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 a block boundary size indicator to indicate a specified block boundary of a block of the second operand; and executing the machine instruction, the execution comprising: loading from a block of the second operand a variable amount of data into the first operand while ensuring only data within the block is being loaded into the first operand, wherein the loading from the block starts at a starting address in the block of the second operand, the starting address provided by the machine instruction, and wherein the loading ends at or before the specified block boundary of the block of the second operand.