The Filter

The Event Filter software unit (FLT) is a processor which can make a test on the data for a single event and return a simple `pass' or `fail'. The test could be a check of whether the event passes an $N_{hits}$ threshold, for example, or whether it is a calibration event. In this way, the filter plays an important part in the programmable event loop with the results of tests being used to steer the processing.

Any number of tests may be defined and used in a SNOMAN job, each one being identified by an integer, $i$, where

• $(i>0)$ is a Programmable test defined in a filter request titles bank FLTR $i$. It is called programmable as it is data-driven; the test is defined at execution time.

• $(i<0)$ is a Hardwired test evaluated by FLT code. Each hardwired test is implimented by a separate routine. Each test has its own private titles bank FLTH -$i$ (titles bank numbers are always positive).