How to Program DMA for Linear Addresses under Windows
ID: Q124727
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The information in this article applies to:
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Microsoft Windows Software Development Kit (SDK) versions 3.1, 3.11
SUMMARY
The process for programming DMA for linear addresses under Windows is
similar to the process for programming physical addresses under MS-DOS. On
byte (8-bit) channels (0-3), program the page register with bits A23-A16
and the base register with bits A15-A0 of the 24-bit address. On word
(16-bit) channels (5-7), program the page register with bits A23-A16 and
the base register with bits A16-A1, shifted right 1 bit (>> 1 in C, or SHR
<reg>,1 in ASM).
MORE INFORMATION
It looks like bit A16 is used twice on word channels. Typically, bits
A23-A16 would be compared with the value 0FEh (0xFE) using an OR operator;
then the page register would be programmed with the result. It is not
necessary to strip off bit A16 in the page register because as the least
significant bit in the page register, it is ignored by the hardware.
As noted above, the base register on word channel transfer is programmed
with bits (A16-A1) >> 1. The hardware generates the address by
concatenating the 7 most significant bits from the page register with the
16 bits from the base register. Because the base is shifted left by one,
incrementing the programmed address actually increments the physical
address by 2. Because the LSB is always zero, the transfers are always word
aligned.
Word channels are actually somewhat of a hardware hack for a DMA controller
that really only supports byte transfers.
Given a 24-bit physical address, the controller would be programmed as
described above. If you have a 24-bit linear address, program it the same
way. The virual DMA device (VDMAD) will reconstitue the linear address from
the page/base registers the same way the hardware would.
There is one caveat to keep in mind when using linear addresses. If the
linear address is not below one megabyte, it will probably be above two
gigabytes (> 24-bits). If it is, it cannot be virtually programmed into the
base register. If the DMA buffer comes from GlobalAlloc() instead of
GlobalDosAlloc(), you need to disable address translation using the Virtual
DMA Specification (VDS) API through INT 4Bh. Then use VDS again to get the
physical address and program the page/base with the physical address.
Additional query words:
3.10 3.11 VDMAD no32bit
Keywords : kb16bitonly
Version : WINDOWS:3.1,3.11
Platform : WINDOWS
Issue type :