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How was early randomness generated?

Many programs make use of randomness, from BASIC guess-the-number games to encryption key generators. This randomness could have been generated in many, many different ways: hardware, software, software seeded by hardware...

What techniques were used to generate randomness in early computers, and were cryptographically secure sources of randomness commonly used?

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Don't forget lookup tables. Those were used long before "true" pseudo-random number generators, and usually worked well enough. The simplest is just copying, say, 256 numbers from your random number codebook (a physical book, yes) to your program, and indexing into this memory with a successively incremented index.

Now, this is obviously nothing even close to randomness required in cryptography, or anything where correctness depends on good randomness (e.g. genetic algorithms, Monte Carlo approximation, ...). But it's good enough for games. As a reasonably recent example, [the original Doom used a "random" number generator exactly like this][1] - 256 pre-defined numbers taken in a loop. This had some visible side-effects (the BFG weapon could "theoretically" inflict much more (or less) damage than it could in practice, thanks to the random generator not being random enough). This was a very common approach in games, since it's extremely cheap (just a single shared "pseudoRandomIndex++" and a memory read from something that's always in CPU cache), and true randomness is seldom required - the main sources of entropy in a typical game are from mostly random ordering of events. BFG in Doom became so heavily biased mainly because it took a lot of those pseudo-random numbers in a sequence.


  [1]: https://github.com/id-Software/DOOM/blob/master/linuxdoom-1.10/m_random.c

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Often the appearance of a lookup table was caused by a simple, cyclic algorithm; a lookup table would take up much too much space!

wizzwizz4
– wizzwizz4 ♦

2017-02-06 17:46:54 +00:00
Commented Feb 6, 2017 at 17:46
@wizzwizz4 No, 256-byte lookup tables were quite common. Any decent PRNG takes more (about 2-3 kiB for MT). You're not suggesting you'd waste precious CPU cycles on something as insignificant as the illusion of randomness, are you? :D Both approaches were used, each in their own niche. Mind you, Doom was already written for computers that had a couple megabytes of memory - 256 bytes were quite a good trade-off for the cheap pseudo-randomness. But lookup tables were used even on old mainframes, if you didn't need something too special.

Luaan
– Luaan

2017-02-06 22:42:09 +00:00
Commented Feb 6, 2017 at 22:42
What retro computing platform were you using that had a CPU cache?

Mark Ransom
– Mark Ransom

2017-02-09 23:20:41 +00:00
Commented Feb 9, 2017 at 23:20
@MarkRansom Anything that had the CPU faster than memory. And of course, where the memory was faster than the CPU, lookup tables were just as effective. IBM 360, anything based on 68k and of course 386+ (Doom is the example I used). I think we've had plenty of discussions on RC about what exactly "retro computing" means :)

Luaan
– Luaan

2017-02-10 08:13:19 +00:00
Commented Feb 10, 2017 at 8:13

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