Embedded Trace Macrocell Architecture Specification: 6.4.1. Trace packet headers

6.4.1. Trace packet headers

The trace packet header indicates the type of trace packet being output on the TRACEPKT pins, and specifies how to interpret the subsequent bytes of the packet. Trace packet header encodings are shown in Table 6.2.

Table 6.2. Trace packet header encodings

Value	Description
bC0A0SS10	Normal data address expected. See Normal Data packets.
bX0X1XX00	Reserved.
bX0X1XX10	Reserved.
bX10XXX10	Reserved.
bX1100X10	Reserved.
bC1101010	Value Not Traced. See Value Not Traced packets.
bC1101110	Context ID. See Context ID tracing.
bX111XX10	Reserved.
bC1A1TT00	Load Miss occurred. See Load Miss packets.
bCTT0SS00	Load Miss data. See Load Miss packets.

Note

Branch addresses are encoded cXXXXXX1, where c is the address continue bit. Branch addresses are always output last in a cycle and are not preceded by a header. See Branch Address trace packets for more information.

Certain bits in the trace packet header encodings shown in Table 6.2 have specific functions, as follows:

C: Informs the decompression tool how many packets are placed on the FIFO in a single cycle. The C bit is set to 1 in every packet except the last packet placed in the FIFO in a cycle. This enables the decompressor to correctly associate packets with cycles (and therefore with instructions).
All headers other than Branch Address have a C bit. A Branch Address packet is always the last packet to be placed in the FIFO in a cycle, and does not require a C bit.
X: Indicates that the value is undefined.
A: Specifies that this is the first data packet for a particular instruction, and that a data address is expected (if address tracing is enabled). This information enables the decompressor to maintain synchronization when tracing through sections of code that cannot be decompressed (any region for which a binary is not available). The A bit is not asserted on Coprocessor Register Transfer (CPRT) packets.
TT: Used as a tag to identify each load miss. For more details, see Load Miss packets.
SS: Used for data value compression. The SS bits specify the size of the data value transferred. Leading zeros are removed from the value as a simple form of data compression. Typically this compression technique is enough to offset the additional bandwidth cost of the header byte. The encodings for the SS bits are given in Table 6.3.

Table 6.3. SS bit encodings

Encoding	Description
b`00`	Value = 0, no data bytes follow
b`01`	Value < 256, one data byte follows
`b10`	Value < 65536, two data bytes follow
`b11`	No compression done, four data bytes follow

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