## Virtual Device Driver Reference for OS/2

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### Configurable Defaults

Default values are set for properties in order to be appropriate in most instances. However, users can change these defaults, if desired. The following defaults can be set in the CONFIG.SYS file:

Default EMS Memory Size Default limit set by specifying the number of kilobytes of memory desired on a "DEVICE=" line in the CONFIG.SYS file. Alternately, the limit can be specified by using a switch, /S=xxx, on the line. For example:

             DEVICE=path\VEMM.SYS 1024

or

             DEVICE=path\VEMM.SYS/S=1024

Default EMS Low OS Map Region Size of remappable conventional memory set by using the /L switch. For example:

             DEVICE=path\VEMM.SYS /L=256

EMS High OS Map Region Size of extra mappable memory (other than the 64KB minimum) set by using the /H switch. For example:

             DEVICE=path\VEMM.SYS /H=32

EMS Frame Location Default frame position set by using the /F switch. Any value in the DOS setting list can be chosen as the default. For example:

             DEVICE=path\VEMM.SYS /F=AUTO

or

             DEVICE=path\VEMM.SYS/F=C800

When devices without virtual device drivers can directly map memory, the Include and Exclude regions are crucial for the virtual EMM device driver. These properties are supplied by another component, and determine which regions can be directly mapped to physical addresses by default when touched by applications.

The virtual DevHlp Mapping services allow the same master page to be aliased in by successive pages in the V86 address space. In particular, EMM allows an application to map in the same 16KB size range of memory repeatedly (for example, a 64KB page frame could consist of the same four pages mapped four times). Since DMA requires contiguous physical memory, I/ O that involves aliased pages, which map to the same physical page, must be handled by breaking up DMA requests.