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* [COFF] [TUHS] for now  to follow up - why a byte is 8-bits
@ 2021-02-03 14:43 clemc
  0 siblings, 0 replies; only message in thread
From: clemc @ 2021-02-03 14:43 UTC (permalink / raw)


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I will ask Warren's indulgence here - as this probably should be continued
in COFF, which I have CC'ed but since was asked in TUHS I will answer

On Wed, Feb 3, 2021 at 6:28 AM Peter Jeremy via TUHS <tuhs at minnie.tuhs.org>
wrote:

> I'm not sure that 16 (or any other 2^n) bits is that obvious up front.
> Does anyone know why the computer industry wound up standardising on
> 8-bit bytes?
>

Well, 'standardizing' is a little strong.  Check out my QUORA answer: How
many bits are there in a byte
<https://www.quora.com/How-many-bits-are-there-in-a-byte/answer/Clem-Cole>
and What is a bit? Why are 8 bits considered as 1 byte? Why not 7 bit or 9
bit?
<https://www.quora.com/What-is-a-bit-Why-are-8-bits-considered-as-1-byte-Why-not-7-bit-or-9-bit/answer/Clem-Cole>
for my details but the 8-bit part of the tail is here (cribbed from those
posts):

The Industry followed IBM with the S/360.The story of why a byte is 8- bits
for the S/360 is one of my favorites since the number of bits in a byte is
defined for each computer architecture. Simply put, Fred Brooks (who lead
the IBM System 360 project) overruled the chief hardware designer, Gene
Amdahl, and told him to make things power of two to make it easier on the
SW writers. Amdahl famously thought it was a waste of hardware, but Brooks
had the final authority.

My friend Russ Robeleon, who was the lead HW guy on the 360/50 and later
the ASP (*a.k.a.* project X) who was in the room as it were, tells his yarn
this way:  You need to remember that the 360 was designed to be IBM's
first *ASCII
machine*, (not EBCDIC as it ended up - a different story)[1] Amdahl was
planning for a word size to be 24-bits and the byte size to be 7-bits for
cost reasons. Fred kept throwing him out of his office and told him not to
come back “until a byte and word are powers of two, as we just don’t know
how to program it otherwise.”

Brooks would eventually relent on the original pointer on the Systems 360
became 24-bits, as long as it was stored in a 32-bit “word”.[2] As a
result, (and to answer your original question) a byte first widely became
8-bit with the IBM’s Systems 360.
It should be noted, that it still took some time before an 8-bit byte
occurred more widely and in almost all systems as we see it today. Many
systems like the DEC PDP-6/10 systems used 5, 7-bit bytes packed into a
36-bit word (with a single bit leftover) for a long time. I believe that
the real widespread use of the 8-bit byte did not really occur until the
rise of the minis such as the PDP-11 and the DG Nova in the late
1960s/early 1970s and eventually the mid-1970s’ microprocessors such as
8080/Z80/6502.

Clem


[1] While IBM did lead the effort to create ASCII, and System 360 actually
supported ASCII in hardware, but because the software was so late, IBM
marketing decided not the switch from BCD and instead used EBCDIC (their
own code). Most IBM software was released using that code for the System
360/370 over the years. It was not until IBM released their Series 1
<https://en.wikipedia.org/wiki/IBM_Series/1>minicomputer in the late 1970s
that IBM finally supported an ASCII-based system as the natural code for
the software, although it had a lot of support for EBCDIC as they were
selling them to interface to their ‘Mainframe’ products.

[2] Gordon Bell would later observe that those two choices (32-bit word and
8-bit byte) were what made the IBM System 360 architecture last in the
market, as neither would have been ‘fixable’ later.
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2021-02-03 14:43 [COFF] [TUHS] for now to follow up - why a byte is 8-bits clemc

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