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"The DEC PDP-10 computer used 36-bit words. It had special instructions which allowed memory to be treated as a sequence of bytes (bytes could be any size from 1 bit to 36 bits). A one-word sequence descriptor in memory, called a "byte pointer", held the current word address within the sequence, a bit position within a word, and the size of each byte."

(from wikipedia)

Faheem Mitha

9:08 AM

Isn't the PDP-10 obsolete?

Kusalananda

@FaheemMitha Yes, but that doesn't make any PDP-10 that might be operational go away :-)

Faheem Mitha

@Kusalananda Fair enough. Do any currently manufactured computers have bytes which aren't 8, though?

Kusalananda

I doubt it, but I don't know about the mainframes.

Faheem Mitha

Kusalananda

Q: System where 1 byte != 8 bit?

All the time I read sentences like don't rely on 1 byte being 8 bit in size use CHAR_BIT instead 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++ regar...

1 hour later…

Faheem Mitha

10:33 AM

@Kusalananda Interesting. Thanks for the link.

Stéphane Chazelas

10:44 AM

@FaheemMitha Mb is megabit. You want MB for megabyte (1e6 bytes). AFAIK, Spaniards also use Mo.

Faheem Mitha

@StéphaneChazelas Oh, ok. Is Mo == MB in practice?

I've tended to use Mb and MB interchangeably in the past without worrying about it. I guess that was wrong.

Kusalananda

11:02 AM

@FaheemMitha I've also used Mb/Gb and MB/GB interchangeably depending on what looks best in the context...

AFAIK, Mbit is more common for Megabit.

Faheem Mitha

I thought MB and Mb were the same. Maybe not.

I guess MB is safer/better. There's a lot of confusion between bits and bytes, anyway.

Kusalananda

And then you only have the MB/MiB difference to worry about.

And I still have to think way too long to remember which one is which.

Faheem Mitha

Sometimes I think it would be better to write Megabytes. Except that nobody does that.

For similar reasons, I tend to write dates in full. Like 10th May 2017.

Kusalananda

@FaheemMitha I have yet to see anyone write "Mebibytes". All spellcheckers complain about its spelling, and everyone who uses "MB" means "2^20 bytes" except for disk drive manufacturers.

Faheem Mitha

11:19 AM

@Kusalananda "Mebibytes"? What's that?

Kusalananda

Exactly

Mebibyte

A mebibyte is a unit of measure. It represents 1024 * 1024 bytes. A mebibyte (a contraction of mega binary byte) is a unit of information or computer storage, symbol MiB. 1 MiB = 220 bytes = 1,048,576 bytes = 1,024 kibibytes The mebibyte is closely related to the megabyte (MB). This can either be a synonym for mebibyte, or refer to 106 bytes = 1,000,000 bytes, depending on context (see binary prefix for background). The two numbers are relatively close together, but mistaking the two has nevertheless led to problems. The unit MiB was defined by the International Electrotechnical Commission (IEC...

Faheem Mitha

@Kusalananda Huh. I don't think I've seen that before.

I thought a Megabyte was a 1000 Kilobytes. Or a million Bytes.

2^20 bytes would be 1048576 bytes.

This is certainly one area where there is much terminology confusion. Yay.

Kusalananda

Factor in bytes with varying number of bits and you're not even sure what any of this means anymore ;-)

Faheem Mitha

@Kusalananda Let's not factor in bytes with varying numbers of bits.

Kusalananda

@FaheemMitha Deal!

Stéphane Chazelas