C Code to calculate PI on my Z80

[Peter Onion]

We've been playing with a bit of code on some of  the museum machines that calculates PI with a very odd algorithm, so I thought I should try it on my Z80.  It only needs integer arithmetic, but it does need 32 bit integers so it's not particularly fast, 3:25 for 400 digits.

PeterO

[S P Dixon]

I love this stuff, do you have details of the algorithm? (I'm just really curious about how it works.)

Shiela

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<Spigot400.png>

[Peter Onion]

Shiela,

We got the original "obfuscated C" code from this paper : https://www.cs.ox.ac.uk/people/jeremy.gibbons/publications/spigot.pdf

We were talking to some visitors to TNMOC about this last Saturday.  It turned out one of them knows the person that wrote that paper !  They commented "That's typical of the things he does!".

I've attached a version for modern C (tested on gcc)  For Hi-Tech C on the Z80 just change uint32_t to long and remove the second #include line.

Have fun,

PeterO

[Martin Giese]

This seems to be the original reference for the Rabinowitz and Wagon “spigot algorithm”:

https://www.jstor.org/stable/2975006

PDF here: http://stanleyrabinowitz.com/bibliography/spigot.pdf

Martin

[Peter Onion]

On Tue, 2025-09-09 at 05:43 -0700, Martin Giese wrote:
> This seems to be the original reference for the Rabinowitz and Wagon “spigot algorithm”:
> https://www.jstor.org/stable/2975006
>
> PDF here: http://stanleyrabinowitz.com/bibliography/spigot.pdf
>
> Martin

Yes, I have that paper in my "Downloads" directory. I also have a version of the PASCAL
code from there translated to ALGOL-60 to run on the ELLIOTT 803, but it is rather slow !

PeterO

[PauldB]

I agree, this stuff is a lot of fun.

Some years back during Covid, Frank P. posted a conversation where he was "experimenting with computing PI on the SC131 using various programming languages and algorithms". There's some interesting stuff there.

Paul

[David Lochlin]

Hi Peter,

 like Sheila, I like these types of programs that calculate important numbers. I copied your code but couldn't get it to compile( I know zero about C) so ChatGPT came to the rescue and now it runs fine on my SC126.

Cheers Dave