Display Standards
— 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 standard.

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 these systems.

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 systems.

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 introduced.

Attempts have been made to define a Super EGA/VGA 800 by 600 standard to solve these problems but little has come of it.

IBM 8514/A

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 itself.

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.

Summary

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.