capabilities. To find out why a VGA can’t generate
as pure a red as an LED, read Michael’s article. If you want to find out
how to flip between 16 different sets of 16 colors, though, don’t touch
that lie within its capabilities. To find out why a VGA can't generate
as pure a red as an LED, read Michael's article. If you want to find out
how to flip between 16 different sets of 16 colors, though, don't touch
that dial!
I would be remiss if I didn’t point you in the direction of two more
articles, these in the July 1990 issue of *Dr. Dobb’s Journal*. “Super
VGA Programming,” by Chris Howard, provides a good deal of useful
information about SuperVGA chipsets, modes, and programming. “Circles
and the Digital Differential Analyzer,” by Tim Paterson, is a good
article about fast circle drawing, a topic we’ll tackle soon. All in
I would be remiss if I didn't point you in the direction of two more
articles, these in the July 1990 issue of *Dr. Dobb's Journal*. "Super
VGA Programming," by Chris Howard, provides a good deal of useful
information about SuperVGA chipsets, modes, and programming. "Circles
and the Digital Differential Analyzer," by Tim Paterson, is a good
article about fast circle drawing, a topic we'll tackle soon. All in
all, the dog days of 1990 were good times for graphics.
### VGA Color Basics {#Heading3}
@ -58,7 +58,7 @@ all, the dog days of 1990 were good times for graphics.
Briefly put, the VGA color translation circuitry takes in one 4- or
8-bit pixel value at a time and translates it into three 6-bit values,
one each of red, green, and blue, that are converted to corresponding
analog levels and sent to the monitor. Seems simple enough, doesn’t it?
analog levels and sent to the monitor. Seems simple enough, doesn't it?
Unfortunately, nothing is ever that simple on the VGA, and color
translation is no exception.
@ -79,9 +79,9 @@ sent out by the palette RAM. (The operation of the palette RAM was
described back in Chapter 29.)
The process is much the same in text mode, except that in text mode each
4-bit pixel value is generated based on the character’s font pattern and
attribute. In 256-color mode, which we’ll get to eventually, the palette
RAM is not a factor from the programmer’s perspective and should be left
4-bit pixel value is generated based on the character's font pattern and
attribute. In 256-color mode, which we'll get to eventually, the palette
RAM is not a factor from the programmer's perspective and should be left
alone.
#### The DAC {#Heading5}
@ -92,24 +92,24 @@ sent to the DAC (Digital/Analog Converter). The DAC performs an 8-bit to
18-bit conversion in much the same manner as the palette RAM, converts
the 18-bit result to analog red, green, and blue signals (6 bits for
each signal), and sends the three analog signals to the monitor. The DAC
is a separate chip, external to the VGA chip, but it’s an integral part
is a separate chip, external to the VGA chip, but it's an integral part
of the VGA standard and is present on every VGA.
![](images/33-01.jpg)\
**Figure 33.1**  *The VGA color generation path.*
(I’d like to take a moment to point out that you can’t speak of “color”
(I'd like to take a moment to point out that you can't speak of "color"
at any point in the color translation process until the output stage of
the DAC. The 4-bit pixel values in memory, 6-bit values in the palette
RAM, and 8-bit values sent to the DAC are all attributes, not colors,
because they’re subject to translation by a later stage. For example, a
pixel with a 4-bit value of 0 isn’t black, it’s attribute 0. It will be
translated to 3FH if palette RAM register 0 is set to 3FH, but that’s
because they're subject to translation by a later stage. For example, a
pixel with a 4-bit value of 0 isn't black, it's attribute 0. It will be
translated to 3FH if palette RAM register 0 is set to 3FH, but that's
not the color white, just another attribute. The value 3FH coming into
the DAC isn’t white either, and if the value stored in DAC register 63
the DAC isn't white either, and if the value stored in DAC register 63
is red=7, green=0, and blue=0, the actual *color* displayed for that
pixel that was 0 in display memory will be dim red. It isn’t color until
the DAC says it’s color.)
pixel that was 0 in display memory will be dim red. It isn't color until
the DAC says it's color.)
The DAC contains 256 18-bit storage registers, used to translate one of
256 possible 8-bit values into one of 256K (262,144, to be precise)