[Coco] Indexed addressing postbyte

[Robert Gault]

David Gacke wrote:


> Hi All,

> 

> 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.

> 

> Thanks very much,

> 

> Dave Gacke

> 

> 

> 

There are about 26 possible postbytes so I don't intend to list all of 
them. Here is are some typical examples.
             postbyte
   opcode    D,R        where R=X,Y,U or S
             1RR01011   X=00 Y=01 U=10 S=11

              ,--R
             1RR00011

             n,PCR
             1XX01101   16 bit offset XX=don't care
Note that bits 5&6 determine the register and the other bits determine 
the function. There may be other bytes after the postbyte as needed for 
addresses.