The PS TAP has two modes of operation. These modes change the available instruction set to control what can be accessed by JTAG interface. During reset and pre-boot, only the IDCODE, IP_DISABLE, JTAG_STATUS, ERROR_STATUS, and BYPASS instructions are functional. The rest of the instructions can become functional depending on the boot mode, eFUSE states, and register settings.
When the PMU processes the boot header, it determines if the system remains in its secure mode or transitions to a non-secure mode.