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A bus is a collection of
wires.
Diagram A is a four bit
bus.
Zeros and ones can be
put on the bus, 0 volts
for zero and +5 volts to
represent a one.
The smallest number that
can be put on a four bit
bus is 0000.
The largest is 1111
which is 15 in decimal
and F in hex.
Therefore sixteen
different numbers can be
placed on this bus, 0000
being the lowest and
1111 the highest.
Rather than draw four
wires, we use the
representation shown in
diagram B.
Many micoprocessor
systems use a eight bit
data bus.
The smallest number that
can be placed on it is
00000000.
The largest number is
11111111 which is
equivalent to 255 in
decimal.
Therefore 256 different
numbers can be placed on
this bus.
255 in decimal is FF in
hex.
A sixteen bit bus is
shown in diagram C.
The smallest number we
can put on this bus is
0000000000000000.
The largest number is
1111111111111111 which
is 65535 in decimal.
Therefore 65536
different numbers can be
put on this bus.
65535 in decimal is FFFF
in hex.
All the registers in the
memory chips have their
own individual
addresses, like house
numbers in a street.
By putting its address,
in binary, on an address
bus we can select any
individual register.
Address buses are
commonly 16 bits, so we
can select any one of
65536 registers.
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