Page:Gametronics Proceedings.djvu/59

 memory can thus be accomplished concurrently with display refresh. To the programmer then, the memory is organized as 16K x 1.

Analog/Digital conversion of potentiometer inputs is accomplished by firing a one-shot at vertical line number zero. When the one-shot times out, the number of vertical lines displayed since line zero will be in the CPU input port.

Vertical retrace is detected by monitoring the state of the vertical address counter and may be used as the signal to begin changing the elements of the display before the next frame. By processing rapidly moving objects and objects near the top of the screen first, some slight jitter may be avoided. Object distortion might also be reduced by writing in the new position before erasing the old.

Fig. 11 outlines the features of a more sophisticated video game called Tank Squadron. This game illustrates the use of the F8 DMA feature discussed above, to provide maximum flexibility and performance and also illustrates how one goes from prototype to production to take maximum advantage of the F8 family.

The block diagram of the prototype of Tank Squadron is shown in Fig. 12. The F8 portion of the system consists of the MK3850 CPU, MK3861 PIO, MK3852 DMI and MK3854 DMA. A brief description of the systems operation follows.

The information appearing on the screen is divided into the fixed background pattern (bombed out buildings, trees and shrubs), and the variable pattern (tanks, mines, shells, score). These two patterns are switched at the appropriate time by a multiplexer going to the video summer.

The playing field ROM (background pattern) is addressed continuously and converted to serial data at the multiplexer. Common video packing techniques are used to reduce the ROM size (½K in this example).

The variable pattern is contained in RAM (1K x 8) where it is read out by the MK3854 DMA device into a high speed FIFO (16K x 8). The Field Select circuitry compares the contents of the FIFO against the horizontal position to determine when to switch the multiplexer to read out the variable field. The horizontal position circuitry also provides the vertical retrace signal back to the F8 through an I/O port to signal the start of the variable field update as required before the next frame.

The initial or starting patterns of the variable field objects are obtained from a 3K x 8 ROM addressable by the CPU. This contains tank positions, mine and shell patterns as well as constant score patterns.