Fwd: Unix History: Why does hexdump default to word alignment?

[geoffrey mendelson]

On Dec 1, 2011, at 12:00 PM, Elazar Leibovich wrote:

> But what did the word mark mean? In my ignorance I thought that work  
> meant to imply amount of bits.

The IBM 1401 and similar series of computers used DECIMAL not binary  
numbers and the word mark was the extra bit turned on to indicate an  
end of word. Actually the word mark was at the beginning of a word so  
the end was really the word mark of the next word after it.

So if you had the number 123456789 in memory and you wanted to address  
it the one would be at the low memory address with the word mark bit  
turned on, and the nine at the high end. You would point the  
instruction to the high address (that of the nine).


If I remember correctly, instructions addressed the ones digit in a  
number, so you could specify as many digits in a word as you needed.  
This was common in 1950's vintage computers as business computers had  
decimal instructions and scientific ones binary instructions (integer  
with the option of floating point on some computers).

The IBM 360 was the first AFAIK to have both. It used instructions  
with the decimal length in them, so although binary words were 32 bit,  
decimal ones, if you want to call them words at all, were up to 31  
digits plus a sign (1-16 bytes).

Turbo Pascal for the IBM PC had a decimal mode were it would store  
numbers as decimal digits and do decimal arithmetic on them. I never  
used TP, so I don't know much more about it. Any Pascal programmers  
out there? Do Linux Pascal compilers have it?

Geoff.


>

-- 
Geoffrey S. Mendelson,  N3OWJ/4X1GM
My high blood pressure medicine reduces my midichlorian count. :-(