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 hold exception conditions, wherein each move specification block is configured to include an operation code field, a block count field, a main storage address field, a storage class memory address field, a block size field and a flags field; for each MSB block of the operation block, obtaining a move specification block of the one or more move specification blocks, and: determining based on the flags field that the obtained move specification block is configured to obtain a list of one or more indirect data address words, the list of one or more indirect data address words located at a first main storage address of the main storage address field of the obtained MSB; for each block of data specified by the obtained MSB, obtaining from the list of one or more indirect data address words a second main storage address, wherein a number of blocks and a block size are determined based on the block count field and the block size field of the obtained MSB; determining based on an operation code of the operation code field in the obtained move specification block that the obtained move specification block is configured to move blocks of data; and based on the obtained move specification block, moving each block of data between the main storage and the storage class memory, wherein an address of a main storage location used for the move of a block of data is obtained using a respective second main storage address and an address of the storage class memory used for the move is obtained using the storage class memory address field.