height of those displayed when 256 bytes are displayed.
One of the main advantages of mapping main memory directly into the monitor or TV raster is the ability to manipulate the display using the normal instruction set. In systems that employ an external frame buffer for refresh, specialized instructions are required to change the buffer contents. The buffer memory also costs more money. With the refresh buffer approach toward animation, you must store two picture patterns in memory and alternately transfer them to the buffer memory. Using the Pixie graphics display described here, you store the same two-picture patterns in memory but you need only change the initial value of R0 to alternately display them. Not only do you save the cost of a refresh buffer, you can greatly simplify the programming.
Construction. The Pixie circuit can be mounted on the original Elf board by relocating the crystal and two capacitors to the center of the board. Now the 1861 IC goes on the upper left of the board, and the output jack on the rear apron of the chassis.
Remove the crystal from the Elf and wire the Fig. 2B frequency divider to pin 1 of the 1802 µP. Then interconnect the two boards exactly as shown in Fig. 2A and B, including the power lines. Jack J1 can be mounted on a small metal bracket and secured to the add-on board with No. 4 machine hardware. Also, mount R1 and R2 on the add-on board via "flea" clips because they may have to be changed for different-value resistors to suit the modulation requirements of the particular monitor you are using.
Sample Display Program. To test the Pixie, load the program given in Table I, starting at location M(0000). When this program is run, a random spot pattern should be displayed on-screen. At this time, you may have to alter the values of R1 and R2 to produce a tight sync lock and the desired modulation level of the spots. These are only level-adjust resistors and play no role in the actual sync or video production. The displayed pattern represents whatever is stored in the Elf's memory. The top eight rows represent the program given in Table I.
You can familiarize yourself with the new graphics ability of your computer if you visualize a grid of 64 boxes wide by 32 boxes deep, assuming a 256-byte memory. Bear in mind that the operating program given in Table I occupies the top eight lines. Since the program ends at memory location M(003B), load 00 into memory location M(003F) to complete that line.
Now, to display the spacecraft shown in the lead photo, load the programs given in Tables Iand II in that order, starting the Table II program at memory location M(0040). Reset and switch to RUN.
If you wish to create your own display, Fig. 3 illustrates how to arrive at the correct hex digits. (In this case, the example used is for a small area of the program in Table II.) Use graph paper to "draw" your picture, shading in the "spots" you want to be white on the CRT screen. Then transfer the line bit pattern into the eight hex bytes per line as shown in Fig. 3.
Conclusion. The Pixie system described here adds video graphics to your Elf microcomputer at very low cost. So far, we have described how the Pixie system can be used to put simple, stationary images on-screen. Accompanying this article is a program that will put the graphics in motion. [Part 4A]