1. A computer program product for executing an instruction to execute a Configure Storage Class Memory command in a computing environment comprising main storage and storage class memory, said 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 an input/output (I/O) subsystem a request block, the request block comprising a command code indicating the Configure Storage Class Memory command and a total size value to specify a requested number of increments of storage class memory to be allocated; based on the command code, initiating a configuration process for configuring the storage class memory, the process configured to allocate the requested number of increments of storage class memory specified in the total size value, wherein the initiating comprises performing one or more validity checks; continuing to perform the configuration process of the storage class memory responsive to the one or more validity checks being successful, wherein the one or more validity checks comprises determining that the total size requested does not exceed a number of storage class memory increments in an initialized state; and storing in a response code field of a response block a response code indicating whether the configuration process has been initiated, the response block having a length code indicating a length of the response block and the response code field.