This section gives a brief description of the formatting process as an aid to generating correct formatting strings.
Each trace record stored in the RAS buffer consists of a header followed by a number of variable length trace data records. The header identifies the major and minor code, time stamp, process ID, etc., and the total length of the trace data for that trace record.
Each MEM32, MEM, ASCIIZ32, or ASCIIZ data statement, coded in the trace source file for a tracepoint, produces an associated data record to be stored in the trace buffer. The data records consist of a 3-byte prefix followed by the trace data. This prefix consists of a status byte followed by the length of the data for that statement. The status byte indicates whether valid data has been traced.
Dynamic trace can only trace data that is resident in memory at the time that the tracepoint is executed. Data may not be able to be traced for two reasons: it resides in a page that is currently paged out or the address specified is invalid. This latter case usually occurs due to tracing indirectly via invalid pointer variables. In either of these two cases dynamic trace sets the status byte accordingly and stores the pointer in the place of the wanted data. No more data is attempted to be traced for this invocation of the tracepoint, but tracing will resume the next time this tracepoint is encountered.
Since the position of these prefix bytes, within a trace record, is dependent on the data being traced and the number of MEM32, MEM, ASCIIZ32, or ASCIIZ statements, the Trace Formatter must be told when to expect the prefix in the trace record. This is the purpose of the %P formatting control. It must be coded in the formatting string at every place a data record is expected.
Note: With ASCIIZ and ASCIIZ32 commands, the prefix must be used to obtain the length of the string since the string is not null terminated.