The high-order four (or six) bits are "modifiers" ('M') for some commands, or are ignored. The commands are:  The data address (8-31) is the 24-bit address of a main storage buffer area to or from which the data is to be transferred. The flag bits (32-36) are defined as follows: The count field (48-63) indicate the number of bytes to be transferred by this command. All eight bits of commands that initiate I/O operations are passed to the device. "The modifier bits specify to the device how the command is to be executed." For example, for the IBM 2305 DASD the "write" command ('MMMMMM01'BX) may have the following values: