Unix and real time control - PDP11 experience

[G.TOMASEVICH]

It is not easy to get real-time behavior out of UNIX.  I have managed to
get a PDP-11/45 with RP04 and two TU10 tape drives, plus two DR11-W's,
to process data at 8 KHz.  The operation is to copy a source file to
one DR11 and read the second DR11 to a destination file.  I had to hack
some drivers into UNIX to get real-time restart of DMA at the end of
each block transfer, and that only works with all interrupts disabled
for a time.  The clock loses ticks during DMA as a result.  The UNIX
scheduler is slow, and the clock interrupt routine is a pig.  There is
no way I could get the DR11W ready state detected and DMA restarted
in 125 microsec without disabling all interrupts.
The hacks are in the utssys.c file, which already looks like a hack.
One is to read the DR11 registers, the second is to write them, and the
third is to disable interrupts, wait for DR11 ready, then write the
registers, enable interrupts, and return the status register value.
In addition, I changed 'plock' to allow any user to lock a process,
I let anyone read '/dev/mem' (an alternative is to let the user change to
group 'sys').  The program reads the segmentation registers and calculates
the addresses of the buffers and then the bits for the DR11W registers.
The main program has a multitasking scheduler to do everything during
a DMA run, which includes operating the DR11-W's, detecting errors,
and reading keys and interpreting commands.
I do not know RT11 at all, but in any case we are replacing our PDP with
a VAX later this year, so I will have to redo the program for the VAX.
I do not know how slow a data rate one needs in order to run with
clock interrupts enabled.  In addition, I avoided writing a real driver
with interrupts.  That would almost certainly fail in my case, because
there is so much stuff done before the operating system even gets to the
device interrupt routine.
The program cannot reliably read from one file and write to another except
right after doing 'fsck -S'; apparently the freelist entropy goes up too
much after a few files have been made and erased.  It is possible to get
real-time copy for a disk file source to a raw disk filesystem (/dev/rrp4)
or to a magnetic tape in raw mode, but the mag tape cannot be at the load
point.  For some reason, tape motion away from the load point is very
slow, then succeeding writes occur fast enough.
The program uses ping-pong buffers which hold .75 second of data;
one pair each for the transmitting and receiving DR11W's.
	George Tomasevich, AT&T Bell Laboratories