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Subject: [std-proposals] Restrict possible values of CHAR_BIT
From: Tjernstrom, Staffan (Staffan.Tjernstrom_at_[hidden])
Date: 2019-10-24 15:10:39


Looking at interactions with (and by extension, programming on) FPGAs, where everything is bit-based, it's common to find things like 14-bit values. We have 14-bit types in our internal type system.

Date: Thu, 24 Oct 2019 17:30:00 +0000
From: Lyberta <lyberta_at_[hidden]>
To: Std-Proposals <std-proposals_at_[hidden]>
Subject: [std-proposals] Restrict possible values of CHAR_BIT
Message-ID: <8e78d84f-6f29-c028-009c-f0e5481dbf0c_at_[hidden]>
Content-Type: text/plain; charset="utf-8"

I propose to restrict possible values of CHAR_BIT to be 8 * (2 ^ n) where n is a non-negative integer.

This gives values of 8, 16, 32, 64, 128 and so on.

I have surveyed the most likely hardware architectures that would not fit with this restriction - computers with 24 bit bytes i.e. where CHAR_BIT == 24.

The most extensive information I could find is on Wikipedia:

https://en.wikipedia.org/wiki/24-bit
https://en.wikipedia.org/wiki/Category:24-bit_computers

It seems like the only candidate that is still relevant is Motorola
56000 series:

https://en.wikipedia.org/wiki/Motorola_56000

However, the specification says that it uses 8 bit bytes:

http://cache.freescale.com/files/dsp/doc/inactive/DSP56000UM.pdf

Therefore, I couldn't find implementations where CHAR_BIT == 24 which means other more exotic cases probably never existed.

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