You will discover sixteen attainable 4-bit command codes, and twelve of them are assigned. With the exception of the unique twin deal with cycle, the least major bit of the command code indicates no matter if the next information phases certainly are a study (info sent from target to initiator) https://nathanlabsadvisory.com/how-nist-800-171-compliance-consulting-in-usa-can-secure-your-business-data/