On 12/12/09 5:29 PM, "Colby Gutierrez-Kraybill" colby@astro.berkeley.edu
wrote:

There were a variety of computers synchronized not to the mains frequency
but to the horizontal retrace or vertical frame rate for video.  That way,
they could do things like DRAM refresh or video buffer updates in a clock
synchronous way.  To a certain extent, even the IBM PC was built like this,
running at 4.77 MHz, divided down by 3 from a 14.3 MHz crystal (which was
divided by 4 to get the 3.58 MHz color burst).  If I had to guess, at the
low end, boxes like the Atari 68K machines, at the high end, 3Rivers PERQ
(but that one sticks as using 2901 bitslice...)

Anything intended to generate video for integration with other video streams
would greatly benefit from being able to be synchronized to the NTSC 59.95
Hz frame rate, and if the video memory is the same as the system ram, then
running the CPU clock at an exact multiple makes designing the memory access
arbiters easier (they can be synchronous), so what you really want is the
pixel rate being a multiple of 59.95 and the CPU clock being a multiple of
the pixel rate, so that wait state generation is easy (or you can do
transparent/hidden access to RAM during a time when you KNOW the CPU won't
be looking at it).  More than one system used the video access to do DRAM
refresh, too.

Talk about dusting of the old brain cells.
I seem to remember that the PDP 11/23s did indeed allow the use of the 60 hz
as an interrupt for precision timing if that can actually be said. The data
general nova 1200 also. Boy thats exposing ones age.

On Sat, Dec 12, 2009 at 8:29 PM, Colby Gutierrez-Kraybill <
colby@astro.berkeley.edu> wrote:

Yes, the whole PDP-11 line used line frequency to update the real-time
clock.
DEC had a real-time operating system, very useful for emulation of analog
process control functions. Of course, an RTOS is more than just the clock.

We lost that anchor to real time in the interval between the PDP-11 and
NTP or SNTP when the microprocessors took over. All crystal clocks; time
of day (social time) set by anybody with a wristwatch.

Bill Hawkins

-----Original Message-----
From: paul swed
Sent: Saturday, December 12, 2009 10:09 PM

Talk about dusting off the old brain cells.
I seem to remember that the PDP 11/23s did indeed allow the use of the 60 hz
as an interrupt for precision timing if that can actually be said. The data
general nova 1200 also. Boy thats exposing ones age.

On Sat, Dec 12, 2009 at 8:29 PM, Colby Gutierrez-Kraybill <
colby@astro.berkeley.edu> wrote:

the

guess

have

internal

In message 3058527A-CC99-4174-BE75-21DD92334155@astro.berkeley.edu, Colby Gut
ierrez-Kraybill writes:

They sure did.  Digitals PDP computers had a counter register
which counted mains-cycles as the only sort of real-time-clock.

--
Poul-Henning Kamp      | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG        | TCP/IP since RFC 956
FreeBSD committer      | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.

I'm not so sure about the Nova 1200. I think all the Novas had the RTC was
on a standard I/O board, along with the serial interface, PTR, PTP. I
remember two crystals, one 16.000 KHz for the clock. The other was for the
Baud Rate generator, somewhere about 1 MHz. A minimal system had 3 cards
(CPU, Memory, and I/O)

-John

=============

In message 2157.12.6.201.67.1260689371.squirrel@popaccts.quik.com, "J. Forste
r" writes:

No, it was an option, but almost everybody bought it, because it was
necessary to run any kind of timesharing kernel (RTOS, DOMUS, etc)

--
Poul-Henning Kamp      | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG        | TCP/IP since RFC 956
FreeBSD committer      | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.

On Sat, Dec 12, 2009 at 11:29:31PM -0800, J. Forster wrote:

No 50/60 Hz interrupt was standard on any DG machine that I

remember. I do remember some on early DEC equipment.  And yes they had a
Xtal RTC.

I do remember the early DG basic IO boards had a big LF crystal

for the baud rate generation, but I think a later version of the design
used a baud rate generator IC and a much smaller (and cheaper) crystal
or canned crystal oscillator along with a real UART.

OS timing was based on classic real time xtal based interrupts,

not the power line.

--
Dave Emery N1PRE/AE, die@dieconsulting.com  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in
celebration of what could have been, but wasn't and is not to be now either."

On Sun, Dec 13, 2009 at 07:39:05AM +0000, Poul-Henning Kamp wrote:

Technically one could order basic IO boards without some of the

functionality - they simply didn't stuff in some of the chips (and more
than one customer just added the requisite chips themselves).  I do
remember that there were some functions on that card that were rarely
stuffed...

I seem to remember that RDOS (the NOVA disk operating system)

DID require the RTC and I certainly don't remember any hacks in the
system code to get around the need for timer interrupts to keep time and
handle delays and timed waits.  It was a real time multitasking kernel,
though only one "process" (mostly) due to the lack of memory management
Long time ago though...

--
Dave Emery N1PRE/AE, die@dieconsulting.com  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in
celebration of what could have been, but wasn't and is not to be now either."