How does the computer know which bits/bytes
to set, clear or read, out of the zillions it can store
in its RAM, on physical drives, and so on?
Every single memory flip-flop is connected to the
computer's output line through a circuit network,
and it sets this output to its bit-state when its
read-input bit is activated.
Imagine you had a computer with only 2 bits of RAM.
Then you could select which bit to read by a control bit
that either activates RAM bit A if 0, or RAM bit B if 1,
using a bit-inverter circuit before A:
control bit = 0 →
read-input A = 1 → output line reads bit A
read-input B = 0
control bit = 1 →
read-input A = 0
read-input B = 1 → output line reads bit B
As this works a little bit like postal codes,
one can say that bit A has the address "0"
and bit B has the address "1".
The address control bit tells the computer
which flip-flop to read, set or clear.
With two more control bits and some simple circuitry
one can realize all three flip-flop operations:
read/write control bit = 0
on/off control bit = 0
= "read: off" (does nothing, idle state)
read/write control bit = 0
on/off control bit = 1
address control bit = 0
= "read: on" from cell "0"
read/write control bit = 0
on/off control bit = 1
address control bit = 1
= "read: on" from cell "1"
read/write control bit = 1
on/off control bit = 0
address control bit = 0
= "write: off" to cell "0" (clears the bit)
read/write control bit = 1
on/off control bit = 0
address control bit = 1
= "write: off" to cell "1" (clears the bit)
read/write control bit = 1
on/off control bit = 1
address control bit = 0
= "write: on" to cell "0" (sets the bit)
read/write control bit = 1
on/off control bit = 1
address control bit = 1
= "write: on" to cell "1" (sets the bit)
Now just add a second address bit,
and we can read from, clear and set
four memory cells (flip-flops).
With a 3-bit address, we can access
eight flip-flops, a byte.
In reality, though, computers do not only
have one single data output line and one
single on/off control line, but use the
read/write control bit to read from,
set or clear whole groups of flip-flops
in parallel all at the same time.
The address then does not refer to
individual bits or flip-flops but, depending
on the microcontroller or computer type:
8 bit: one byte at a time
16 bit (2 bytes): one word at a time
32 bit (4 bytes): a double word at a time
64 bit (8 bytes): a quad word at a time
There are always as many write lines
as there are read lines in a computer.
An 8-bit computer, for instance, has an
8-bit write port (bit write on/off lines) and
an 8-bit read port (memory output lines).
An 8-bit computer can use a 3-byte address
(256×256×256 = 16,777,216 = 16×1024^2)
to access up to 16 megabytes of RAM,
a 16-bit computer can use twice as much, 32 MB.
A 4-byte address for a 16-bit PC suffices for 8 GB RAM,
while a 64-bit PC can use it to access up to 32 GB RAM,
to just give a few examples.
Learning Programming with Eas — the Tutorial by Molaskes
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