The core memory inside a Saturn V rocket's computer

Ken Shirriff now looks at the Saturn V Launch Vehicle Digital Computer (LVDC)

The Launch Vehicle Digital Computer (LVDC) had a key role in the Apollo Moon mission, guiding and controlling the Saturn V rocket. Like most computers of the era, it used core memory, storing data in tiny magnetic cores. In this article, I take a close look at an LVDC core memory module from Steve Jurvetson’s collection. This memory module was technologically advanced for the mid-1960s, using surface-mount components, hybrid modules, and flexible connectors that made it an order of magnitude smaller and lighter than mainframe core memories. Even so, this memory stored just 4096 words of 26 bits.
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The core memory module provides an interesting view of a time when 8K of storage required a 5-pound module. While this core memory was technologically advanced for its time, the hybrid ULD modules were rapidly obsoleted by integrated circuits. Core memory as a whole died out in the 1970s with the advent of semiconductor DRAMs.

The contents of core memory are retained when the power is disconnected, so it’s likely that the module still holds the software from when the computer was last used, even decades later. It would be interesting to try to recover this data, but the damaged circuitry poses a problem so the contents will probably remain locked inside the memory module for decades more.

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“Just”? Why, that’s more than three times the memory of an Altair 8080 from a decade later, after expanding it to be able to load a BASIC interpreter!

I very much liked the tension between the two designs: MIT’s AGC and IBM’s LVDC. See footnote 7 in the article and the linked page

Physically, the LVDC was about twice the size (2.2 cubic feet vs 1.1 cubic feet) even though they were both about 70 pounds. The LVDC used 138 Watts and was liquid-cooled, while the AGC used 55 watts and was cooled by conduction. The LVDC used 26-bit words compared to 15 bits in the AGC. One big architectural difference was that the LVDC was a serial computer, operating on one bit at a time, while the AGC operated on all bits in parallel

According to MIT, however, the AGC could run a guidance program 10 to 20 times faster than the LVDC, use half the memory, and provide more accuracy (by using double precision).

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