6502.org

For circle plotting in anything controlled by a 6502, it is probably easier to use sin'x = cos x and cos'x = -sin x directly; this gives you

x0 = r
y0 = 0

then

x1 = x0 - d * y0
y1 = y0 + d * x0

x2 = x1 - d * y1
y2 = y2 + d * x1

etc

A suitable value for d might be (surprise!) 1/256.

You're talking Cordic, right? (I am aware of it, but not knowledgeable in it.) But with the tables, taking the SIN of an angle is even faster than the single multiplication in the Cordic algorithm.

Nope, not cordic... it's just a simple approximative generator for sin & cos. By using dx=dy=1/256, you don't need to do multiplication or division - only subtraction and addition.

The disadvantage is that the values for x,y will keep getting smaller with this method, so if you plot x,y repeatedly, you get a spiral.

If you do it like this:

x = x - d * y
y = y + d * x

Then you get an ellipse instead of a spiral.

We're going in circles!

Y'all got my head spinning!

I added another page to the 6502 primer. It starts with a whole-computer schematic of a really basic 6502 computer, then after that it's mostly circuits to interface real-world stuff to the computer-- hence the name, "circuit potpourri." I have more to add, but I'm going to take a little break before continuing.
http://wilsonminesco.com/6502primer/potpourri.html

What's there so far:

• First: a very basic whole-computer schematic
• Connecting I/O devices
  • Using the 6522's shift register for tons of output bits
  • Using the 6522's shift register for tons of input bits
  • Using the 6522's shift register for both input and output
  • SS22: Using the 6522's shift register for a link between computers
  • Converting to and from higher-voltage logic
  • driving 12V relay coils
  • High-voltage shift registers
  • Interfacing to I²C
  • Keypads and keyboards
  • Displays
  • Printers
  • Digital-to-analog (D/A) converters, or DACs
  • Digital potentiometer
  • Analog-to-digital (A/D) converters, or ADCs
  • A note about A/D and D/A jitter (and other performance considerations)
  • Jeff Laughton's circuit tricks, for ultra-fast I/O (single-cycle!), and re-mapping op codes
  • Non-typical power-supply circuits run by the 6522 and digital components

Several circuits have accompanying 65c02 assembly driver code, either inline or linked in a separate .asm file.

6502 gets another promotion! My site is featured as "website of the day" at http://www.eeweb.com/, a professional engineers' resource website, about 2/3 of the way down, in the middle column.

Well done! Should we prepare to welcome some new members?

Nice Garth!

That, and the article mentions that the 6502 continues to be produced in high volume.

Congratulations are in order.