MIPS Tutorial 35 - Bit Manipulation Video Lecture | MIPS Assembly Programming Simplified - Electronics and Communication Engineering (ECE)

hello everybody I'm back again I'm glad
to be back and today I have a new topic
to show you and I think it's going to be
very useful to you and I'm going to
explain this topic to you by presenting
a problem and it is write a function
that clears beat zero so the function is
called clear bit zero and it takes its
parameter value in a 1f in register a1
and it clears the first bit or bit zero
in that value and then it's going to
return the new value or the result in V
zero so for example let's say that you
give it a number 10 11 in decimal is 11
in binary 10 11 so after you clear the
first base or bit zero this one is the
first bit from the right so it's going
to become a 0 so you 1011 becomes 10 10
so as you can see here binary
representation 10 11 is 11 in decimal
becomes 10 10 which is 10 in decimal so
how can we do this how can we solve this
problem in order to clear the first bit
we need to use another number called a
mask and the mask is just a number that
we make it's gonna allow us to clear the
first bit and that number we is going to
be this one right here umm why well
because we're going to use be wise and
operation we're going to use be wise end
so let's say that we take let's go down
here this is 11 in decimal but let's say
that we take this 11 and we have the
mask and um when we do one
be west and 0 we get 0 so this is going
to be cleared but the other numbers here
they will remain the same because the
mask has all ones here so 1 B 1 and 0 is
0 so you're going to get 0 & 1 0 0 1 0 0
1 0 all the way to here and then 1 + 1
is 1 0 1 is 0 1 long is 1 well the first
B you have 1 and 0 so this is 0 so the
first bit has been cleared so you can
see that 1011 when you do the be wise
end with a mask it's going to give you
10 10 and that's going to do the trick
so now that you know that we need a mask
how can we make a mask so let me just
move this here and move this here so you
can focus more on the idea to make the
masks we can take negative 1 because the
binary representation of negative 1 is
all ones and after that we want to be we
want the first bead to be a 0 so we can
simply shift this to the left by 1 and
this is going to give us the first day
to be a 0 ok so this is how we make the
mask and in assembly in MIPS assembly we
haven't we have an instruction called
cheeseless logical I already talked
about this instruction in one of my
previous tutorials but in that tutorial
I showed you that she's less logical
allows you to multiply but in this case
we're gonna use it for multiplication
we're going to use it for bit
manipulation so how do I know that the
beat the binary representation of
negative 1 is all ones well as you
probably know in computers
many computers negative numbers are
represented in two's complement so to
complement means that if you have a
positive number in order to get a
negative version you just have to flip
all the bits and not one and that's
going to give you a negative version so
for example these right here in decimal
this is number one so if I want to get
negative one I flip all the bits so all
the 0 becomes 1 and other ones to come 0
and this is one compliment but I have to
take two's complement so the next step
is just to add one to it so I'm going to
add one to it and what I get is all ones
so that's why negative one is all ones
in binary hopefully this is not too
confusing you know at the main idea is
that we want to make a function that you
give it a number and the function is
going to clear the first bit in the
number so 1011 becomes 1010 because we
clear the first bit and what we do is
that we make a mask and we use a be wise
and operation and you saw that if we
create if we could make a mask that has
all one's but the first values of zero
when we do be wise end all these values
in the in the original number will not
be affected they will remain the same
you can see that all these values remain
the same in the result but the first
bill if it is a one is going to become a
zero so it's going to be cleared if it
is a zero is going to remain a zero okay
so that's the main idea we're going to
use the sheath-knife logical operation
and it's going to allow us to our make
our mask after we take negative 1 and we
shift by to the left by one and
now I'm going to open Mars and what I'm
going to do is I'm going to jump into
the code to show you how all these
simple methods in assembly so I'm going
to make a new file I'm gonna save it to
my desktop and I'm going to call it big
clearing that ASM you can call it
whatever you want but I'm gonna say and
I have my data section as usual in the
data section I'm going to have a new
line current a new line string so when I
have this because I want to display my
number a nicely I want to be able to see
the number nicely on the screen so this
is this is for good looks then we're
going to have my text section my main
procedure and remember that it's always
good practice to tell the operating
system that this is the end of the
program so loading media delta v is 0
comma 10 cisco so this is just tang is
the code number to end the program and
the cisco is to indicate this cisco to
the operating system so initially I'm
going to do loaded media that assign a 1
