SL811的0X04寄存器读入时值是什么意义？

我在看CYPRESS网站上下的SL811的例子程序是有如下疑问：

在usbXfer()函数中：

SL811Write(IntStatus,INT_CLEAR);// clear interrupt status
result  = SL811Read(EP0Status); // read EP0status register
remainder = SL811Read(EP0Counter); // remainder value in last pkt xfer
//-------------------------ACK----------------------------
if (result & EP0_ACK) // Transmission ACK
{
// SETUP TOKEN
if(pid == PID_SETUP) // do nothing for SETUP/OUT token
break; // exit while(1) immediately

// OUT TOKEN
else if(pid == PID_OUT)
break;
// IN TOKEN
else if(pid == PID_IN)
{ // for IN token only
wLen  -= (WORD)xferLen; // update remainding wLen value
cmd   ^= 0x40;     // toggle DATA0/DATA1
dataX++; // point to next dataX
//------------------------------------------------
// If host requested for more data than the slave
// have, and if the slave's data len is a multiple
          // of its endpoint payload size/last xferLen. Do
// not overwrite data in previous buffer.
//------------------------------------------------
if(remainder==xferLen) // empty data detected
bufLen = 0; // do not overwriten previous data
else // reset bufLen to zero
bufLen = xferLen; // update previous buffer length
//------------------------------------------------
// Arm for next data transfer when requested data
// length have not reach zero, i.e. wLen!=0, and
// last xferlen of data was completed, i.e.
// remainder is equal to zero, not a short pkt
//------------------------------------------------
if(!remainder && wLen) // remainder==0 when last xferLen
{ // was all completed or wLen!=0
addr = (dataX & 1) ? data1:data0;// select next address for data
xferLen = (BYTE)(wLen>=wPayload) ? wPayload:wLen; // get data length required
if (FULL_SPEED) // sync with SOF transfer
cmd |= 0x20; // always sync SOF when FS, regardless
SL811Write(EP0XferLen, xferLen); // select next xfer length
SL811Write(EP0Address, addr);           // data buffer addr
SL811Write(IntStatus,INT_CLEAR);// is a LS is on Hub.
SL811Write(EP0Control,cmd); // Enable USB transfer and re-arm
}
//------------------------------------------------
// Copy last IN token data pkt from prev transfer
// Check if there was data available during the
// last data transfer
//------------------------------------------------
if(bufLen)
{
SL811BufRead(((dataX&1)?data0:data1), buffer, bufLen);
buffer += bufLen; }

//------------------------------------------------
// Terminate on short packets, i.e. remainder!=0
// a short packet or empty data packet OR when
// requested data len have completed, i.e.wLen=0
// For a LOWSPEED device, the 1st device descp,
// wPayload is default to 64-byte, LS device will
// only send back a max of 8-byte device descp,
// and host detect this as a short packet, and
// terminate with OUT status stage
//------------------------------------------------
if(remainder || !wLen)
break;
} }

其中，当PID为PID_IN，也就是HOST读数据时，用ramainder读入了寄存器0x04的值，然后进行的动作我就读不懂了，清哪位大虾解释一下，当读入数据的时候是怎么控制的。

没有人知道吗？
手册上面写得实在是不清楚，请指点一下吧，多谢了！

真的没有人知道吗？
失望

读入的寄存器0x04的值应该表示buffer中还没有发送完的数据！