Lecture 57: 8085 Microprocessor (Contd.)

Lecture 57: 8085 Microprocessor (Contd.)


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So, conditional call so so so far whatever call and return we have seen; so that where
unconditional. So, the call instruction will definitely take the control to the subroutine
and return instruction will definitely take you back from the subroutine.
Sometimes we need some conditional call and return instructions like the CC instruction.
So, call subroutine if carry flag is set similarly, CNC is call subroutine if carry flag is not
set so like that we can have some conditional calls. Similarly, RC is return from subroutine
if carry flag is set and RNC return from subroutine if carry flag is not set then we have got
this 0 flag so CZ call if 0 flag is set, so like that we can have. So, all these condition
codes that you are that are allowed with this conditional branch instructions. So, they
are all allowed with this conditional call instructions also.
But they are much less practiced compared to this branch instructions, because that
makes the code complex and difficult to follow while reading. So, that is. So, normally what
is done is that we do some jump some conditional branch checking and then ah put the call instruction
on top of that ok. We try to make it as straight line as possible ok. So, that way it has to
be done . Now, how to write a subroutine? Like what
should be the what are the general guidelines for writing a subroutine. So, proper subroutine
is only entered with a call and exited with an ret instruction. So, that is it has got
a single entry point, so do not use a call statement to jump into different points of
the same subroutine should has it is ok. So, what we mean is the guideline is like
this see assembly language programs. So, since that control is entirely in the hand of the
programmer. So, if this is my subroutine and suppose my subroutine starts at location say
say 2000 it starts at location 2000 . Now we should issue a instructions like call 2000
so that is fine some . So, this subroutine suppose it spans from 2000 to 3000. Now it
is highly possible that I put a call instruction for location 2500.
That is possible because 2500 is somewhere in between so the call this particular call
will start from this point onwards. So, processor does not differentiate between whether this
is a subroutine or not, but the processor will not differentiate between whether this
is a subroutine or not; so it will always take you from this address onwards. So, 25
so if you whatever address you mention so it will take you to that address and it will
go from there, but this so it is , but as you can understand that if you have got several
such entry points inside the subroutine then, the branching becomes this understanding the
program becomes difficult. So, this is exactly what is done here the
done by the programmer. So, programmer has to be careful, and has to ah take into consideration
that I should not branch into there the location inside a program. So, do not use a call statement
to jump into different points of the same subroutine. So, that you should not do and
you should similarly you should have a single exit point. So, somewhere here in the program
I put an statement in the subroutine I put a statement like jump to say 1500; that takes
it outside the subroutine. So, we should not do that because then we have got some exit
so which is not really very valid one. So, there should be only one return statement
from any subroutine. So, these are the guides from the software
engineering angles so as per as the processor instruction execution is concerned so these
are the not at all any restriction. So, the programmer is absolutely free to do all these
things , and assembly language programs also they have got the highest level of freedom,
so they can write programs in in any in any style that they feel like ok. So, just similarly
this if push and pop we have seen that there are some rules like push and pop should be
done in the reverse order and all. So, those rules if they are followed properly, so that
will avoid any confusion that can arise due to this push pop instructions so that that
is there . Next will be looking into another very important
concept which is known as Interrupt. So, interrupt is a process where an external device can
get the attention of the microprocessor. So, in our day to day life also interrupt is very
common, like say suppose we are sitting in a room and reading a book and then all of
a sudden the door bell rings. So, we have to go and see like what is attention give
attention to why the door bell has ring somebody has come or not etcetera.
Now, the first thing that we should do is that; we should ah keep a note like a like
the which page of the book I was reading. So, that I can come back and start reading
from that so that is the first point; then even before that maybe we were reading one
line we were in the middle of a line. So, we first finish of that line and then a note
down the page number maybe by means of a bookmark or so, so we keep a note now the page that
we are reading. And then go to attend the the door call and then after doing whatever,
after talking to the person who has arrived and then maybe we again come back and start
reading the book. So, that whole thing is an interrupt process.
So, the similarly the processor when it is in it is normal execution what it is doing?
So, it is just getting the next instruction from memory and executing it. After that it
is again getting the next instruction and executing it. So, there is no way by which
I can tell the processor see something emergency has happened. So, you need to look into this
part and do the do the operation. So, so that until and unless the processor is asking for
some input output type of operation. So, we are not getting any chance to tell the processor
that; something extraordinary has occurred and you have to take care of this you have
to do something for this. So, this is the process of interrupt. So,
this is a interrupt is a process where an external device can get the attention of the
microprocessor. The process starts from the IO device and the process is asynchronous.
So, it starts from the IO device means the IO device will tell the ah microprocessor
that; there is something ah emergency that has occurred and it has to ah it has to process
that operation. And if it is asynchronous because when this interrupt will come; so
there is no such time there is no such fixed timing for that. So, so processor works in
a synchronous fashion so it works at the with the clocks and or the as some clock frequency
and all, but this interrupt can come at any time .
