There are several approaches to increasing the density of a graphics display. Fig. 2-17 shows the block diagram of a 256×256 dot graphics video display made by Matrox, a Canada-based firm. This company makes plug-in video displays for many of the computers on the market, including the popular hobby S-100 and Intel SBC buses. The display in Fig. 2-17 is for S-100 or SBC bused microprocessors using the 8080 or Z-80 chip. With raster scan dot matrix, we can easily mix in text from a separate alphanumeric video board. The X and Y registers are ports for loading with 8-bit coordinates for dots. The third port turns the dot on or off. This unit is the Matrox ALT-256**2. Fig. 2-18 shows screen output from the ALT-256**2 board running in a IMSAI 8080 computer. As seen in the block diagram of Fig. 2-17 the Matrox unit uses port addressing, which means the display is fed X, Y point pairs through its input ports from the 8080. Thus no display RAM is required. Text can be mixed in from a separate alphanumeric board.

The secret to how this circuit works is a single 65,536-bit dynamic RAM memory chip which simply stores all the points on the screen that are on as logic 1s and everything that is off as logic 0s. The X and Y bytes are fed to the display and the memory address multiplexers use them to locate the proper bit and store it in the memory. Meanwhile the RAM chip is scanned constantly and its output is sent to the screen.

Fig. 2-19 shows a Mona Lisa painting that has been reproduced by using three Matrox ALT-256**2 devices and a special video digitizing camera. There are three bits of gray scale in this photo, and each Matrox board processes one of the three bits. The outputs of the three boards are then summed and sent to the video monitor.


Fig. 2-16. The 68047 vdg


display memory partitioning.


Fig. 2-17. A 65,536-bit Pixel display using port addressing.
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