> On Aug 17, 2015, at 10:48 PM, Jay Jaeger <[email protected]> wrote:
>
> BR level is the bus request level for an Interrupt. BR 4 is typical.
On Unibus machines, more of the BR levels were used. The rule of thumb was BR4
for slow devices (like terminals and printers), BR5 for fast devices (disks and
tapes), BR6 for real time critical devices (clock, also DECtape because you had
to respond to a “read block number” interrupt fast enough to start reading the
block before it passed over the heads). BR7 could in theory be used by devices
but I don’t believe it ever was in practice.
paul
