Technical The Internal Components Graphics Cards

Graphics Cards

The personal computer display adapters we are all using today is as the result of a many advances of architecture starting from the original CGA (Colour Graphics Adapter) and MDA (Monochrome Graphics Adapter). The display adapter you are using also supports all of the preceding display adopters and maintains exact backward compatibility. Display adopters are available as separate adapter boards or integrated on the system's motherboard.

The display connects to the PC through a high density 15 pin D shell style connector, not to be confused with the game port. In addition to this connection, most display adapters provide a Video Feature Connector (VFC) that allows images from other sources to be merged with the graphics images. Live TV can be mixed with computer generated images and some display adapters support a TV out connector to send the computers display to a television screen. There are still several different versions of the connector that merge the data at different rates and colour depths or stages in the mixing.

The PC's display adapter performance has been driven by none other than Bill Gates's Microsoft Windows GUI's (Graphical User Interface) not to mention the PC's bus architecture advances in design and speed. Early ISA display cards became a major performance bottleneck as data rates increased, screen refresh rates went higher, more colours appeared and the display got bigger. Today's display adopters are designed to slot into the PCI or AGP bus.

The major components of the PCs display adapter are the AVGA (Accelerated Video Graphics Array) chip, display memory, RAMDAC and a clock generating device.
The controller chip attaches directly to the bus and generates horizontal and vertical sync signals, controls and provides software access to the display memory and supports graphics functions such as line draw, area fills, arcs, circles, block moves and colour manipulation. Normally the software instructs the graphics card to perform these functions without the CPU's intervention.
The RAMDAC (Random Access Memory and Digital to Analog Converter) converts the images data from the display memory in to analog RGB signals that a CRT monitor understands and can use the memory to manipulate colour depth. It is common for manufacturers to incorporate the clock generator device and the RAMDAC with the AVGA in to a single chip.
The display memory is a major factor in determining the overall performance of the display adapter. The amount of memory determines the maximum screen resolution and colour depth. For example a display setting of 800x600 pixels in 256 colours (1 byte) uses 480,000 bytes of display memory. Displaying the same size of image at a colour depth of 16.8 million colours (3 bytes) requires 1,440,000 bytes.

The bandwidth which is always the bottleneck on graphics cards today, is measured in megabytes per second (MBs). Three devices access the display memory and share the bandwidth. The refresh, the CPU and the Graphics Accelerator. The display must be refreshed often enough to prevent flicker. At 76 times per second (76 Hz rate), a true colour display (24 bits per pixel) at a resolution of 800x600 pixels requires a display memory bandwidth of 109.44 MBs and a resolution of 1280x1024 in true colour and refresh rate will roughly transfer 393.216 MBs of data in and out of the display memory.

32 bit DRAM memory are capable of a 200 MBs bandwidth. If approximately half of the display memory bandwidth was available to the display refresh and the other half for the CPU and graphics accelerator, a 800x600 true colour image refreshed at 76 Hz would push the 32 bit DRAM to the limit. To meet higher bandwidth requirements 64 bit DRAM designs were used on display adapters offering a bandwidth of up to 400 MBs, which at 1280x1024 pixels and a 76 Hz refresh struggles at 16 bit or higher colours.
VRAM, Video Random Access Memory is one of the most common types of dual port memory used in display adapters. The memory has two ports, which effectively doubles the bandwidth. One port is used for refreshing the display and the second port for updating the display. VRAM is able to support a bandwidth of 800 MBs which is still stretched to the limit on a high quality monitor.
SDRAM (Synchronous DRAM) designed to deliver bursts of data at very high clock speeds of over 100 MHz is the most popular display memory used on most higher performance display adapters. However games have pushed the edge further and newer memory technologies were needed as better Graphics Accelerators emerged. Faster than SDRAM is DDR memory which manages to increase bandwidth without needing to increase the clock speed.

As a result of increasing memory speeds and advances in the Graphics Accelerator and RAMDAC a new bus design was added to the PCs architecture, which provides an Accelerated Graphics Port (AGP) to a new faster range of display devices with fast access to system memory.

