Specifications

Operating System Routines
IOC$LOADALTMAP
Description
A driver fork process calls IOC$LOADALTMAP to load a previously allocated
set of alternate map registers with page-frame numbers (PFNs). This enables a
device DMA transfer to or from the buffer indicated by the contents of UCB$L_
SVAPTE, UCB$W_BCNT, and UCB$W_BOFF.
IOC$LOADALTMAP confirms that sufficient alternate map registers have been
previously allocated. If not, it issues a UBMAPEXCED bugcheck. Otherwise, it
loads the appropriate PFN into each map register and sets the map register valid
bit. It clears the last map register. This last invalid register prevents a transfer
overrun.
If the system does not support alternate map registers, the routine exits with
SS$_SSFAIL status.
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