The DSKSYNC register is used to synchronize the input stream.  This is
highly useful when reading disks. If the WORDSYNC bit is enabled in
 ADKCON , no data is transferred until a word is found in the input stream
that matches the word in the DSKSYNC register.  On read, DMA will start
with the following word from the disk.  During disk read DMA, the
controller will resync every time the word match is found.  Typically the
DSKSYNC will be set to the magic MFM sync mark value, $4489.

In addition, the DSKSYNC bit in  INTREQ  is set when the input stream
matches the DSKSYNC register. The DSKSYNC bit in  INTREQ  is independent
of the WORDSYNC enable.