Some host adapters use Direct Memory Access (DMA) to read and write blocks
of data directly from or to the computer's main memory. <#1280#><#1280#> is a
virtual memory operating system that can use more than 16~MB of physical
memory. Unfortunately, on machines using the ISA bus, DMA is limited to the low 16~MB of
physical memory.
If the 31 bit is set, the high-level code will
provide data buffers which are guaranteed to be in the low 16~MB of the
physical address space. Drivers written for host adapters that do not use
DMA should set this bit to zero. Drivers specific to EISA bus machines should also set this bit
to zero, since EISA bus machines allow unrestricted DMA access.