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With both counters in place, the number of bytes in the buffer is now indicated by the difference between the M0 and M4 counts. The M4 count needs to be increased whenever the M4 produces or consumes data in the USB bulk buffer, so that the two counts remain correctly synchronised. There are three places where this is done: 1. When a USB bulk transfer in or out of the buffer completes, the count is increased by the number of bytes transferred. This is the most common case. 2. At TX startup, the M4 effectively sends the M0 16K of zeroes to transmit, before the first host-provided data. This is done by zeroing the whole 32K buffer area, and then setting up the first bulk transfer to write to the second 16K, whilst the M0 begins transmission of the first 16K. The count is therefore increased by 16K during TX startup, to account for the initial 16K of zeros. 3. In sweep mode, some data is discarded. When this is done, the count is incremented by the size of the discarded data. The USB IRQ is masked whilst doing this, since a read-modify-write is required, and the bulk transfer completion callback may be called at any point, which also increases the count.