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include vdmad.inc
 
VxDcall VDMAD_Lock_DMA_Region
 
The VDMAD_Lock_DMA_Region service attempts to lock a region of memory for a
DMA transfer. It is called before a DMA transfer is started, that is, before
the physical state is set for a channel and before it is unmasked.
 
Parameter  Description
────────────────────────────────────────────────────────────────────────────
 
ESI        Specifies the linear address of the actual DMA region.
 
ECX        Specifies the numbers of bytes in the DMA region.
 
DL         Specifies the alignment. This parameter can be one of the
           following values:
 
           Value  Meaning
           ─────────────────────────────────────────────────────────────────
           1      region must be aligned on 64K page boundary
 
           2      region must be aligned on 128K page boundary
 
Return Value
 
The carry flag is set if the lock failed. The ECX register contains the
number of bytes that are lockable in the region (starting from ESI), and the
AL registers contains one of the following error values:
 
Value  Meaning
────────────────────────────────────────────────────────────────────────────
1      DMA_Not_Contiguous: region not contiguous
 
2      DMA_Not_Aligned: region crossed physical alignment boundary
 
3      DMA_Lock_Failed: unable to lock pages
 
The carry flag is clear if the lock is successful. The EDX register contains
the physical address of the DMA region the region has been locked.
 
Comments
 
The service first verifies that the region is mapped to contiguous pages of
physical memory, then it determines whether the region results in a DMA bank
(page) wrap.
 
On AT class machines each channel has a base address register and a page
address register. The base address register is incremented after each byte
or word transferred. If the increment of this 16-bit register results in the
roll over from FFFFh to 0, then the transfer wraps to the start of the DMA
bank because the page register is not updated. Normally MS-DOS watches for
this condition and adjusts Interrupt 13h parameters to split transfers to
avoid this wrap, but MS-DOS does not account for the difference between
linear and physical addresses under Windows, so VDMAD checks again to
prevent wrap from occurring.
 
If these checks pass, the service calls the memory manager to lock the
physical pages.
 
This service does not check to see if the region is within some physical
maximum constraint. If the region is lockable, then it locks the memory, and
it is up to the caller to check to see if the physical region is acceptable.
If the region is not acceptable, then the caller should unlock the region
and perform a buffered DMA transfer.
 
Uses
 
EAX, ECX, EDX, Flags
 
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