On 2016-01-02 13:07, Mark Pizzolato wrote:
On Saturday, January 2, 2016 at 3:28 AM, Johnny Billquist wrote:
On 2016-01-01 23:08, Mark Pizzolato wrote:
On Friday, January 1, 2016 at 1:02 PM, Johnny Billquist wrote:
Um. What does 7B and 7P stand for here? They are not "normal"
designation for 7 bits with some parity... The letters I know of are
N - None, S
- Space, M - Mark, O - Odd, E - Even.
And if you are using anything that manage to print something visible
when outputting a NUL, that sounds like something broken somewhere
else.
Sounds like the current simh code is trying to cover up when people
have an error somewhere else in their setup?
Actually, no. Some simulated systems have very particular
expectations of the terminal devices which are connected to the
system. They'll output 7bits of data with a parity bit. In general
the parity bit isn't useful for essentially any modern system running
the simulator so 7P and 7B allow the parity bit to be stripped off
before the data is delivered to the local session. Besides parity
bits, as Dave Bryan points out some of these simulated systems presume
they're talking to ASR Model 33 teletype terminals. When talking to
these slow mechanical devices the simulated system often will insert
NUL or RUBOUT or other characters into the output character stream as
non-printing delays to accommodate the slow mechanical behavior of the
terminal when it may be repositioning the print head. If you happen
to be running a simulator with a real Model 33 teletype terminal then
you would want to run the console port in 8B mode so all of the generated
characters and parity bits get to the terminal, otherwise most folks will want a
filtered mode.
I would like to point out that this is in no way ASR-33 specific. Using NUL
fillers for delays was needed on *lots* of terminals, including even a VT100
when running at higher speeds and doing some operations (like a linefeed).
So NUL characters was used a lot of the time. And there are also lots of other
control characters defined in ASCII. And programs sometimes use them. And
they have defined behaviors.
The details of the filtering behavior are very configurable.
Didn't say anything else.
And simh deciding to filter them all out is
basically wrong. If you have a specific environment, in which you are not
actually using a proper ASCII conformant terminal, then fine, have some way
for simh to accommodate such brokenness. But don't assume it, and don't
enforce it, and don't break everyone else because of it.
Since the behavior is configurable, simh isn't deciding or forcing anything on
the user. If you want every bit without any filtering configure the port/mux
to 8B.
Meanwhile, having the output data filtered allows the "User visible" data to
interact with SEND/EXPECT commands that may be defined in the simh
environment in meaningful ways without having to be burdened with the
potentially variable filler/invisible characters that may have been injected
into the output data in somewhat arbitrary places.
simh is deciding, by having a default. And, for some reason, the default
on a pdp-11 is to filter stuff, while on a VAX not, for example.
And I personally find that wrong. On a real system, you have an ASCII
console. And several non-printing characters are output during
operations, and they all have meanings and actions. Filtering them out
should not be the default.
Not only that, but it's definitely not obvious or easy to realize that
it is happening. I found out several years ago, but I don't remember
exactly how anymore. But I was definitely annoyed at it before I found
out how to fix it. I had since forgotten all about it, until Paul
obviously also got struck by it the other day, figured it out, and
posted about it.
Anyway, it's not a big deal. I think simh is doing it wrong, but since I
figured out how to fix it for me, I mostly ignore such annoyances. But
other people obviously will also hit the same issues...
Johnny
--
Johnny Billquist || "I'm on a bus
|| on a psychedelic trip
email: b...@softjar.se || Reading murder books
pdp is alive! || tryin' to stay hip" - B. Idol
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