1.  void* init_ring(void* this, uint64_t alignment, IOMapper* mapper, ...) {
2.      ...
3.      IOOptionBits options = kIOMemoryTypeVirtual | kIODirectionOutIn;
4.      IOBufferMemoryDescriptor* desc =
5.          IOBufferMemoryDescriptor::inTaskWithOptions(kernel_task,
6.                                                      options,
7.                                                      capacity,
8.                                                      alignment);
9.      ...
10.     IODMACommand* cmd = IODMACommand::withSpecification(
11.         IODMACommand::OutputLittle64,  //outSegFunc
11.         0,                             //numAddressBits
12.         0,                             //maxSegmentSize
13.         0,                             //mappingOptions
14.         0,                             //maxTransferSize
15.         1,                             //alignment
16.         mapper,                        //mapper
17.         0);                            //refCon
18      ...
19.     cmd->setMemoryDescriptor(desc, true);
20.     ...
21. }
function 0xFFFFFFF006D1C074