IBM 4300 Manual page 131

Condition Code: The code remains unchanged.
Program Exceptions:
Access (fetch, operand 2; store, operand 1)
Programming Notes
1. An example of the use of UNPACK is given in
Appendix A.
2. A field that is to be unpacked can be destroyed
by improper overlapping. To save storage
space for unpacking by overlapping the
operands, the rightmost position of the first
operand must be to the right of the rightmost
position of the second operand by the number
of bytes in the second operand minus 2. If
only one or two bytes are to be unpacked, the
low-order positions of the two operands may
coincide.
7-38 IBM 4300 Processors Principles of Operation