The Basics

When the PCs operating system or an application communicates with you through the monitor, it first builds up a message in a virtual screen in the PCs system memory. The message is now treated as a block of memory which is formatted and transferred to the display adapters memory as a pattern of pixels that represents an image or text. The display adapter reads the formatted message out of its display memory and displays it on the screen. Several alternative paths of communication are used by older DOS applications or games that bypass the operating system and communicate directly with the PCs display subsystem. The BIOS and hardware had to be exactly backward with all earlier PC architectures for this to work which is why software that worked with the original PC will still work with the latest display adapter hardware.
Images are formed on the monitor by forming a pattern of dots of varying intensity and colour. The smaller the dots and with enough colour the image looks naturally formed. If you look closely at the screens image you will see the individual dots that form either an image or text.

Pixels, Dot Pitch, Resolution and Colour Depth

The dots on the screen are known as pixels (picture elements). The dot pitch of the monitor determines the smallest displayable pixel size. In a CRT display the pixel is a red, green and blue phosphor dot. An electron beam strikes the phosphor and causes it to emit light. A range of colours is emitted by by varying which phosphor dots are struck and at what intensity. The dot pitch of a CRT is measured across the shortest distance between two dots of the same colour. If you know the dot pitch and size of the screen you can calculate the maximum resolution in pixels that can be displayed.
The number of colours that can be displayed on a colour monitor capable of showing a continuously variable colour image with more than 16.8 million colours is dependent on the function of the display adapter. Different colour depths depend on the amount of display memory dedicated to each pixel. One byte is used to represent 256 colours for each pixel, 16 bits or 2 bytes per pixel allows up to 65535 colours and 3 bytes, 24 bit colour can display 16.8 million different colours per pixel. 8 bit colour is better known as pseudo colour, 16 bit mode as high colour and 24 bit mode is called true colour. The digital colour is converted to an RGB analog signal by a DAC built into the display adapter. Usually the DAC (Digital to Analog Converter) is 8 bit and produces 256 different colour intensity levels and requires a 24 bit value.
If all the colours are not used and only say 256 are made available out of the 16.8 million the conversion happens through a colour palette device built into the display adapter. A colour look up table (CLUT) memory scheme employing a 256 word 24 bit memory block is used to store true 24 bit true colour RGB values. The 8 bit colour value is applied as an address to this block of memory and the data at this address is sent to the DAC. Any 8 bit value can be converted to any of 256, 24 bit values stored in the CLUT memory. The 256 colours available from the 16.8 million can be defined by simply changing the 24 bit codes in the 256 CLUT memory addresses.

Display adapters actually use 32 bits in 24 bit true colour mode, where the high order byte of the 32 bit word is used to read the next pixel of data in the series. This simplifies the display adapter design and reduces the bandwidth needed to display an image. Display memory is usually manufactured in 1, 2 or 4 MB sizes. Memory accounts for the largest cost of the display adapters. There is a mode called Packed Pixel mode that enables the use of the least amount of memory for a 24 bit colour image.

Text Characters

Early PC display adapters contained special hardware that scanned text characters out of a character generator ROM. The pixel patterns of characters were stored in the ROM. Today's PCs are All Point Addressable display systems (APA). Each pixel of the text character image is addressable and can ve modified to change the image. Today's PCs still support character generator modes to support DOS based text software. Instead of storing the characters bitmap patterns in ROM they are stored in the display adapters RAM which allows changing of the character font style and size.
APA character is used by Microsoft Windows instead of ROM character generator modes. Bitmaps of text characters are stored in off screen display and fetched as needed by the processor or controller. The monochrome bitmapped image is then sent to a colour expansion feature in the display adapter which adds a specified background and text pixel image colour which is then expanded into the display adapters displayable region.
Character bitmap patterns are generated either by storing the text character as a series of bits per pixel in memory called a bitmapped font, which take up a lot of memory but are fast and require no processing, or as scalable outline fonts. A set of mathematical expressions and coefficients that represent that represent a series of lines and curves that when attached to an end form the outline of a character. The processor can then expand or shrink the character and generate the size of bitmapped font from the outline. Particular fonts are typically stored four different outlines files. Standard, bold, italicised, super and subscript modes.

VESA (video Electronics Standard Association) grouped together a set of standard video BIOS calls to support a range of display modes known as the Super VGA modes. Display modes of up to 1280 by 1024 pixels with 24 bits per pixel colour depth and refresh rates up to 75 Hz were defined.
When the operating system changes mode it sends a mode code to the display adapter BIOS which then sets up the display adapter to support the mode. The VESA standards committee expands the modes to maintain VESA standards.

