1 June 1998
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
- Color Depth
- Refresh Rate
- Bus interface
- Accelerator
- 640x480 (VGA)
- 800x600 (SVGA)
- 1024x768
- 1280x1024
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.
- 4 bits - 16 colors
- 8 bits - 256 colors
- 16 bits - 32 or 64 thousand colors
- 24 bits - 16Million (Truecolor)
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.
A 24 bit (Truecolor) depth allows a specific one byte value to be assigned to Red, Green, and Blue signals to the display. A 15 bit color value can be divided into three 5 bit fields with a value of 0 to 31 for R, G, and B. An 8 bit value, however, cannot be reasonably divided into three separate numbers. The adapter holds a table of 256 entries. Each entry has a value for R, G, and B. Different application can compete to program this table with different values. This creates a problem when the two applications appear on different windows of the same screen.
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.
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.
Everyone knows that you cannot reliably run a CPU chip rated for 166 MHz at a speed of 200 MHz. This this called "overclocking" and though it may work for a while, you run the risk of damaging the chip. However, video systems come with a variable clock rate.
The refresh rate is defined as the number of times that the screen must be rewritten per second. The higher the resolution, the larger the number of dots that have to be written in every refresh cycle. The greater the color depth, the more work that has to be done per point. Increase any of these values and the system has to speed up the clock to support it.
The video adapter chips have a maximum speed. Separately, each display has electronics that can run at some maximum clock rate. Each version of Windows comes with display driver tables that can identify the maximum internal clock speed of each type of adapter card and display.
If you go to Control Panel - Display - Settings, Windows gives the current settings for resolution, color depth, and refresh rate. The model of display monitor establishes limits. If you increase the value of the resolution, the system may reduce the refresh rate to keep the chip speed within tolerance.
If you install Linux, the XFree86 display management software is a bit less sophisticated about identifying and protecting adapter cards and display monitors. The programmer has to configure the limits at a much more primitive level. Make an error and the display could overheat.
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.
Most modern PC display adapter cards can process commands as quickly as the CPU can send them. The most common factor that limits performance is the I/O bus.
The first display adapter cards plugged into the old 8 MHz ISA bus. That was terrible. Around the time of 486 systems there was a short period in which adapter cards use the 33 MHz VESA Local Bus interface. However, as the PCI bus became a standard feature of all systems, the display adapters also switched over to PCI.
The most recent development has been the AGP port on Pentium II systems. PCI supports a 4 byte transfer transfer with every tick of the 33 MHz clock, for a total of 133 megabytes per second. AGP supports a 4 byte data transfer twice in every cycle of the 66 MHz mainboard (CPU-memory) clock. Thats four times the throughput of the PCI bus.
PCI is fast enough for Windows business applications. AGP will only produce a noticable improvement if it is used for 3D Engineering applications. That is why it is currently available only on Pentium II systems.
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Back Copyright 1995 PCLT -- Introduction to PC Hardware -- H. Gilbert