4.5.1. A-sync, alignment synchronization packet

Periodically, the PTM outputs an A-sync packet. Whenever the PTM is enabled, the first packet output is an A-sync packet. In addition, an A-sync packet is output:

The A-sync packet is a sequence of five or more A-sync headers, b0000 0000, followed by the binary value b1000 0000, as Figure 4.1 shows. This is equivalent to a string of 47 or more 0 bits followed by a 1.

Figure 4.1. A-sync alignment synchronization packet

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This bit sequence can occur only as the result of an A-sync packet. To synchronize, the decompressor must search for this sequence. Trace capture devices are usually byte-aligned, but this might not be the case for sub-byte ports. Therefore the decompressor must realign all data following the A-sync sequence if required.

After an A-sync packet, the next byte is always a header, that can be of any type.

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