1. A computer program product for executing a machine instruction in a central processing unit, the computer program product comprising: a computer readable storage medium readable by a processing circuit and storing instructions for execution by the processing circuit for performing a method comprising: obtaining, by a processor, a machine instruction for execution, the machine instruction being defined for computer execution according to a computer architecture, the machine instruction comprising: at least one opcode field to provide an opcode, the opcode identifying a load to block boundary operation; a register field to be used to designate a register, the register comprising a first operand; at least one field for locating a second operand in main memory; and executing the machine instruction, the execution comprising: only loading bytes of the first operand with corresponding bytes of the second operand that are within a block of main memory dynamically determined based on a specified type of block boundary and one or more characteristics of the processor.