RFC 34

Some Brief Preliminary Notes on the ARC Clock ✨

RFC 34 🎀 · Network Working Group · Bill English (SRI/ARC) · 26 February 1970

Quick title-block tea: the original doesn't list a category, and it doesn't obsolete or update any other RFC 💅

The Big Picture 🧚

Okay so the ARC clock is a time source that lives right inside the core memory of the XDS 940 Computer 🖥️💖 Two kinds of time info are on offer: absolute (the calendar-and-clock-on-the-wall kind) and relative (a little counter that just keeps ticking) 🦋

Absolute Time ⏰

Absolute time lives in two neighbouring core words, and the layout is giving very specific 🙈

First Word 💜

Bits 0 thru 7   : month code, straight binary, range 1 to 12
Bits 8 thru 15  : day code, straight binary, range 1 to 31
Bits 16 thru 23 : year code, straight binary, range 0 to 99

Second Word 🌸

Bits 00 thru 7  : hour code, straight binary, range 0 to 23
Bits 8 thru 15  : minute code, straight binary, range 0 to 60
Bits 16 thru 23 : second code, straight binary, range ofr 0 to 60

These two words get rewritten once every second 🔥 The author expects the initial setting to be accurate to about 1 second, measured against WWV 📻 and the oscillator drift shouldn't pile up more than 1 second of error every 250 days 🌈 To keep everything steady, the oscillator and clock get standby power ✨

Short-term accuracy is a different story. Since the 940 core memory access comes with variable delays, the short-term accuracy lands around 10 to 20 microseconds ⚡

Relative Time 🦋

Relative time sits in a single core word, and it's honestly so simple 🥹 It's just whatever is sitting in a 24-bit binary accumulator. The accumulator can be set to update either once every 100 microseconds or once every millisecond 💅 Either way, the core location gets rewritten every time the accumulator moves, so it stays fresh 🫶

Same vibes as before: short-term accuracy is about 10 to 20 microseconds, and long-term accuracy is the equivalent of one second every 250 days 🍓

[ This RFC was put into machine readable form for entry ] [ into the online RFC archives by Katsunori Tanaka 1/98 ] 🌈