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> with optimal placement of code on the drum being seen as a major programming skill.

I remember reading an article/blog post from a programer of that time detailing how he optimized some code by taking into account the physics of the drum (such as mechanical inertia, angular momentum, etc.). I don't know how to locate it now but it was amazingly low-level.

You may be thinking of the Story of Mel:

http://www.catb.org/jargon/html/story-of-mel.html

https://en.m.wikipedia.org/wiki/The_Story_of_Mel

The words are 36 bits. 3 12 bit characters (the analogue to a byte)

Wikipedia: "The size of the byte has historically been hardware dependent and no definitive standards existed that mandated the size – byte-sizes from 1[3] to 48 bits[4] are known to have been used in the past."

12 3 bit characters.

12 "character" words of 36 bits.