Some of this interrupts are maskable interrupts because you may we can make the processor,
you you can say that ok it will wait for some time and some of them and non maskable interrupt
so they cannot be delay. In terms of our day to day life, so suppose this micro oven whistle
goes; that means, the say the food is ready. So, you know that even if if you keep the
food there for some time, nothing is going to happen ok. So, you can it is it is a maskable
interrupt so we can do it later it can be delayed. And somebody the making a door pressing
the doorbell maybe we need to attend it immediately. So, that has to be done immediately so that
is non maskable. Then interrupts can also be classified into
vectored interrupt and non-vectored interrupts. So, vectored interrupt means the address of
the service routine is hard wired. So, the processor knows the address of the subroutine
to be executed, when this interrupt occurs. And the non-vectored interrupts so address
of the subroutines need to be supplied externally . So, what we mean by this vectoring maybe
we can explain further so it is like this. Ah see ah so also this is the if this is suppose
suppose this is the program that the processor was executing. So, this is the program P that
the processor was executing when the interrupt occurred. So, when the processor was at this
particular line, the interrupt has occurred ok so then interrupt has come .
Now, what the processor will do ? So, processor has to process this interrupt, and processing
this interrupt means again executing some routine. So, what is done; depending upon
this type of interrupt maybe this interrupt is ah a to to that the user has pressed a
key and that has generated an interrupt or user has moved the mouse and that has generated
interrupt. So, what the processor is supposed to do; is to read the character that has pressed
or read the current mouse location coordinate of the current mouse location for some purpose.
So, that so the for those operations that is, reading the key where the content of the
key or reading the mouse position etcetera. So, there is there are routine. So, this may
be the keyboard routine, which will be ah which will be stored which will be reading
the keyboard. This may be the mouse movement detection routine .
So, if this interrupt is a keyboard interrupt so this these routines are already loaded
into the memory. So, one possibility is that the processor already knows that for the keyboard
interrupt so, what is the address; from which the from where the program be executed. So,
when this keyboard interrupt comes, the processor immediately branches to this address so that
is one possibility. And the other possibility is that the processor
does not know ah where to go. So, in that case in that case the the device that has
generated the interrupt should tell the processor that; what is the ah address of the service
routine the corresponding service routine, maybe for the mouse it does not the processor
does not know like where to go, if the mouse interrupt occurs
In that case the processor should ah the, the the the this address of this mouse routine
should be provided by the mouse device itself. So, it should tell the processor ok this is
the address. So, once the mouse device tells the processor that this is the address, then
the processor will go to that particular address. So, this is the difference between the vectored
interrupt and non-vectored interrupt. So, in case of vectored interrupt, the processor
knows the ah service address service routine address where to go, and in case of non-vectored
interrupt so processor does not know the service routine where to go. So, in that in the second
case the the the processor has to get the address from the device.
So, this is the difference between this vectored and non-vectored interrupt. So, in case of
vectored interrupt the address of service routine is hard wired, so it is it is fixed
by the designers, and in case of non-vectored interrupt so this needs to be supplied externally
by the device . So, in terms of an interrupt, so it can be
considered to be an emergency signal and microprocessor should respond to it as soon as possible,
so as soon as possible it should be responded to. When the microprocessor receives the interrupt
signal it suspends the currently executing program jumps to the interrupt service routine
to or it is interrupt service routine so this is also known as ISR in short, to respond
to the incoming interrupt and each interrupt most will most probably have it is own ISR.
So, some of in some cases some of the interrupts we grouped together to have a common ISR,
but in general each interrupt should is of different group as a result it should have
different types, so it should have a different service routine.
So, if you try to look in to the ah spaces, the the steps that it should take for responding
to an interrupt. So, so the steps that are needed for responding to an interrupt. The
first thing that the processor does is that; so when interrupt occurs the first thing that
it should do is to complete the current instruction .
As you know that every instruction, may be spanning over number of clock cycles depending
upon the type of instructions so it may span over a number of clock cycles . So, it should
complete the current instruction. After completing the current instruction, the next thing that
it should do is to save the return address, save the return address onto step .
And then it will branch to the to the interrupt service routine. Now for in case of vectored
interrupt the interrupt service routine address is known so that case; branch to external
this interrupt service routine is easy or this ah it is basically a call basically a
call instruction. So, this program counter values will be saved onto the stack and it
will be going to the interrupt service routine, but in case of non-vectored interrupt, so
this ah ah ISR ISR address is not known. So, in that case it will go into ah into some
other it will be the it will be doing something extra to get the interrupt service address
and from where it will execute after getting that address it will go to the branch branch
to that ISR. Now so, this is the 3 things that the processor does before going to the
ISR. Now if we look into ; how do we respond to an interrupt. So, this maybe ah this maybe
immediate or delayed depending upon whether the interrupt is maskable or non maskable,
and whether interrupts are being masked or not.
