From coff at tuhs.org Thu May 7 07:58:48 2026 From: coff at tuhs.org (Aron Insinga via COFF) Date: Wed, 6 May 2026 17:58:48 -0400 Subject: [COFF] [TUHS] Re: DECtapes under the UNIX room floor In-Reply-To: References: <98F84673-74C5-4C38-8078-05B1452113C5@archibald.dev> Message-ID: <9c21baea-2b95-45d3-8d76-f7f1bd04ea1f@insinga.com> Hence the term "jiffy" for a power line frequency clock period: https://hackersdictionary.com/html/entry/jiffy.html With the possible exception of a system with only interpreted languages you need some sort of clock interrupt in order to implement time-sharing.  So there had to be one to allow TSS/8, the various PDP-6 time-sharing systems, and the PDP-11 time-sharing systems such as RSTS(/E) and Unix.  (This is actually a reply to the Unix mailing list, but the connection may be too thin.) In related news, https://blog.tephra.me/pdp-11-dl-11-repair-uart-2/https://archive.org/details/ [Remove '-2' from the URL to get the previous post which was actually about the serial interface.] At the time, I was told that the [never completed]  "minnow" or "tiny" PDP-10 would be the KT20 instead of the KT10 (like the KS10 etc.) because there was an existing option named KT10.  IIRC that person believed it was perhaps a clock, but I see now that it was the "KT10 Memory Protection and Relocation" option ('T' for time-sharing??) for the KA10.  The clock was actually the DK10. https://ftp.mirrorservice.org/sites/www.bitsavers.org/pdf/dec/pdp10/KA10/PDP-10_InstallationMan.pdf https://archive.org/details/bitsavers_decpdp10peealTimeClockMaintenanceManualJul73_2436338/mode/2up - Aron On 5/6/26 14:19, Clem Cole via TUHS wrote: > On Wed, May 6, 2026 at 4:12 AM Thalia Archibald via TUHS > wrote: > >> >> Was there a precedent for the choice of sixtieths of a second? >> > Yes, DEC used line frequency from very early on in their processors. For > the original PDP-11s (/20, 40, 45, 70), the KW11-L (not be confused with > the KW-11P) was a single high card > http://www.bitsavers.org/pdf/dec/pdp11/1140/EK-KW11L_TM-002_KW11-L_Line_Time_Clock_Manual_Jul74.pdf > [there is a picture of in on gunkies: > https://gunkies.org/wiki/KW11-L_Line_Time_Clock ], the functionality was > added to the DL11-W, so when it was used as the console KL11 it also acted > as a KW11-L > > > > The purpose is to produce interrupts at a rate of 50 or 60 Hz, driven from > the AC power provided to the CPU's power supply [which is why UNIX is > configured appropriately]. > > If I understand the history correctly, the PDP-5 (1963): is the first DEC > machine to offer a Type 137 Real Time Clock, which could be configured to > trigger interrupts at the power line frequency (60Hz or 50Hz). This design > directly influenced the later PDP-8. PDP-8 (1965): Supported the DK8-EA > (and later the DK8-L and DK8-P) real-time clocks. The DK8-L Line Frequency > Clock was functionally identical to the KW11-L, providing a flag and > interrupt every 16.6ms or 20ms. PDP-6 (1964): As a large-scale system, it > had a central clock (the Type 701) that provided several fixed frequencies, > including a line frequency signal for system timekeeping. PDP-7 (1965): > Offered the Type 175 or Type 144 real-time clocks. Like the PDP-5, these > could be set to line frequency for basic task scheduling. From coff at tuhs.org Thu May 7 16:28:43 2026 From: coff at tuhs.org (Lars Brinkhoff via COFF) Date: Thu, 07 May 2026 06:28:43 +0000 Subject: [COFF] [TUHS] Re: DECtapes under the UNIX room floor In-Reply-To: <202605061813.646IDYhk082929@ultimate.com> (Phil Budne via TUHS's message of "Wed, 06 May 2026 14:13:34 -0400") References: <98F84673-74C5-4C38-8078-05B1452113C5@archibald.dev> <20260506083214.un4cgnw7xzkevbm3@illithid> <202605061725.646HPaad081924@ultimate.com> <202605061813.646IDYhk082929@ultimate.com> Message-ID: <7wv7czlomc.fsf@junk.nocrew.org> Phil Budne wrote: > CTSS (the progenitor of both Multics and DEC timesharing systems) had > a "chronolog" device (attached as a magtape drive?) that supplied the > current time, month and day (but not year!), while even large (million > dollar) DEC timesharing systems in the 1970's and 80's expected a > human operator to be present to enter the date and time. Fun fact. Early ITS had a modem device, and used it to dial out to CTSS to get the time and date from the login prompt. Later ITS got its own clock device with a separate power supply.