bqt at update.uu.se
Tue Aug 18 11:30:30 CDT 2015
On 2015-08-18 14:46, Paul Koning wrote:
>> On Aug 17, 2015, at 10:48 PM, Jay Jaeger <cube1 at charter.net> 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.
To put things another way. Think of BR level as "interrupt priority". A
bit coarse, since position on the bus further refined interrupt priority
within a BR level. (Closer to the CPU -> higher priority than farther
from the CPU, for devices on the same BR level.)
And then the CPU could choose to block all interrupts at a specific BR
level and lower.
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