1. A computer program product for executing a Start Subchannel instruction 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: responsive to determining that a subchannel identified by the Start Subchannel instruction is an Asynchronous Data Mover (ADM) subchannel, performing: obtaining an operation request block from main storage, the operation request block comprising an address of an operation block; based on the address of the operation block, obtaining the operation block from main storage, the operation block consisting of a request block, a response block, and one or more move specification blocks (MSBs), wherein the request block comprises an MSB count field having a value indicating the number of one or more MSBs included in and referenced by the operation block, wherein the response block is configured to include an operation code field, a block count filed, a main storage address field for addressing main storage, a storage class memory address field for addressing storage class memory, a block size field and a flags field; performing for each MSB of one or more of the MSBs comprising a flags field having a first value, a method comprising: obtaining a move specification block of the one or more move specification blocks; based on the operation code in the obtained move specification block, determining that the obtained move specification block is configured to release blocks of storage class memory; determining a size of the block of storage class memory to be released and a number of blocks of storage class memory to be released based on the block size field and the block count field of the obtained move specification block; and based on the obtained move specification block, releasing the blocks of storage class memory associated with the storage class memory address of the storage class memory address field of the obtained MSB.