|Parameter||Dflt 4096/8192||Min||Max 4096/8192||Auto|
For AT systems, MAXDMAPAGE is set by default to ``4096'' for the benefit of devices that are restricted to DMA into the lower 16MB of memory (also called 24-bit DMA). Such devices include most DMA-capable ISA devices (such as diskette drives), as well as some EISA devices performing standard mode transfers. If you have DDI5 device drivers, you must set MAXDMAPAGE to ``4096''.
Setting MAXDMAPAGE to ``0'' disables the kernel's support for restricted DMA. Do this only if the you are sure that every device on the system either doesn't have DMA capability or is capable of accessing the entirety of physical memory through DMA.
Your system can have a PCI or EISA bus but still have an ISA controller, a controller that uses standard mode DMA, or a driver that programs the controller into standard mode. If any of these situations apply, then you must not set MAXDMAPAGE to ``0''. If you have DDI5 device drivers, you must set MAXDMAPAGE to ``4096''.
When MAXDMAPAGE is non-zero, and memory is present above the threshold specified, the kernel will reserve a portion of its memory for exclusive use by devices with a DMA access restriction (see DMA_PERCENT). Setting MAXDMAPAGE to ``0'' preserves this memory for other uses (for example, for files and processes).
Devices that support DMA but are not restricted in their ability to access memory (for example, EISA devices performing extended mode transfers) and PCI devices are not affected by MAXDMAPAGE. MAXDMAPAGE does not apply to 32-bit DMA requirements.