关于iorequest的问题

如果我想分多次传输一个大文件，下面这段代码中iorequest.usize的值应该是一次传输的数据长度还是总共传输的长度？十分感谢指点！
ioRequest.uAddressL = 0;
ioRequest.bAddressH = 0;
ioRequest.uSize = threadParam->uiLength;
ioRequest.bCommand = 0x81; //start, read

ioBlock.uOffset = 0;
ioBlock.uLength = sizeof(IO_REQUEST);
ioBlock.pbyData = (PUCHAR)&ioRequest;
ioBlock.uIndex = SETUP_DMA_REQUEST;

bResult = DeviceIoControl(hDevice,  //读寄存器
IOCTL_WRITE_REGISTERS,
(PVOID)&ioBlock, //输入缓冲区
sizeof(IO_BLOCK),
NULL,            //接收缓冲区
0,
&nBytes,     //实际接收数据个数
NULL);