SFXGE_TX_DESCOFFSET
mapping_off & SFXGE_TX_DESCOFFSET;
#define SFXGE_TX_DESCMASK (~SFXGE_TX_DESCOFFSET)
stmp->stm_off = dmac.dmac_laddress & SFXGE_TX_DESCOFFSET;
ASSERT((dmac.dmac_laddress & SFXGE_TX_DESCOFFSET) == 0);