Patrick O'Callaghan wrote:
On Wed, 2009-09-30 at 10:55 -0400, William Case wrote:
I'll take that information to the bank. To state it another way just
to make sure I've got it. A typical physical address goes to, or
points to, 8 + 8 + 8 + 8 + 8 + 8 + 8 + 8 cells arranged side-by-side
in a line on an individual DIMM/DRAM stick.
+8 more if it's parity memory, or more for ECC memory.
I suspect that by thinking of address as divided into bytes rather
than a single 64 bit word (dword, qword, -- pick your author) there is
a natural division for instructions, numbers and characters within the
'word'. Or, is there some physical reason why it is thought of as 8 +
8 ...
It's the data that's stored in units of 8 bits. When addresses are
stored then of course the same applies. When they're on the address
lines of the memory bus, they may be in groups of 16 or 32 or 64
(depends on the bus design). None of this matters to you as a
programmer.
Note that the pedantic name for a group of 8 bits is "octet". A "byte"
is the number of bits required to represent a character in some
encoding. Nearly all modern machines have 8-bit bytes and are
byte-addressed, so we tend to equate "byte" with "8 bits", but I've used
machines where addresses refer to 12-bit words, and the DEC-10 famously
had 36-bit words and a configurable byte size, usually set to 6.
The GE-600/6000 line had 36 bits also, and could use 6 or 9 bit characters. We
used ASCII with the unused high bit as end of string. MULTICS was developed on a
GE-645, leading to UNIX, leading to Linux. Sort of.
Nomenclature: 8 bits is a byte, 4 bits is a nybble, 2 bits is a quarter. That's
probably only funny to old programmers in the USA, sorry.
--
Bill Davidsen <davidsen@xxxxxxx>
"We have more to fear from the bungling of the incompetent than from
the machinations of the wicked." - from Slashdot
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