— Then and Now
by Lloyd Borrett
Technical Cornucopia, September 1990
Today, there are many choices when it comes to selecting
the visual interface that will allow you to see what your PC
is doing. Selecting the right combination is vital if you
are going to maximise the benefits of your PC and provide an
ergonomically acceptable solution.
IBM Monochrome and Colour Adapters
When the PC was introduced in 1981, IBM offered two video
adapters: the Monochrome Display Adapter (MDA) and the
Colour Graphics Adapter (CGA).
The MDA is designed for use with a monochrome monitor
that displays 80 columns and 25 rows of alphanumeric text.
The CGA supports either a RGB display (a monitor with
separate input signals for red, green, and blue) or a home
television set (which uses a NTSC composite video signal).
The CGA can display graphics information on a dot-by-dot
basis as well as alphanumeric text.
Even though both the MDA and the CGA can display 25 rows
of 80 column text, most people find the MDA's display easier
to read. This is because the 720 dots wide and 350 dots high
resolution of the MDA is significantly higher than the 640
dots wide and 200 dots high resolution of the CGA.
Thus early on the choices were simple. If you needed
sharp readable text you installed the MDA. If you needed to
display charts, diagrams and other graphics information, or
use colour to enhance text, you installed the CGA. If you
needed both, and had the money, you installed both!
Hercules Graphics Card
1982 saw the introduction of Hercules' solution to the
problem of displaying readable text and dot-by-dot graphics
on the same monitor. The Hercules Graphics Card (HGC) can
display graphics and alphanumeric text on the same
monochrome screen used by a MDA.
The ability to display a combination of readable text and
monochrome graphics is sufficient for many applications, so
many users find the HGC an economic option. Today there are
very few monochrome systems sold that don't use the HGC
IBM Enhanced Graphics Adapter
A different response to the demand for better text and
graphics resolution was released by IBM in early 1985. The
Enhanced Graphics Adapter (EGA) can be configured to emulate
a MDA or a CGA, but what makes the EGA "enhanced" is that it
also can display things the others can't.
Unlike the MDA, the EGA can produce dot-by-dot graphics
on a monochrome display. However, this graphics mode of the
EGA is not compatible with Hercules (HGC) graphics. The EGA
has similar compatibility problems with its own CGA mode and
the real CGA adapter.
However the biggest reason that the EGA is called
"enhanced" is the ability to generate 16-colour alphanumeric
or graphics images with 640 dots wide by 350 dots high
resolution. To be able to exploit this EGA mode an EGA
capable RGB display monitor had to be used.
Finally it was possible to have acceptable colour text
and graphics resolution on the one display by combining an
EGA display card with an EGA display monitor.
IBM Multi-Colour Graphics Array
The introduction of the IBM PS/2 Model 30 saw the Multi-Colour
Graphics Array (MCGA) as the video subsystem integrated into
the mother board of the system. The MCGA resembles the CGA
in many ways, but the MCGA has much better resolution (a
maximum of 640 dots wide by 480 dots high) and improved
colour display capabilities.
A significant difference between the MCGA and the earlier
display adapters is that the MCGA generates analog RGB video
signals, whereas the others produce digital RGB signals.
Thus a wider range of colours can be displayed. The MCGA can
display as many as 256 different colours at once from a
palette of 262,144 colours.
To show the output of the MCGA video subsystem an analog
colour display or analog monochrome display can be used.
With a monochrome monitor, the MCGA can display as many as
64 shades of grey.
IBM Video Graphics Array
The introduction of the IBM PS/2 Models 50, 60 and 80 at
the same time as the Model 30 saw a different Video Graphics
Array video subsystem integrated into the motherboard of
The VGA is similar to the EGA, so some programs written
for the EGA will run unchanged on the VGA. The VGA is
capable of higher display resolution modes such as 720 dot
wide by 400 dot high in text modes, or 640 dot wide by 480
dot high in graphics modes. The VGA also can generate the
same 256 colours at a time and 64 levels of grey as the MCGA
and uses the same analog monochrome or colour monitors.
