[Up] [Previous] [Next]

16 Oct 1995

Video Adapters

Today, most systems are sold with a display adapter that connects to a PCI or VESA "local bus", supports some Windows accelerator, and provides SVGA resolutions. The "local bus" means that the CPU can send data to the card at high speed. The "accelerator" means that the display adapter can draw lines and boxes and can move windows and scroll text itself. Resolution and number of colors are determined by the amount of video memory, and refresh rate is determined by the quality of the components. All these items need to be explained in detail.

Display adapters are characterized by


. Resolution refers to the number of dots on the screen. It is expressed as a pair of numbers that give the number of dots on a line (horizontal) and the number of lines (vertical). Four resolutions are in common use today

A computer display is essentially a high resolution TV set. It generates colors by combining amounts of Red, Green, and Blue (an "RGB" connection). In current use, these colors are controlled by three wires in the display cable. Each has a variable amount of voltage represented by a number from 0 to 255. This produces a theoretical 16 million possible colors. Complete control of color ("Truecolor") may be needed for displaying photographs, but ordinary applications get along with far fewer.


The Color Depth (number of colors) is determined by the number of bits assigned to hold color value.

The display adapter stores a value (4 to 24 bits) in memory for every dot on the screen. The amount of storage needed is determined by multiplying the number of dots (resolution) by the memory required for each dot.

The original VGA display had a resolution of 640x480 and supported 4 bit color. This required only 256K of memory.

An SVGA adapter with 512K can generally support resolution up to 800x600 and 8 bit (1 byte per dot) color.

An SVGA adapter with 1 megabyte of video memory can support 1024x768 resolution at 8 bits, or 800x600 resolution at 16 bits. In some systems, it can also display 1280x1024 in 4 bit color.

Additional memory is required for greater resolution or more color depth. However, not all systems support more video memory.


The Refresh Rate determines the speed that the display uses to paint the dots on the screen. The original VGA displays ran at 60Hz, but some people complained that this produced a flicker. International standards now require a rate of 70Hz. A "multisynch" monitor can adapter to refresh rates in a range, typically 60-75Hz. A utility program is typically provided on diskette to set the refresh rate on the display adapter for various resolutions.

However, a multisynch monitor generally needs adjustment when first connected to a new adapter card or run at a new resolution or rate. IBM avoids this problem by precisely matching adapters and displays within its product families. This means, however, that an older IBM display may not work on a non-IBM system. This became more trouble than it was worth, so today IBM generally produces only multisynch monitors like all the other vendors.

Combinations of high refresh rate, high resolution, and maximum color depth may overtax the chip that converts numbers in the display adapter memory into voltage levels in the wires of the display cable. This chip is called a RAMDAC. Faster RAMDAC chips are available, but they are expensive. Any attempt to run the RAMDAC too fast (to use a higher resolution or refresh rate than the chip supports) can damage the display card. Consult the manual provided with the display adapter to determine the allowable combinations.

A display will struggle to keep up with the signal sent by the card. If the signal is beyond the capability of the display, it can be damaged. To avoid this problem, operating systems will often require the user to identify the make and model of the display before allowing higher resolutions and refresh rates to be selected. If the model is not one listed in the table of supported displays, the manual that accompanied the display may provide a list of supported modes.


An accelerator chip on the video card can draw lines and boxes, fill in background color, scroll text, and manage the mouse pointer. These functions significantly improve the performance of Windows and OS/2. Before accelerators, a video adapter simply mapped the display memory to an area of the PC memory. The PC program would calculate the location of the line, and then would change the color dot by dot (byte by byte) in this area of memory.

With an accelerator, the CPU only has to send the video adapter a command to draw a line (and the starting point, ending point, width, and color of the line). The CPU is not required to calculate the bits in the line, and the amount of data that has to flow from the CPU through the I/O bus to the adapter card is greatly reduced.

An application program sends a sequence of requests to Window. Each request creates a window, button, box, menu, or writes some text. Some commands can be simply passed on to the accelerator chip for exection. A display adapter requires a Windows Driver routine. The Driver knows which commands the chip can handle, and which have to be turned into bits (or lines) on the CPU. Every video card has a Windows 3.1 driver. However, it is a good idea to make sure that the card also has a Windows 95, Windows NT, OS/2, and maybe a Linux XWindows driver.

The first generation of accelerator cards ran on the ISA bus, which limited data transfer to 16 bits in realtively slow cycle times. Then VESA Local Bus cards came along that could tranfer data in 32 bit chunks at a faster clock rate. Most Pentium machines today have a PCI video interface that transfers data at 64 bits and a very fast clock speed. Video performance is so good across the board that it is better to select a middle of the road adapter that every operating system supports rather than a very fast adapter with limited software.

In current use, the most popular accelerator cards are based on the "S3" family of chips, which combine good performance and low cost. Low end 486 machines with a VESA Local bus may have the S3 805 chip. Pentium machines would have the S3 864 chip. There are many other popular chip sets, but check the list of supported cards for each operating system to make sure that drivers are available.


An accelerator card reduces the amount of data that must be transferred between the PC and the display adapter. Nevertheless, display performance may be limited by the transfer rate. Originally, display cards plugged into the standard ISA bus. Since that bus is limited to transfer two bytes of data at a time, and is clocked at only 8Mh, this became a bottleneck. Today, most ISA bus machines provide one or two slots of Local Bus sockets. A Local Bus video card can transfer four bytes of data at a time, and it can operate at the 25 or 33Mh clock rate of the CPU and memory instead of the slower clock rate of the I/O bus.

For Pentium and PowerPC machines, an even faster alternative is provided by the PCI bus. The PCI interface allows 8 bytes of data to be transferred in one operation.

In many cases the desktop PC models come with a built-in Local Bus video adapter based on one of the S3 chips. This should be perfectly adequate for typical serious use under Windows or OS/2. Computers that are used as a disk or database server are often locked in a closet or run unattended. It may be tempting to save money by using a low cost video adapter on a server. However, if there is any monitor displaying performance data or status on the screen, then the choice of display adapter can effect performance. When any program writes to the screen (even when the display is powered off) the CPU must wait until the data or commands have been transferred to the display adapter. By using a low speed adapter (such as an old VGA), the CPU is forced to wait longer. The apparent CPU utilization goes up, and the performance of the server may degrade. The CPU is being used up by the wait states required to move a large block of data two bytes at a time across the old ISA bus. Therefore, while it may make sense to save money by using an inexpensive display screen on a server, it is still a good idea to get a high speed adapter card.

Over the last year there have been no new developments in video technology. Most vendors are using better RAMDAC chips and now support higher refresh rates. However, video speed is so high that improvements would not be noticed, and the display screen technology has not changed enough to make large screens any more affordable.

Continue Back PCLT

Copyright 1995 PCLT -- Introduction to PC Hardware -- H. Gilbert

This document generated by SpHyDir, another fine product of PC Lube and Tune.