Men in black always want popcount (it's been added to various architectures over the years at their request) so this makes sense given his background.

IIRC it also introduced index based addressing. Before that, people just wrote self modifying code to index arrays...

As bit-serial machines, popcnt etc would have been a bit easier than parallel machines. (Heck, I'm glad the world moved on from using teleprinter code for numbers and instructions; hideous isn't it?!)

to save you time, this is a summary of the links various people here are posting:

popcnt of a word counts the number of bits that are "1"

https://www.talkchess.com/forum3/viewtopic.php?t=38521

the NSA wanted to do cryptanalysis on intercepted messages, and since the CDC 6000 had 60-bit words, one word was enough to store most alphabets they were interested in. They were able to:

    Split a message into lines
    Set a bit for each unique character they encountered per line
    Use popcount to count the distinct characters
    Use the count as a hash for further cryptanalysis

(of course for all we know this could be NSA disinformation ;)

That's the answer to the NSA question. There are other uses described here https://vaibhavsagar.com/blog/2019/09/08/popcount/ including for Hamming codes and Neural nets and more.

Semi-related: one thing I liked about the video game Turing Complete [1] is that, once you've built the computer and implemented its (binary) machine code ... it doesn't give you an assembly language, or suggest one!

It has you start programming in binary, and then just tells you how to make your own mnemonics to substitute for having to write out each 8-bit input. So you end up picking them based on what commands appear most often in practice.

One instruction I ended up creating was JNEQCI, "jump if not equal, comparing to immediate". (jump if not equal where the first operand is treated as a register and the second as a constant/immediate value)

[1] from LevelHead, and similar to nand2tetris in that you build a computer from its circuits and then program it

Personally, I think the modern typesetting discards a lot of the essential value of the original. My interest in this manual stems not from any desire to program a Mark I, but because I want to connect with the past and understand what programmers of that era had to deal with. A big part of that was documentation that was not pristine, that was full of hand-written notes and symbols that they had to somehow make sense of nonetheless. It's like the difference between a real vintage airplane and a modern replica. The replica looks superficially the same, but when you look at the details it feels, well, modern. There's something radically different about flying a plane with period [1] vs modern [2] avionics.

[1] https://media.defense.gov/2015/Aug/03/2001266310/780/780/0/1...

[2] https://www.scalewings.com/sw-51-mustang/

Thanks.

> in the order they'd be input into the machine

That is the usual why for little endian. Same reason why little endian byte ordering is used on the 8008 and successors. The processor needs the lower byte first, to calculate the carry over to the high byte. Bit serial machines (like the Mark 1 and most other early computers) operate in a similar manner, just one bit at a time, from least to most significant digit.

I've read that since Arabic goes right-to-left, decimal numbers are little-endian there.

That is indeed little-endian. And cursed.

Some of the control codes we use in ttys today came from teletype machines that were already around for five to ten years in 1950. Also when the 132 character wide terminal standard was born. Thats’s another valid interpretation of desktop.

My parents wrote their graduate theses this way too, in the 1950s, except that the spaces were for chemical formulas.

It says very clearly on the title page it's for the Mark II, and in the text he says the advice is based on experience with the Mark I, so it's definitely for the Mark II.

Good catch. It's for the Mark II and seems to be written based primarily on his experiences with the Mark I. I do not know if you are tripping.