AVGA Standard

Once software started using the higher resolution modes the performance of the Windows GUI in APA mode started to degrade. Windows acceleration hardware was added to SVGA chips. The AVGA accelerator hardware did not need to be compatible between devices and was isolated from the user and Windows by providing a driver. The Windows driver interfaces to the Windows API (Application Program Interface) and converts Windows graphics commands to the Windows accelerator hardware in the AVGA chip. Each display adapter manufacturer must provide a unique driver to support its specific accelerator hardware functions. GUI accelerators at the heart of the AVGA chip performs graphic image creation and manipulation independent from the CPU. Functions such as Bit and Block Transfer, line draw, area fills, and colour expansion are provided in the graphics engine.
Display memory is mapped into the lower 1 MB space of the system memory address space in a 128 or 64 KB block. The AVGA controller maps small blocks at a time into this region where the CPU can access the APA pixel images.

The Graphics ROM BIOS comes with each display adapter subsystem. The ROM contains a minimal amount of software to support setting up the AVGA controller. The BIOS software is also an interface for the AVGA hardware to a standard set of DOS functions. To improve system performance in DOS applications, the system BIOS can be copied from ROM to system memory which can be accessed much faster. This is called shadowing.
The DAC function can be built into the AVGA chip or on a separate chip for higher performance, along with the CLUT RAM (RAMDAC). The speed of the RAMDAC can limit the system screen resolution and refresh rates. The DAC speed is independent of the colour depth and accepts all colour modes directly from the display memory or from the CLUT RAM.

DPMS (Display Power Management System) gained popularity from concerns over energy conservation. The display adapter controller and BIOS software can signal the monitor attached to the display adapter to enter several stages of power down states.
DDC (Digital Data Channel) is a feature that allows the monitor to inform the display adapter subsystem about the monitors capabilities and enables the display adapter BIOS to select characteristics that are compatible the monitor. DDC2 level 2 permits bi-directional communication between the display adapter and the monitor over a serial interface and is referred to as the ACCESS.Bus standard. DDC2 also form part of the Plug and Play standards.

3D Graphics images can be broken down into two areas with completely different requirements. Traditional 3D rendering is dominated by Open GL from Silicon Graphics and is the industry standard API for most 3D software. The second area is 3D games and multimedia were there is need for real time rendering at 25 or 30 frames per second.
3D rendering is basically a geometry transformation process (floating point intensive) and a scan line rendering process (CPU intensive). 3D images are represented by a data structure that defines the vertices of a mesh of connected polygons in a 3D space. Transforming a 3 dimensional image for a specific viewing position and lighting is very processor intensive. 3D Geometry Transformation Engine hardware can perform many simultaneous floating point operations. The hardware is used to perform pixel level operations and to build 3D images. Once a 3D image is created with a mesh of polygons it has to be transformed to a 2D representation on the flat surface of the display and then shaded. 3D scan line rendering is an extension of the hardware graphics engine.

As fast and wide as the PCI bus is graphics cards were consuming most of its bandwidth which is shared between all the PCI adapters. For 3D graphics to model enormous texture mapping and object shading lots of high speed memory is required to avoid frame rate dropping and jerky action. The PCI bus bandwidth is not up to it.

The AGP (Advanced Graphics Port) was introduced by Intel as a separate connector operating off the CPUs bus. The chipset acts as the glue between the processor, 2nd level cache, system memory, the AGP display adapter and the PCI bus. AGP operates at the speed of the processor bus, the frontside bus. At a clock rate of 66 MHz the PCI clock speed is doubled and the maximum possible bandwidth is around 256 MB per second. AGP cards support data transfer during the up and down clock cycle which doubles the clock rate and peak transfer and is known as X2. Using pipelining and queuing commands, called SBA (Sideband Addressing), peak transfer can be sustained almost 100% of the time.
AGP display adapters can use system memory as if it is actually on the graphics card. DIME (Direct Memory Execute) is handled by a device called a graphics GART (Graphics Aperture Remapping Table) which handles the system memory addresses in small chunks and presents them to DIME enabled graphics adapters as if they are part of the display memory. Dime allows much larger textures in graphics. AGP2.0 specifies a X4 transfer mode that delivers a maximum bandwidth between the AGP display and system memory of 1 GB per second.

AGP cards draw more power than PCI and older bus devices and the computer system requires a larger Power Supply Unit PSU) to deliver the higher current.

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