Q124727: How to Program DMA for Linear Addresses under Windows

Article: Q124727
Product(s): Microsoft Windows Software Development Kit
Version(s): WINDOWS:3.1,3.11
Operating System(s): 
Keyword(s): kb16bitonly
Last Modified: 27-OCT-2000

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The information in this article applies to:

- Microsoft Windows Software Development Kit (SDK) versions 3.1, 3.11 
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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 virtual
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

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Keywords          : kb16bitonly 
Technology        : kbAudDeveloper kbWin3xSearch kbSDKSearch kbWinSDKSearch kbWinSDK310 kbWinSDK311
Version           : WINDOWS:3.1,3.11

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