John Summerfield wrote:
David Boyes wrote:
That generates an interrupt in the host, and then the host issues a
read.
This kind of approach, where the terminal generates an interrupt for
each key-press, wouldn't be so expensive.
I beg to differ. I/O interrupts on a 390 are *unbelievably* expensive if
the amount of data transferred is one byte. A LOT of stuff happens --
but, if you limit it to recovery/ermergency work, then there won't be a
lot of such I/O.
everything in the architecture is designed to minimize that happening.
You'd have to have a continuously running channel program to make this
only painful rather than impossibly so. Cf the history of the original
BusTech network adapter and the Lippke driver for it -- the same
physical hardware was literally almost 5 times faster if you didn't use
the 8232 I/O model but a continuous channel program.
Many years ago, I wrote a program to drive some remote 3270 terminals: a
small TP monitor, I used a general poll, and the work was offloaded to
the 3704 running Emulation Program.
This is, of course, the wrong example. What I'm really talking about is
a channel-attached device that, like a local 3270 controller, generates
an interrupt when a terminal user presses an attention key. Unlike a
3270 terminal where only some keys (the function keys, the program
access keys) require attention, in this case all keys do.
And unlike a local 3270 controller which has real devices connected, the
devices on this would be remote (presumably ssh) connexions.
However, the controller doesn't require being polled, any more than a
local 3270 does. The only time a channel program needs to be run is when
the device signals that one is necessary.
It should be capable of running in burst mode.
--
Cheers
John
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