How It Works: Display Adapters Part III

Video RAM and RAMDAC

Video RAM

In addition to a specialized video processor and BIOS, video cards also use a small amount of their own RAM, which is built directly onto the video card. Several different types and densities of video RAM have been used for local video memory, some of which were standard DRAM used for system memory. Others were specially designed for graphics adapters.

FP RAM (Fast Page Ram)- Basic type of DRAM memory. Can only allow one access at a time. Average latency is 80-70ns

EDO (Extended Data output)- Improved type of paged RAM that can allow overlapping reads and writes, which slightly decreases net latency to 60-50ns

VRAM/WRAM (Video Ram/Windows RAM)- Specifically designed for graphics subsystems. Both are dual ported memories that allow simultaneous reads and writes, allowing both the video card and the CPU to access the RAM at the same time. This design made the RAM very fast because it had little to no latency (dead time when the RAM cannot be accessed), but they are very expensive types of memory. WRAM has slightly more bandwidth than VRAM.

MDRAM (Multi-Bank DRAM)- Specifically designed for graphics subsystems. MDRAM incorporates clusters of small 32KB memory chips, allowing manufactures to use non-traditional binary memory amounts. The point of allowing varying amounts of memory was to more closely match frame buffer requirements. Memory is typically clustered in even binary amounts (1MB, 2MB, 4MB), but frame buffers use memory inefficiently. By using smaller banks, the amount of installed memory can be made to more closely match frame buffer requirements with less wasted RAM.

SDRAM/SGRAM (Synchronous DRAM/Graphics Ram)- Very fast memory, runs at the same speed as the host bus (or video processor in this case). Faster clock speed reduces latency. Both SDRAM and SGRAM fall into this category, all the while being fairly inexpensive. SGRAM was specifically designed for graphics applications, and has a higher overall bandwidth, but is more expensive than SDRAM. Both function in the same way overall. No other type of RAM is currently used on current video cards. Due to variations in current video technology, SDRAM/SGRAM is normally clocked at a different speed than the video chip.

Digital to Analog Converter

All VGA and up video cards use a "RAMDAC" (Random Access Memory Digital to Analog Converter). This component is very important since it is responsible for converting the digital signals into an analog signal to be displayed on the monitor. Early VGA cards used an external RAMDAC chip, which was mounted onto the video card PCB (PC Board). Later generations incorporated the RAMDAC directly into the accelerator chipset, cutting down on manufacturing costs (Since less external components were required), and improving performance.

Speed is of the essence for the RAMDAC, since it is essentially responsible for driving the display. The RAMDAC is the last step in the video acceleration process before the information is sent out to the display, so its speed is going to determine the overall quality of the output, both in terms of resolution and refresh rate. A slower RAMDAC will generate a grainy, or fuzzy image because it's not properly translating the digital information that is being fed by the accelerator, which causes degradation in visual quality. Running a RAMDAC at a faster clock speed usually translates into a clearer, shaper image displayed on the screen.

Prior to PS/2 systems, a RAMDAC was not required since CGA and EGA outputted the video data to the monitor in digital form. The benefit to using an Analog output with VGA was that analog allowed for a higher degree of color generation. Before, color intensity was fired in digital on and off, which only allowed for 8 different colors to be displayed at once. Using a variable amplitude analog output, the CRT's guns can be fired at varying intensities, which provides for a greater spectrum of colors and intensities.