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 non-transitory computer readable storage medium readable by a processing circuit and storing instructions for execution by the processing circuit for performing a method comprising: based on 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 a number of MSBs associated with a request associated with the operation request block, wherein the response block is configured to hold exception conditions, and wherein each move specification block is configured to include an operation code field, a block count field, 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; obtaining a move specification block of the one or more move specification blocks; based on a value of a flag of the flags field of the obtained move specification block comprising a first value, performing: 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 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.