Timeline

Monochrome Display Adapter(MDA)

First generation IBM PCs used a monochrome display adapter, which could display, as the name implies, only one color, and only text. Basic monochrome adapters were capable of 720X250 resolution, and could only display basic text (Alphanumeric, or A/N) symbols.

MDA display cards fit into a standard ISA slot, and contained no memory or BIOS of their own. Video BIOS functions were integrated into the system BIOS, with a small chunk of system RAM reserved for the frame buffer. To further conserve on costs, an LPT (printer port) was integrated into the MDA card. This served to leave another expansion slot open that would have otherwise been occupied by a parallel printer port card.

Hercules graphics released their own video adapter called, aptly enough, the "Hercules Graphics Card (HGC), which could not only display sharper text than the IBM MDA, but also had some graphics capability.

Color Graphics Adapter (CGA)

The CGA graphics card was introduced about the same time as the MDA was. Unlike the MDA, which was only capable of displaying alphanumeric characters, the CGA video was able to display text, and also use "All Point Addressable" (APA) pixel addressing. Text-only cards like the MDA could only address the screen pixels in a character box (9X14 dot for the MDA). The ability of the CGA card to address each pixel individually is what gave it its graphics capability.

In A/N mode, the CGA card could display either 40X25 (40 column by 25 lines), or 80X25 16-color text. In APA mode, the card could generate either 320X200 or 640X400 2 color text, from a palette of 16 colors. One very annoying drawback to the CGA Adapter was its tendency to create flicker and snow on screen. Staring at a flashing image for hours on end was aggravating, which ended up being one of the major reasons that MDA was preferred over CGA. Just like MDA, CGA cards did not contain their own video BIOS or RAM. They were controlled by the motherboard BIOS and used a reserved 128KB page of system RAM for frame buffer.

Enhanced Graphics Adapter (EGA)

EGA was introduced in 1984, shortly after the release of the now famous IBM AT. EGA was considered the second generation of PC graphics standards, with a package that consisted of three parts: the graphics board, memory expansion board, and memory module kit. The entire set cost over $1,800 at launch time, but the system was modular, which meant that you could purchase the graphics board, then add the memory expansion system later if you so required it. EGA continued to use the standard 9 pin DB-9 connector, but the pin arrangement was slightly changed from the MDA/CGA to accommodate the EGA system's separate color intensities, thus making EGA incompatible with the older MDA/CGA monitors. To use EGA graphics, you had to match it to an EGA display. EGA was backwards compatible with MDA and CGA, though. When connected to those types of displays its abilities were further enhanced when using an EGA display

The initial card, sans memory expansion kit, came with up to 256KB of onboard video memory, and also used its own 16KB ROM BIOS. In order to speed video BIOS operation, the BIOS can be "shadowed" to main memory. Shadowing simply means that the video BIOS code is copied to a reserved area of memory and executed from there because system RAM is faster than a ROM chip. A majority of EGA cards came equipped with the full 256K of memory on the board, so no expansion was required. Several manufacturers poured their own EGA display adapters onto the market, signaling a decrease in IBM's hold on the market.