Thanks! It's weird that a) Bill Atkinson also used all-caps for his mnemonics, as did I when I grew up on the MC68k; b) how it brings me back to the Amiga (yes, I know this code pre-dates the Amiga, but still).
Don't worry, 68k assembly is easier to read than x86 assembly. This code is nicely structured and heavily commented. Don Knuth called Atkinson's QuickDraw and MacPaint code some of the best ever written.
Of course, if I'd actually read the code before linking to it, I'd have realized that this file doesn't really answer avgarrison's question because the algorithm is in DrawArc.a (see Luc's post earlier in this thread for an archive with the rest of the files).
Edit: Luc already linked to the source archive in this thread, so I changed my link specifically to the RRects.a file.