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The CPU can put a binary
number on the address
bus, to select an
individual register in
the ROM or ram or the
I/O.
The arrows on this bus
show that addresses go
one way only.
Data at the selected
address can be put on
the data bus.
The CPU can also put
data on this bus which
can be written into a
register of ram or i/o.
It is not possible to
write data into ROM
(read only memory).
This is shown by the
single arrow on the ROM
data bus and double
arrows on the other two.
The control bus
instructs the chips to
do various things, such
as when to read or write
etc.
The clock tells all the
chips when to change
what they are doing.
Like the drill sergeant
who shouts "LEFT, RIGHT,
LEFT, RIGHT".
The crystal control the
speed of operation.
In simple systems the
crystal frequency is
1,000,000 Hz.
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