Third Party Enhancements
While MCGA and VGA were built into the motherboard of the
IBM PS/2 systems, it wasn't long before third party
manufacturers had VGA compatible cards out that could be
installed into the 8 bits slots of IBM PC, XT, AT and
compatible systems. Later higher performance 16 bit versions
became available for use in 286, 386SX, 386 and 486 based
In order to make it easier to choose which monitor to
buy, NEC introduced MultiSync monitors. These monitors could
be connected to any of the existing display cards and thus
it was no longer necessary to buy a new monitor when a
system was upgraded to a higher resolution display card.
One way for third party manufacturers of EGA and VGA
compatible products to gain an increased share of the
marketplace was to offer extra features. Most thus tried to
add HGC and true CGA modes to their cards with various
levels of success.
Even more third party manufacturers added "super
resolution" 800 dots wide by 600 dots high modes. Because
there was no common standard for these extra modes the card
manufacturers would have to provide their own unique
software drivers so that these modes could be used with the
popular software packages. This introduced the problem of
having to be constantly creating and distributing new
software drivers as new programs and updates were
Attempts have been made to define a Super EGA/VGA 800 by
600 standard to solve these problems but little has come of
IBM announced the 8514/A at the same time as the MCGA and
VGA, but the 8514/A had one important difference. It was not
backward compatible with any previous display adapter.
Instead, it was based on a graphics coprocessor capable of
executing rudimentary graphics primitives on the board
The 8514/A is similar to a math coprocessor in that it
takes graphics commands and executes them. The 8514/A
supports line drawing, filled rectangles, area fills and bit
block transfers. These graphics primitives can also take
advantage of some simple image processing related display
features. Higher level graphical primitives, like real-time
font rendering, circles, arcs, etc., have to be handled by
the PC's CPU, which performs the calculations and uses the
8514/A to display the results.
The 8514/A also brought a new resolution of 1,024 dots
wide by 768 dots high with 16 colours into the limelight. A
memory upgrade can bring the card up to 256 colours at that
same resolution. Unfortunately, the 8514/A's 1,024 by 768
resolution was interlaced and prone to flicker.
More Third Party Enhancements
Recently we have seen third party manufacturers of VGA
cards offering "enhanced" 1,024 by 768 resolution versions.
These cards are not 8514/A compatible and again need board
and application software specific software drivers for you
to be able to exploit the "enhanced" resolution.
This is fine if you are running the most popular software
packages, but we're seeing that these manufacturers are
having trouble keeping up-to-date as the software changes.
Few currently have Windows V3.0 compatible drivers, and even
fewer have OS/2 Presentation Manager compatible drivers.
The system CPU has to update over five times as much
graphics memory on these new VGA cards at the higher
resolutions, and this delay can get very noticeable on
slower CPUs. Fortunately, in most cases these new cards are
being installed into the faster 386 and 486 power-houses
with the muscle to spare for updating the display.
We are also now starting to see affordable third party
8514/A compatible display cards. And there are constant
rumours that IBM will make an 8514/A or similar interface
standard on the next range of PS/2s.
Graphical User Interfaces
Colour graphics based applications and graphical user
interfaces are becoming more popular, and it's getting to
the point where the standard VGA screen resolution of 640 by
480 just isn't good enough. Multi-tasking graphical user
interfaces such as Microsoft Windows and OS/2 Presentation
Manager need better resolution.
Also a 1,024 by 768 image when displayed on the typical
14-inch monitor is far from ideal. This is especially true
when trying to use multiple windows on the one screen.
The strong trend towards graphical user interfaces will
force the following minimum standards for displays:
- 1,024 by 768 resolution, 256 colour
- graphics coprocessors to relieve the main CPU of the
increased graphics workload
- 16-inch monitors
Keep this in mind as you plan your future system
purchases. Purchasing the wrong system unit, graphics
adapter and display monitor combination will severely limit
your ability to exploit fully the benefits of the new
graphical user interface standards.
Saturday, 15 October 2011