"Hans Franke" <Hans.Franke(a)mch20.sbs.de> wrote:
   I have never
really understood the purpose of the M1 pin or cycle. Can
 anyone in simple terms explain it's purpose? 
 M1 serves various purpose on the Z80 Bus: 
 
Hi
 On one system I worked on, it had an input board that didn't
have M1 decoded properly. It was a real funny problem,
It would trash the interrupt vector when executing different
address locations, when the interrupt hit. Adding debug
code would move the problem around. It took 2 weeks to
track this one down.
Dwight