The Header Data Register (HDR) specifies a default header pattern that is pre-pended to the beginning of all subsequent SDR commands. The Header Instruction Register (HIR) specifies a default header pattern that is pre-pended to the beginning of all subsequent SIR commands. The header commands have a set of counterpart trailer commands (TIR, TDR) that are described in a following section. A header can be removed by setting its length to 0.