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Hi Peter

I don't know the answers to all of your questions, but I'll try to answer what I do know.

IODMACommand::withSpecification() will requires a static C++ function as the first argument. I currently use the provided kIODMACommandOutputHost32, which is defined as

class IODMACommand : public IOCommand
{
...
    #define kIODMACommandOutputHost32   (IODMACommand::OutputHost32)

    static bool OutputHost32(IODMACommand *target,
        Segment64 seg, void *segs, UInt32 ind);
};

So, in theory at least, you should be able to provide your own function.

I then call gen32IOVMSegments thus:

        UInt64 offset = 0;
        IODMACommand::Segment32 segments[1];
        UInt32 numSeg = 1;
        result = cmd->gen32IOVMSegments(&offset, &segments[0], &numSeg);


So my sequence is something like (obviously with error checking etc)

mem = IOMemoryDescriptor::withAddress()
cmd = IODMACommand::withSpecification(....)
cmd->setMemoryDescriptor(mem, false)
cmd->prepare()
cmd->gen32IOVMSegments()
cmd->complete()
cmd->clearMemoryDescriptor(false)
cmd->release()

cheers
peter