Changes the current active scan chain.

4 bits.

SCAN_N as the current instruction in the SHIFT-DR state selects the scan path select register as the serial path between DBGTDI, and DBGTDO.

During the CAPTURE-DR state, the value 1000 binary is loaded into this register. This value is loaded out during SHIFT-DR (least significant bit first), while a new value is loaded in (least significant bit first). During the UPDATE-DR state, the value in the register selects a scan chain to become the currently active scan chain. All additional instructions, such as INTEST, then apply to that scan chain.

The currently-selected scan chain changes only when a SCAN_N instruction is executed, or when a reset occurs. On reset, scan chain 0 is selected as the active scan chain.