com_scratch
scr0 = com_read_reg(sc, com_scratch);
com_write_reg(sc, com_scratch, 0xa5);
scr1 = com_read_reg(sc, com_scratch);
com_write_reg(sc, com_scratch, 0x5a);
scr2 = com_read_reg(sc, com_scratch);
com_write_reg(sc, com_scratch, scr0);
#define com_uer (com_scratch)
#define com_uir (com_scratch)