comma 11 so I'm putting number 11 into a
1 and then I want to show that number 2
screen so I'm going to say jumping link
show number and show number is a
function that I haven't made yet but I
will make it down here later but for now
I just want you to see the main phone
so I'm going to finish the main function
and then I'm going to work on the on the
other functions so display level to the
screen then I'm going to pass 11 play 1
again it's ready there a one has 11 but
just to make it clear I want to pass 11
to a walk again put it there and I want
to jump the link or call the functions
clear b0 which I haven't made yet but I
will make it so this function here takes
11 as an argument in a 1 and it's gonna
clear the first bit so this is going to
become 10 because 10 11 becomes 10 10
and 10 10 is 10 in decimal then
zentangle binary is Tang in decimal and
it's going to return that 10 then you
value in v-0 so I'm going to move down
to dollar a1 the value returned by clear
bit zero the values is zero and then
when I jump a link call the function
show number so this is going to show the
number that has to they be zero cleared
so this is going to be 10 this is gonna
show 10 okay so now I'm going to come
down here I'm going to write my function
show number remember that this function
expects the argument for the number to
be displayed in dollar a1
so loaded media members will come before
this is the system call to display text
and in this case I want to display the
new line so I put a new line to the
address here load address to a zero and
then Cisco and then I want to display
the number loading media another semi
circle no one move to a zero the value
in a one so I'm going to pass the
argument to a zero and I'm going to
display that number and finally jump
registers Alice IRA to go back to the
Colin function so this function is just
going to display a new line right here
then it's going to display the number
that you passed in a one right here and
then it's just going to go back to the
to the calling function so it's going to
jump to the next instruction here's how
you call it here it's going to come back
here that's this right here now we're
going to go to the main part of this
tutorial and this is clear vis 0 this is
the function clear bit 0 first of all
I'm going to use the register s0 for my
mask so mask is dollar s 0 but because
I'm going to use this register here by
convention I have to save it to a stack
because it is Kali saved so I'm going to
say add immediate loss on SP the loss on
SP coma negative for allocating space in
the stack for one value because it is 4
bytes I say 4 and I say negative because
the stack grows downward emits so slow
work dollar s 0
$0 SP okay
and now we're going to go to the main
part here first we want to make a mask
so make the mask initially I'm going to
say load admitted and assign s 0 comma
negative 1 so now I have negative 1
again 0 remember that negative 1 is all
ones so negative 1 in decimal is all
one's in binary but now I want to shift
it to the left because I want the first
value to be a zero so I'm going to use
my instruction shift left logical so I'm
going to say she's less logical dollar s
zero dollar s 0 comma 1
so now dollar s 0 gets the value of s 0
shifted by 1 to the left so now F 0 has
this all one's except the first one
which is a 0 and this is my mask and now
we come down here we have the original
number we have the mask and then we just
have to use be wise end to clear the
first bit so we can say end dollar give
0 I'm putting this in this 0 because
this is the return value I'm going to
return the register with $0 a $1 s 0 so
the value in the argument which is 10 11
with the mask that I made which is all
ones except for the first page which is
a 0 and B wise end what I get is the
number the original number already
passed in but with the first base equal
to 0 I and I'm gonna get it here so I
can just retrieve it from Maine later
and that's that now I just need to
restore the value of s 0 that is
before this is just convention remember
that because this I'm using an S
register you need to restore the value
and then we stole a stack so there speak
over four and then just jump register
dollar RA and now this should be good
we're going to test it and we're going
to say wrong assemble execute and as you
can see we have eleven first but then
after we clear the first bit we get ten
so it's working properly so a quick
recap show xi ER screen now clear the
first building 11 that's going to give
you 10 and I now display that 10 to the
screen and end the program show number
show newline and display the number that
you passed in a 1 and go back and for
beat 0 clear bit 0 just because we're
using a stroke just store that value in
the stack and now make a mask negative 1
so one's shifted by 1 to the left and
then use be wise end to clear the first
bit and then restore the value and go
back so I hope this was useful it's been
a long time I was very busy and you're
still very busy but I wanted to make a
tutorial um because it's being a while
and next time hopefully I will make more
tutorials about bit manipulation
algorithms sorting algorithms thing
assembly and maybe dynamic memory
allocation and thank you for watching
these tutorials and I hope that they
were useful
and see you next time