Architecture and Modes of Operation of 8086 Microprocessor Video Lecture - Computer Science Engineering (CSE)

[Music]
hello and welcome back in our last video
we introduced the basics of what aa
microprocessor is and where it gets used
in this one I'm going to be talking
specifically about the architecture of
the eight zero eight six and its various
modes of operation basically the first
question that one should understand is
how do you define a process of people
talk about 8-bit 16-bit 32-bit
processors what exactly do they mean by
this in this case it really talks about
the internals of the process of how how
many bits does the register of a
processor have how many bits does it
expose to the outside world in terms of
the data bus so on and so forth so in
the eight zero eight six it is a 16-bit
processor because it has 16 data lines
that are exposed to the external world
all internal registers are 16 bits long
- and if you were to talk of a 32 bit
processor you would have the same thing
but 32 bits of internal registers and 32
bit data bus in the case of the 8:08 6
well the data bus is itself 16 bits long
the address is 20 bits wide and
therefore you can actually address up to
one megabyte of RAM or ROM as you will 2
to the power 20 is one megabyte let's
now look at the insides of this
processor there are two major chunks to
this - two groups of functions that this
processor performs one is called the
execution unit and the other one is
called the bus interface unit the
execution unit really executes severy
thing that you will for the processor so
it consists of the arithmetic logic unit
which performs the arithmetic and the
logical operations required of any
program it consists of a set of
registers that are necessary they cut
they call the general-purpose registers
including the accumulator then the
beauty of this particular design 48 0 8
6 is that the registers can be addressed
either
8-bit register or as a 16-bit register
in the case of the 8-bit register you
have a l or a h
that is the accumulator low or the
accumulator high and if you wanted to
address this as a 16-bit register you
would call it a X so on and so forth for
BX CX and DX all of these are general
purpose registers used by the ALU to
perform arithmetic and logical
operations if you now move to the bus
interface unit you first have a set of
segment registers these segment
registers are used to address memory
space either ROM or Ram or the i/o space
by the processor and there is an address
compute engine within this which
converts the logical address that is
held by the segment registers into a
physical address that is exposed to the
outside world interestingly there is a
set of internal memory if you will that
holds what's called the instruction
queue and we will talk about this a
little later this is the insides of the
processor all of this together access
the outside world either through an
address bus the data bus or the i/o
ports let's look at the bus interface
unit a little more detail alright so it
handles all the data and addresses
required by the processor to access the
external world now typically bus
instructions are fetching in fetching
instructions they're about reading or
writing so on and so forth and the
instructions are transferred to what's
called the instruction queue and we
spoke about this six locations of memory
inside the processor which is used for
holding six consecutive instructions why
is this required really the reason is
that the processor the the execution
unit of the processor is much faster
than the peripheral devices like memory
ROM or Ram in the earlier days the
memory was almost as fast as the
processor but from the eight zero eight
six family on
the processor became much faster than
the external peripheral devices and
therefore what the architects of the
processor did was to build a queue
inside the processor which permitted you
to store in advance specific
instructions that would be executed in
the future this whole concept is called
pipelining what you are really doing is
filling a pipe full of instructions that
the execution unit can access on the fly
and as and when instruction is executed
the pipe gets filled by external from
the external memory automatically now
why does this help the process of the
primary reason is the execution unit is
then able to perform at optimum speed
and only when you need to do a jump
instruction of some kind or the other
does the pipe have to be refreshed
completely otherwise it is a sequential
flow of instructions in the main program
let us now focus on the segment
registers the segment registers are each
16 bits long and really performing
specific operations are used for
specific operations and the the segment
registers of different kinds are used to
calculate the addresses within the
memory space of 1 megabyte how is this
done right let's look at this figure
this is the 1 megabyte memory space of
the processor 0 0 0 0 hex 2 F F F F F
hex this is a complete address space let
us assume that the code segment register
the CS points to 1 2 3 4 hex and then
from there because it is a 16-bit
register you can access 64 kilobytes of
memory location but one two three four
hex is actually translated into one two
three four zero hex by the address
compute engine so what really you're
doing is shifting the value in the code
segment register by 8 bits to the left
padding it with a zero to act calculate
the
physical address of the start of the
code segment the instruction pointer in
this example points to five six seven
eight hex is then added to this shifted
code segment value to get the final
physical address location of one seven
nine D eight hex this is the way as two
16-bit registers are used to calculate
one twenty bit physical address and this
physical address is outputted on to a
zero to a nineteen twenty bits long the
segment registers have various users the
code segment the code segment register
for example as the name implies points
to the code within the whole memory
space and you just simply by changing
the value in the code segment register
you could switch across this complete
one megabyte of memory anywhere you
wanted and as long as you really had RAM
or ROM in that space you could have
different kinds of codes being executed
at will
this map shows you the usage of four
different segment registers the data
segment register is at the bottom the
code segment register is at the absolute
top you have the extra segment and the
stack segment registers in between
somewhere you as the designer you as the
programmer are free to map this anywhere
within that one megabyte space the
execution unit consists as I said of a
bunch of general-purpose registers the
ax register is the accumulator and
remember the ax is the sum of a l and a
H each of which is a 8-bit register and
then you have the BX register which is
the memory pointer into the data segment
you have the CX register which is a
counter register and as we go along you
will understand the use of this counter
register you then have the DX register
which is used specifically for divide
and multiplication operations you then
have two special registers called the
index registers which are the source
index register and the destination index
register used specifically for string
operations
on top of this you have something called
the pointer register one is the base
pointer value and this is pointing to
data within the stack segment the other
one is the stack pointer so the stack
pointer points to the top of stack
whereas the base pointer points to a
location within the stack that contains
data that the program wants to use the
last register is called the flag
register and it is used by the processor
to maintain status and to interpret
status of various assembly language
instructions the flag register is bit
addressable so each bit has a meaning
this is kind of in brief the
architecture of the eight zero eight six
let us look at the two modes of
operation that the 8 0 8 6 provides to
you as a designer there is it's called
the minimum mode and the maximum mode in
the minimum mode you cannot add any
other coprocessor to your design it is
called a single processor system whereas
in the maximum mode you are allowed to
add additional coprocessors like for
example the 8 0 8 7 arithmetic
coprocessor the in the minimum mode the
8 0 8 6 is responsible for generating
all the control signals on its own and
in the maximum mode you could use
additional bus controllers to do this
for you in the minimum mode you
primarily use this in very simple
applications whereas in the maximum mode
where you have fairly complex
applications for example a personal
computer that's based on the 8 0 8 6 you
would use the maximum mode how do you
decide whether the processor is acting
in the minimum mode or the maximum mode
this is really defined by pin number 30
on the processor and if you pulled a pin
number 30 high that is 2 5 volts you
enable the minimum mode if you pulled it
to ground that is low you enable the
maximum mode obviously many of the pins
have a common function
across both modes and by studying the
pinout diagrams of the 8:08 six you will
begin to appreciate what those pins are
and where you to use them so this brings
us to the close of this section on the
architecture of the eight zero eight six
and its modes of operation in our next
video we are going to be looking at the
example of how to read bus timing
diagrams what our bus timings how to
read them and how to use them
effectively when you are designing a
system till then I would urge you to
read up a little bit more on the
architecture of the eight zero eight six
try and understand where you would use
the minimum mode and the maximum mode I
have given you a few pointers and of
course do read up on our next video
bus timing diagrams till then thank you
you