DIL
chips supply relative small amounts of memory even though they might sound big
when measured by the total number of bits they provided, For example, a
256K DIL chip is 256K bits - not bytes –
of storage and so you need nine of them make a full 256Kbytes of storage. Why nine? Surely a byte is
only eight bits? Since the first IBM PC, it has been
the rule that ever byte
of memory has an associated error detection of parity
bit. This isn’t used to store data , it is simply stored as a
check that was stored hasn’t changed due to power fluctuations,
cosmic rays or just to detect a bad, chip. So although eight bits make one byte, most
PCs use nine bits to sore a byte with
error checking, see the technical box, Parity. In practice it is
possible for a machine to dispense with
parity checking and so only need
eight bits to store a byte, but this is still relatively uncommon.
So
256Kbits DIL chips store 256K bits and you need nine of theme for 256Kbytes of storage and 1Mbit DIL ships store 1024K bits and you also need
nine of them to form 1 Mbyte of
storage. The only complication is that
you will occasionally come across chips that store 256Kx4 bits. These are simply reorganized 1Mbit chips so that two
chips can be used to provide 256Kx4 bits of storage. This type of chip is
sometimes used an as economical way of
providing that first 1 Mbyte of storage
on 386SX machines. Notice that in this case another type of chip has to be used to supply the
nine parity bit. This explains why some 386SX machines need two 256Kx4bit chips
and one 256Kx1Bit chip for every
256Kbytes of memory fitted.
Installing
DIL chips is usually simply a matter of plugging them into standard DIL
sockets. This is made easier if the legs
of each chips are straightened using
pliers or by pressing against a rigid
surface. If you are at all worried about the mechanical process of
inserting chips then it would be worth
buying a chip insertion tool. The
automatically straightened the legs of
each chip and pushes it into the socket evenly.
You must make sure that you insert each chip the correct way round. One end of a DIL chip is usually
marked by a small semi-circular indentation or a small dot. The should be
lined up with a similar mark on the chip
socket or main board. Some chip sockets
are marked by semi-circular indentation and some by having one corner flattened.
If you insert a DIL chip chance that the
chip will be permanently damaged. For advice about handling chips read the technical box, Static and
handling precautions before even ordering the new chips that you need
As
well as standard DIL sockets you may also come across combination sockets.
These come in two types – dual 18/16-pin
and 20/18-pin. The 18/16-pin sockets can accept either 16-pin
256Kx1bit or 18-pin 1Mx1bit chips.
The 20/18-pin sockets can accept
either 20-pin 256Kx4bit or 18-pin
1Mx1bit chips. You should be very
careful to make sure that you use
the correct section of the socket for each type of chip and this can be
difficult on crowded main board.
The
most common cause of failure in upgrading DIL memory is bent pins. After
inserting each chip you should always
check that all of its pins are
correctly home in the socket. It can be very difficult to see a bent pin
because the bent usually happens at the
point that the leg would otherwise disappear
in to the socket. This makes it look as if the chip is correctly inserted when it isn’t It can also
sometime make an intermittent
contact with the metal of the socket resulting in it working sometimes
but not always. The only sure way of checking for bent pins
is to remove the chip and examine it. A chip extraction tool can help
with this job just as a chip
insertion tool can stop it happening.
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