DEBUG.UB bit gets reset on single stepping

[fbedard]

Code to single step in OpenOCD is wrong:

//retval = arc_jtag_read_aux_reg(&arc32->jtag_info, AUX_DEBUG_REG, &value);
value = SET_CORE_SINGLE_INSTR_STEP; /* set the IS bit */

retval = arc_jtag_write_aux_reg_one(&arc32->jtag_info, AUX_DEBUG_REG,
value);

<<<<

In this case all bits that are already set in DEBUG and which are writable will be reset to the zero. Notable example is DEBUG.UB.

STAR 9000810473