A system and method for performing an operation within limits upon a
shared value stored in an actual value location. The method provides a
plurality of memory locations, two of the memory locations being first
and second reservation memory locations, and two of the memory locations
being limit memory locations containing limit values. An operand is
obtained and the operation is performed upon an affected reservation
register using the operand. A resulting value is compared to the limit
values in the limit memory locations and if the resulting value is not
within the values in the limit memory locations, an inverse operation is
performed to restore the affected reservation register and a failure is
reported. Else the operation is performed to update the actual value
location, the operation is performed to update an unaffected reservation
register, and a success is reported.