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Donald Knuth Clarifies

by Abdur-Rahmaan Janhangeer

🎉 At 85 years of age, Donald Knuth was kind enough to reply to my email.

In April, while writing “Merge Sort and Its Early History” ( ) , I stumbled upon the MOVEIN instruction.

I could not find it in assembly programming references.

Google returned I think few passing mentions.

Even, more weirdly, folks at asm on IRC were unaware and as confused as myself.

This was at a stage where assembly was being defined, forget about that, even the architecture of the computer, the Von Neumann architecture was not popular. Neumann was himself trying out ideas.

So, I asked Donald Knuth, the author of the paper what it meant, here is his answer.

I guess Osmar Mansoor, you were right, it was an interesting piece!

Dear Abdur-Rahmaan,

Please excuse the fact that I haven’t had time to look at your email message from April until today.

In that program, MOVEIN is not an instruction; it is a symbolic name for the address where a dynamically changing instruction will be stored. (His computer was a LOT different from what we have now!)

Line 41 of the program says that we should reserve one “storage tank” (RST 1) to be called MOVEIN. Line 46 puts a PIK instruction into that storage tank. Eventually, control is transferred so that the machine executes that PIK instruction (which moves a block of words into a sequence of short tanks called BUFFER).

Thanks for your interest in history, and for your patience.

Best wishes, Don Knuth