[Simh] serial input on PDP-11: tti polling slow

[J. David Bryan]

On Thursday, November 20, 2008 at 10:35, Jörg Hoppe wrote:

> - How can I get a faster TTI?

I am not familiar with the PDP-11.  However, from looking the code in 
"pdp11_stddev.c", it appears that if you set the TIME register to a value 
other than zero, that value will be used as the poll time instead of the 
line clock.

Poll times are counts of simulated instructions.  So, if you do:

  sim> deposit tti time 1000

...then the input poll will occur every 1000 simulated instructions.

I don't know how fast the PDP-11 simulator is, but the HP 2100 simulator 
executes about 47000 simulated instructions per real millisecond on a 2 GHz 
machine.  Beware of setting the time too low; the operating system may not 
be expecting interrupts that quickly and may crash.


> - What is SimHs approach for simulated baudrates?

It depends on the individual device simulation, but the general approach is 
to ignore the programmed rate and operate terminal output devices as fast 
as possible, while ensuring that interrupts do not occur so quickly as to 
overload the operating system in use.  This rate is generally a few hundred 
simulated instructions per character.  Because terminal input generally 
occurs only at "human speed," it is usually synchronized with a real-time 
clock and set at 50-100 polls per second to limit overhead.


> - What is that 24 bit "TIME" register documented for TTI?

See above.

                                      -- Dave