MC_REGB_SET
csr |= MC_REGB_SET;
csr &= ~MC_REGB_SET;
csr |= MC_REGB_SET;
csr &= ~MC_REGB_SET;
mc146818_read(sc, MC_REGB) | MC_REGB_SET); \
mc146818_read(sc, MC_REGB) & ~MC_REGB_SET); \