The size and location of an envelope of a data block are included in the posting to a second device of a descriptor list entry for the data block, thus allowing the second device to read the data block without having to first read the descriptor list entry. This envelope may be the same size and location of the data block, or this envelope may be larger than the data block. For example, as the size of the posted register may not be large enough to also store all of the bits required to specify the exact size and position of the data block, a larger data block envelope is defined without specifying the exact low order bits of the size and/or location of the data block envelope.