[Verba] Measuring information

[Stuart D. Gathman]

Here is a review of terms for counting bits (binary digits), extended to
large numbers.  This applies to byte addressable computers, which includes
Mac and Intel.

bits	term	abbr
   1 	bit
   4 	nybble
   8	byte
1024	kilobit	 Kb
2**20	megabit	 Mb
2**30	gigabit	 Gb

bytes	term	 abbr
1024	kilobyte KB	(2^10 not 10^3)
2**20	megabyte MB
2**30	gigabyte GB
2**40	terabyte TB
2**50	petabyte PB

Now you have a low level idea of what is meant by "128 MB ram" or
"this disk holds 500 GB" or "my network speed is 100 Mb / sec".

NOTE: marketing brochures and magazine articles often confuse bits and bytes,
so you don't know whether the transfer rate is really Mb/s or MB/s.

A typical computer today has "registers": very fast hardware that holds
and operates on values.  At the machine code level, there will be
instructions to load a value from memory into a register, operate on
registers, and store registers to memory.  For instance, "C = A + B" when
compiled to machine code might look something like this:

	ldint	r1,A
	ldint	r2,B
	add	r1,r2
	stint	r1,C

The size of a register is called the "word size".  The IBM XT and AT had
16 bit registers.  The Intel 386 through Pentium have 32 bit registers.
The Mac has always had 32 bit registers.  The DEC 20 I used in college
had 36 bit registers.

-- 
	      Stuart D. Gathman <stuart at bmsi.com>
Business Management Systems Inc.  Phone: 703 591-0911 Fax: 703 591-6154
"Confutatis maledictis, flamis acribus addictis" - background song for
a Microsoft sponsored "Where do you want to go from here?" commercial.