NAME

>16MB_ISA − support for more than 16 MB of memory on ISA bus systems

DESCRIPTION

Some system and device boards only support DMA transfers to physical addresses below 16 MB (low memory).  On machines configured with more than 16 MB, the operating system arranges for such transfers to be made only to low memory by allocating low memory when possible for such transfers and preferring high memory (>16 MB) for other purposes.  When no low memory is available, buffered transfers (block device I/O) are done through buffers pre-allocated from low memory and the data is copied to/from high memory.  Raw transfers (character device I/O) are done to pages forced into low memory, possibly by swapping out pages and processes. 

The operating system attempts to determine at boot time whether the system board or any configured devices have the 16 MB restriction.  If so, the memory is divided into two pools, which incurs a small additional overhead during system operation, and file I/O buffers are restricted to low memory (and the number of buffers is thereby restricted).  Currently, the configurations that are not considered to have the restriction have an EISA bus or a PCI bus with the Intel 82378 SIO chip, and no bus-mastering ISA boards (such as an Adaptec AHA-1540 or a Western Digital WD7000-ASC). 

If an ISA bus-mastering board is to be loaded after system boot via the idload(1M) command, it can only be done if the system is already using two memory pools. The dual-pool mechanism can be forced by setting the DMA24BIT tunable parameter. 

SEE ALSO

idload(1M), idtune(1M), kconfig(1M), stune(4), aha1540(7), wdasc(7). 

\*U  —  Version 1.0