cube1 at charter.net
Tue Aug 18 12:26:53 CDT 2015
On 8/18/2015 11:33 AM, Johnny Billquist wrote:
> On 2015-08-18 15:09, Jay Jaeger wrote:
>> On 8/18/2015 7:46 AM, 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.
>> Yes, but this discussion regards an RLV11.
> It was possible to use several BR levels on a Qbus as well, but in
> practice it was not done.
> And I suspect the diagnostics wants to know so that they can set the PSW
> correctly on interrupts. Just using the default number should be good
> enough in the common case.
Yes, but from what I could tell from its manual, the RLV11 could ONLY
use BR4 - had no jumpers to do anything else - it was designed in the
days of the LSI bus, the QBus precursor, which only had the one signal.
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