Additional 68HC11 Registers?

[Ross Myers]

>I found both the XGDX and XGDY opcodes in my prom.  My prom is a 93 Z28 A4
>prom.  What does that instruction do?
>
> Roger
>

XGDX (8F) - Exchange Double Accumulator and Index Register X (Phew...)
XGDY (18) - As above but Index Reg Y

Both exchange the contents of the double accumulator ACCD and the contents
of Index register X (or Y).A common use for XGDX is to move an index value
into the double acc to allow 16-bit arithmetic calculations on the index
value before exchanging the updated index value back into the X or Y
register.

Taken from the HC11 Ref Manual.

Ross Myers