Path: utzoo!attcan!uunet!snorkelwacker!apple!fox!portal!cup.portal.com!cliffhanger
From: cliffhanger@cup.portal.com (Cliff C Heyer)
Newsgroups: comp.sys.ibm.pc
Subject: SEGMENT:OFFSET Madness
Message-ID: <25821@cup.portal.com>
Date: 11 Jan 90 16:05:58 GMT
Organization: The Portal System (TM)
Lines: 47

I wish Microsoft and MSDOS book writers would get their 
terms straight. For example, lets take "segment:offset". 
The sensible thing to do would be to use these words 
they way they are usually used in the english language, 
rather than invent "new" meanings. Specifically, segment 
means a 64K "segment" and offset means "offset within a 
64K segment." But I guess this was "too easy" for 
whoever made the word decision. So I'll take this 
opportunity now to set the matter straight.

First of all, "segment" in segment:offset is NOT a 
segment. In fact, the number is meaningless in itself. 
The same goes for "offset". Let me explain.

To get the "real" segment, you must multiply (base 10 
this example) the so-called segment by 16 and add the 
offset. Then you must subtract 65,536 from this total 
repeatedly and stop just before you get a negative 
number. Then number of times you subtract is the "real" 
segment number, and your remainder is the "real" offset 
address.

In Hex, you multiply the so-called segment by 10h and 
add the offset. The first hex number of your result is 
the "real" segment, and the "real" offset is the 
remaining four hex numbers. To be true to standard 
"segment:offset" word use, 39D3:0ECE should be written 
3:ABFE or 3:44030 base 10.

If people are smart enough to invent computers, they 
ought to be smart enough to get word use straight. I 
don't know what to call segment:offset, but certainly 
NOT segment:offset. 

I realize that it is more efficient to convert 
"segment:offset" in it's present form to a 20-bit real 
address than to store the "real" segment number 1-16 in 
the segment register. This is because currently only a 
bit-shift need be done to multiply the segment by 10h. 
With the real segment number in there, you'd have to 
multiply it by 65536 each time to get the segment 
address and consume more CPU cycles. However, this does 
not justify continued use of the same words.

Perhaps an astute observer could offer and explanation 
that would more easily allow conceptualization of what 
the current "segment:offset" really represent.