Control Unit Operation Video Lecture - Computer Architecture & Organisation

in this video we are going to explain
what is the role of the control unit
let us first say that this control unit
is one part of the CPU and is sitting
next to the data path now the main
purpose of the control unit as the name
suggests is to make sure that the rest
of the chip or the rest of the CPU is
performing the right applications or the
right operations
so let's first consider what was the
purpose of the data path now this
circuit over here was in charge of
processing data and performing certain
operations but it also had certain
control signals that needed to be
provided like for example the address of
the register to be read sometimes if the
bank of registers allows we will provide
two of these addresses another one was
the operation in the ALU that needs to
be executed another control signal will
be how to select operand one of the ALU
or select operand 2 and we can have many
more signals that change the behavior of
the rest of the CPU namely the data path
so this is precisely the responsibility
of the control unit it needs to create
all these units while these signals that
modify the behavior of these data paths
to perform the right set of operations
now how does the control you need to
know what kind of operation needs to be
executed well it has one very important
register
typically known as AI R or IR typically
stands for instruction register
now this instruction register what it
contains is the instruction that is
being executed by the CPU it is very
important because this instruction will
have certain encoding in binary of
course one zero ones and zeros and now
from the content of these ones and zeros
over here what this control unit needs
to do is one of the most important steps
which is decoding this instruction this
instruction need to be the code and from
this decoding it uses the steps to carry
the operation so this control unit can
be seen as a little bit like the
conductor of an orchestra there are a
lot of circuits over here that can carry
multiple operations but they need to be
control or they need to be told what to
do the control unit is in charge of
looking at this instruction over here
and decoding the message that is
containing these zeros and ones and as a
consequence providing the right signals
over time this is not a one single time
instant but across several time units to
provide all the control signals so that
this circuit over here execute the
operation encoding in this register now
another detail detail that need to needs
to be explained is that where is this
instruction axle coming from this
instruction is actually part of a
program and this program is stored in
memory or in RAM
which means that this control unit prior
to starting the coding design
instruction over here what it needs to
do is get the content from outside the
CPU and go to memory and read certain
position and store its content here so
even before providing all these control
units for the data path to perform the
write operation first before that the
control unit needs to provide control
signals so that the memory content is
obtained from the right location in
place on the instruction register now
the next question is how does this unit
know what location needs to be loaded
with the following instruction and this
brings us to the other very important
circuit or register within the control
unit which is typically called as the PC
the PC stands for
for encounter it is very important he
has a sets of zeros and ones over here
as well and the value of this register
is precisely the memory location into
which the control unit needs to perform
a read operations such that the
following instruction to be executed get
loaded in instruction register
so as we can see for an operation such
as for example adding these two
registers on the data path the steps
that the control unit would follow would
be this one's first read the instruction
from memory in location given by the
power encounter the PC once this
instruction has been obtained from the
position stated by the PC it is a store
in the I our next step is to decode this
instruction in other words we need to
know exactly what kind of zeros and ones
are here in their meaning in order to
interpret the next operation to execute
now let's assume for this example that
this combination of zeros and ones are
encoding an operation such that two
values in two registers in the data path
need to be added now the following steps
that then the control unit will do is
provide all the control signals at the
right time synchronize with the clock
such that we read the operands from the
register Bank
then we need to also select the right
operation in the ear al you store the
result once the result has been stored
at the output of the ALU it needs to be
written in another register and finally
the control unit needs to prepare the
execution of the following instruction
and the following instruction is in
allocation which is next to the one that
occupied instruction just executed so in
order to do that it needs to increase or
update the value of the program counter
so as we can see out of all these steps
over here these ones are specific for an
instruction that is performing an
addition
now once instruction has been finished
we increase the value of the program
counter and these instructions are
repeated or these steps are repeated by
the control unit so the control unit is
constantly performing this loop with the
difference that depending on the
instruction that is the code here
instead of these steps over here we
could be executing some other set of
steps for another instruction i2 or
another set of steps which could be
shorter for instruction i3 the important
conclusion to understand exactly what is
the role of the control unit is that it
constantly executes these steps in a
loop in an iterative manner and it needs
always to rely on the program counter to
go to memory and get the next
instruction put in an instruction
register decode and find out what it
means and then provide the right signal
so that the data path performs the right
operation
let's give another example let's suppose
this instruction i2 here it's an
instruction to store the value of
register r1 in memory but in position 0
X 3 5 8 a for example so in this case
the control unit would go to memory will
obtain the instruction bring it to the
instruction register this instruction is
being obtained from the location pointed
to by the program counter we decode the
instruction as we did here and now
instead of following these steps which
were part of the addition instruction
then we follow in this other step which
needs to read the data from register 1
r1 as we say in the operation then we
also need to read the address from the
instruction and by others we mean this
value 0 x3 5 8a and finally write data
in memory now these three steps over
here would be exactly the analogous of
these four steps we described previously
previously for these operations but they
would be executed by the control unit
when the instruction to a store r1 in
memory is received
once this instruction is executed these
loop continuously increase and update
the PC and the control unit goes again
to read instruction from memory in the
location pointed to by the PC and this
loop repeats itself forever