SG_GET_LOW_DMA 0x227a. Assumes 3rd argument points to an int and places 0 or 1 in it. 0 indicates the whole 32 bit address space is being used for DMA transfers on this file descriptor. 1 indicates the memory below the 16MB level (on i386) is being used (and this may be the case because the host adapters setting has been overridden by SG_SET_FORCE_LOW_DMA,1 .