REG(DATA_INSTRUCTION_FIELD_BUFFER)  // (df << 3) | if_buffer
REG(DATA_INSTRUCTION_FIELD_SAVED)   // (df_saved << 3) | if_saved
REG(HALTED)
REG(INT_ENABLE)                     // (int_enable_delay << 1) | int_enable
REG(INT_PENDING)                    // only meaningful if interrupts disabled
REG(TT_BITS)                        // see TT[IO]_* consts in isa/isa.h
REG(TT_INT_ENABLE)                  // (status_enable << 1) | (int_enable)