shape table code question

[Jonno Downes <jonnosan@gmail.com>]

Howdy

Can anyone point me at an explanation of how the vector bytes in  a
shape table are interpreted?

I've seen various explanations of how to create shape tables, and they
all talk about creating vectors, then coding that vector into 3 bit
sections where
first bit of 3 : 0=dont plot, 1=plot
remaining 2 bits of 3: 00=move up, 01=move right, 02=move down,
11=move left

then you take 2 of these 3 bit codes, prepend with "00" and you have
an 8 bit byte to put in your shape table.

Which all makes sense, except I am currently looking at the Beage Bros
"Apple Mechanic" manual (http://beagle.applearchives.com/the_software/
vintage_beagle_bros_softwar/apple_mechanic.html ) and there is a table
on page 46 "Apple Shape Table Vectors" that shows the high 2 bits can
be nonzero, and all 8 bits in the byte are significant.

Can anyone explain exactly how the vectors are decoded?

Cheers

Jonno