Unibus question

[Activmaker]

Hi,

I am reading documentation related to unibus.I learned many things about priority and arbitration . But I don't understand priority levels <4 . How it works ?

Thanks,
Actimaker.

 

[jlang]

All the levels below 4 are the same. They don't mask bus requests.
Dec wanted 4 interrupt priority levels. To control 4 levels requires 3 bits in the PS register.
ps=7 all requests masked
ps=6 7 is allowed
ps=5 6,7 allowed
ps=4 5,6,7 allowed
ps=3 4,5,6,7 allowed
ps=2 3,4,5,6,7 allowed
ps=1 2,3,4,5,6,7 allowed
ps=0 1,2,3,4,5,6,7 allowed

BUT... there is no hardware for levels 1,2,3 so ps=0 through 3 all act the same: 4 and up enabled.

joe