[Coco] Indexed addressing postbyte

[James Dessart]

On 8-Aug-04, at 8:43 PM, David Gacke wrote:

> Could someone explain to me how the postbyte for indexed addressing
> breaks down.
>
> The book I have isn't clear (to me) on how to determine what it's
> supposed to do.

I have the Motorola manual, and it seems to me that the specifics are 
mostly in the final 5 bits. Except in the case of a 5-bit offset, and 
then you have the first bit 0, then the register you're indexing from 
as two bits, then the offset itself. The rest all have 1 as the first 
bit, followed by the register (except for PC-relative, where the two 
register bits don't matter), then the 5 bit code indicating the 
specific mode.

For those 5-bits, the first indicates whether or not it's an indirect 
indexing or not. The final 4 bits are the same for the same operation, 
regardless of indirection (hope that makes sense). I haven't been able 
to figure out a pattern in them yet (from the minute or two I've had it 
in front of me just now...)

As for the rest, I can scan in the table for you, if you'd like.

James