AR_MCI_GPM_WRITE_PTR
value = MS(REG_READ(ah, AR_MCI_GPM_1), AR_MCI_GPM_WRITE_PTR);
offset = MS(REG_READ(ah, AR_MCI_GPM_1), AR_MCI_GPM_WRITE_PTR);
gpm_ptr = MS(REG_READ(ah, AR_MCI_GPM_1), AR_MCI_GPM_WRITE_PTR);