3. PERMITTED VALUES

Each of the two numbers in the event code (i.e. TYPE and QUALIFIER) is described by 1 to 4 hexadecimal digits (0,1,2,...9,A,B,C,D,E,F). The user can assign any code number she/he wishes except for numbers <0> that are reserved for special purposes. This means <1> as the minimum and <FFFF> as the maximum values. For those that are not familiar with hexadecimal numbers we recommend ( ref 1, ref 2, ref 3, ref 4).
The value of time intervals is coded in decimal scale and can be comprised between 0 and 9999. Negative values are not allowed.

Event <0,0> (which may be written also <00,0> , or <0,00> etc...) means the null event. It could be used to describe very long intervals or to mark the time at which a comment is recorded in the file. Two examples will illustrate these usages.

Suppose you wish to limit yourself so as to use at most two digits to specify the interval between successive events. What happens if you have occasionally more than 99 time units without any event? It would be a waste of space to code all the times in 3 digits if long periods of silence occurred rarely. You could use the null event <0,0> to describe this silence. For instance, a file that includes the following data:

      1,1,47
      1,5,32
      0,0,99
      1,2,17
      ...

means that event <1,1> occurred 47 time units after the previous event, then event <1,5> occurred 32 time units later and then event <1,2> occurred 116 (99+17) time units later.

Another use of the null event is to mark in the file the point at which something which is not an event occurred. Suppose that you wish to record the time at which you inserted an important comment in the file. You may do it in the following way:

      1,1,47
      1,5,32
      0,0,65
      'Ann and Mike just arrived'
      'break time: John goes to dinner'
      1,2,11
      ...

This record will be interpreted as saying that 65 time units after event <1,5> occurred the comment was inserted, while event <1,2> occurred 76 time units (65+11) after event <1,5>.

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