DJGPP FAQ -- Using DMA with DJGPP

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18.14 Using DMA with DJGPP

Q: How do I use DMA with DJGPP programs?

Q: I want to use DMA, but I don't know how to get the physical address of the buffer I allocate for that purpose.

A: The main problem in using DMA with DJGPP is how to get the physical address of a buffer, which is required to program the DMA controller. In protected-mode environment, memory addresses that your program manipulates are actually offsets from the base address of the data segment. You can obtain the base address of the data segment by calling the __dpmi_get_segment_base_address library function and add it to the address of your buffer, but the resulting address is a logical address, translated into a physical address by the memory-mapping unit which is part of the CPU.

You have two alternatives to get the physical address of your buffer:

Allocate the buffer in conventional memory, below the 1MB mark. This memory is mapped 1:1 by all DPMI servers, so the linear address is equal to the physical one. You can allocate a buffer in conventional memory using the library function __dpmi_allocate_dos_memory.
This method has a disadvantage of using conventional memory which is at a premium, and is therefore generally ill-suited for large DMA buffers.
Use VDS, the Virtual DMA Services API. This is implemented by a bunch of functions of interrupt 4Bh; see Ralf Brown's Interrupt List for the details. This method has a drawback that it needs a real memory manager, such as EMM386 or QEMM, to run, since only memory managers and Windows support the VDS API. In other words, if you use VDS, your program won't work on a system where CWSDPMI(Note: CWSDPMI has an experimental VDS support in its sources, but the distributed binary was compiled without it. Contact Charles Sandmann if you want to try to enable VDS support in CWSDPMI.) is used as the DPMI server, allocating memory by raw XMS calls or via HIMEM.

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