How to Program DMA for Linear Addresses under Windows (124727)

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.