I'm very upset. Why are the extended registers write only???
Sincerly,
Bebé Del Grito
Laredo, Texas
The original Commodore NMOS chip designers used internal tristate busses to wired "or" registers for read back, but in the modern process this is not allow and read back paths are made of MUX trees.
MUX's grow very large and get slower for every input you add to them. Adding every register for reading would've pushed the die size over the budgeted size.