Video Display Controller - Alternatives To Using A VDC Chip

Alternatives To Using A VDC Chip

Note that many older home-computer did not use a VDP-chip, but built the whole video display controller from a lot of discrete logic chips, (examples are the Apple II, PET, and TRS-80). Because these methods are very flexible the video display generators could be very capable, (or extremely primitive, depending of the quality of the design) but also needed a lot of components.

Many early systems used some form of an early programmable logic array to create a video system, examples include the ZX Spectrum and ZX-81 systems and Elektronika BK-0010 but there were many others. Early implementations are often very primitive, but later implementations could result in fairly advanced video systems like the one in the SAM Coupé.

These systems could thus build a very capable system with relatively few components, but the low transistor count of early programmable logic meant that the capabilities of early PLA based systems often were less impressive than those using the video interface controllers or video coprocessors that were available at the same time. Later PLA solutions, like those using CPLDs or FPGAs could result in much more advanced video systems, surpassing those built using off the shelf components.

An often used hybrid solution was to use a video interface controller (often the Motorola 6845) as a basis and expand its capabilities with programmable logic or an ASIC. An example of such a hybrid solution is the original VGA card, that used an 6845 in combination with an ASIC, that is the reason why all current VGA based video systems still use the hardware registers that were provided by the 6845.