System where 1 byte != 8 bit? [duplicate]
All the time I read sentences like
don't rely on 1 byte being 8 bit in size
use
CHAR_BITinstead of 8 as a constant to convert between bits and bytes
et cetera. What real life systems are there today, where this holds true? (I'm not sure if there are differences between C and C++ regarding this, or if it's actually language agnostic. Please retag if neccessary.)
Solution 1:
On older machines, codes smaller than 8 bits were fairly common, but most of those have been dead and gone for years now.
C and C++ have mandated a minimum of 8 bits for char, at least as far back as the C89 standard. [Edit: For example, C90, §5.2.4.2.1 requires CHAR_BIT >= 8 and UCHAR_MAX >= 255. C89 uses a different section number (I believe that would be §2.2.4.2.1) but identical content]. They treat "char" and "byte" as essentially synonymous [Edit: for example, CHAR_BIT is described as: "number of bits for the smallest object that is not a bitfield (byte)".]
There are, however, current machines (mostly DSPs) where the smallest type is larger than 8 bits -- a minimum of 12, 14, or even 16 bits is fairly common. Windows CE does roughly the same: its smallest type (at least with Microsoft's compiler) is 16 bits. They do not, however, treat a char as 16 bits -- instead they take the (non-conforming) approach of simply not supporting a type named char at all.
Solution 2:
TODAY, in the world of C++ on x86 processors, it is pretty safe to rely on one byte being 8 bits. Processors where the word size is not a power of 2 (8, 16, 32, 64) are very uncommon.
IT WAS NOT ALWAYS SO.
The Control Data 6600 (and its brothers) Central Processor used a 60-bit word, and could only address a word at a time. In one sense, a "byte" on a CDC 6600 was 60 bits.
The DEC-10 byte pointer hardware worked with arbitrary-size bytes. The byte pointer included the byte size in bits. I don't remember whether bytes could span word boundaries; I think they couldn't, which meant that you'd have a few waste bits per word if the byte size was not 3, 4, 9, or 18 bits. (The DEC-10 used a 36-bit word.)