Re: [stella] officially offtopic: Apple II mouse pointer in vblank?

Subject: Re: [stella] officially offtopic: Apple II mouse pointer in vblank?
From: zu03776 <zu03776@xxxxxxxxxxxxxxxxxx>
Date: Thu, 19 Feb 2004 23:35:22 -0600
Completely OT at this point, since this is Stella and not an Apple ][
list, but...didn't the Apple ][ have "shapes" which could be defined by
line segments and then displayed, rotated, scaled, and so forth? I
recall typing in a program to do this kind of thing, but perhaps I was
typing in the software to do the shapes as well as the shapes

Shape Tables. And I think you're right: they were a vector format, although my memory is very furry about this.

They were a vector format. I searched for my Apple II programmer's guide without success -- but here's what I remember (salt recommended).

The commands were packed three to a byte -- the first two commands used three bits, and the third command used two bits. Two of the bits indicated a direction to move, and the third bit indicated pen up or pen down before moving.

As example, 000 was move up, 001 was move down, 010 was move left, 011 was move right; 100 was dot and move up, 101 was dot and move down, 110 was dot and move left, 111 was dot and move right (or some similar arrangement). The third command in a byte was assumed to be a move, since the high bit of the third command was assumed to be zero.

The bizarre thing was that a zero byte indicated end of shape; however, a zero byte was also "move up three" (x00 000 000). So, the 'official' solution was to move left, right, and up. Hence, shapes were generally drawn from top to bottom where possible.

I was going to use the shape table to make a shooting gallery game, but it was just too tedious. Not to mention the lack of hardware collision detection. :)

If you really want more information, I'll make a more dedicated effort to finding the reference guide. Several people reference Beagle Brother's "Apple Mechanic"; Apple II images are available at your local mirror sites.

Archives (includes files) at
Unsub & more at

Current Thread