Some of the interrupts are maskable interrupts some of them are non maskable interrupt like.
In case of ah processors, we can say the power failure is a non maskable interrupt so, so
that if if there is a powerful then that is very severe thing because entire system is
going to come down. So, that way that is a non maskable interrupt. And some other interrupts
may be maskable so that is the user may think that ok I should not I do this program should
not be disturbed by ah all these interrupts ok. So, that way you can mask out the interrupts
so that; goes interrupts will not come ah will not disturb the program being executed.
So, this interrupt maybe maskable interrupt maybe non maskable. So, if it is a maskable
interrupt then user has a choice to mask out the the interrupt. So, if the interrupts are
masked then ah the they will not be responded immediately till ah the user again unmask
the interrupts. So, there are 2 ways of redirecting the execution are to the ISR, depending on
whether the interrupt is vectored or non-vectored. So, vectored interrupt means the, the microprocessor
already knows the ah ISR address ok, as I already said. And in case of non-vectored
interrupt the device will have to supply the vector to the microprocessors so that is the
non-vectored interrupt. In case of 8085 processor, the maskable interrupt
process in 8085 is controlled by a single flip flop inside the microprocessor, which
is known as the interrupt enable flip flop. So, this is controlled by 2 instructions,
enable interrupt and disable interrupt EI and DI. So, it has got 8085 as a single non
maskable interrupt and this is the so this non maskable interrupt, so this these interrupt
cannot be affected by this EI and DI instructions. So, the they are always enabled so whatever
program is being executed, if you if it ah if it finds this interrupt has occurred non
maskable interrupt has occurred, that will always be serviced by the microprocessor.
So, this is the this it has got this maskable and non maskable interrupts by and, this maskable
interrupts can be masked by DI, instruction they can be unmasked by EI instruction. So,
8085 has got 5 interrupt inputs one of the first one is the INTR. So, INTR is the it
is a non-vectored interrupt . In case of 80850 so there is only one non vectored interrupt,
which is ah INTR. And the INTR is maskable using the EI and DI instruction pair. So,
it can give this so if if the processor thinks or the user thinks that I should not be destroyed
by INTR for executing some part of my code. So, it can put a DI instruction at the beginning
of that code and then EI at the bottom of the code.
So, it is like this; suppose this is a piece of program that I want to execute, and out
of this code so ah in this part so I do not want to get disturbed by the ah maskable interrupts.
So, what you do; you put a DI instruction here and then EI instruction here. So, therefore,
during this part of execution only the non maskable interrupts, so they can the interrupt
the program; maskable interrupts so they will not be able to ah mask the interrupt the system
. So, in case of ah this INTR, so this is a
non-vectored interrupt. So, this read the ISR address is not known to the processor
directly and then this this is also a maskable because it is the by means of this EI DI pair.
There are some more instructions RST 5.5 RST 6.5 and RST 7.5 they are automatically vectored.
So, they are vectored means their ISR addresses are known. So, the it will be the processor
will jump to a particular address. And all these RSTs they are maskable ok so
all of them are maskable. And they they can the trap is the only non maskable interrupt
in 8085 and trap is automatically also automatically vectored ok or trap is basically similar to
RST 4.5. So, to summarize so INTR RST 5.5 6.5 and 7.5 they are maskable interrupts and
trap is a non maskable interrupt. And this then this INTR as per as vectored non vectored
classification is concerned; INTR is a non-vectored interrupt and 5.5 RST 5.5 6.5 7.5 and trap
so they are all vectored interrupts. Now, how this vectoring takes place ok? So,
for the vectored interrupt so, there is something called the interrupt vector table or IVT.
So, any processor that is supporting vectored interrupt should define some part of memory,
which is called the interrupt vector table or IVT. And the specific locations in the
IVT are dedicated to different vectored interrupts, and what the processor is expecting is that;
in that location so you should put that the interrupt service routine should start from
that location. So, this interrupt an interrupt vector is a pointer to where, the ISR is stored
in the memory. All interrupts vectored or otherwise are mapped onto a memory area called
interrupt vector table. This these interrupt vector table is usually located in the memory
range 0 0 that is 0 0 0 0 to FFF that is first 56 bytes
And the purpose of this interrupt vector table is to hold the vectors that; redirect the
microprocessor to the right place when an interrupts arrives. And IVT is divided into
several blocks east block is used by one of the interrupts to hold the vector, hold it
is vector. So, if you have got a n number of interrupts then this range of this 256
bytes. So, it is divided into n such blocks and each block will have the ah the it will
hold the vector for the interrupt, one vector for the interrupt .
So, in case of 8085 processor so, the for the non-vectored interrupt process that mean;
when it is not non vectored, so, how does it operates? So, this is the vectored process
should be enabled using the EI instruction, the EI instruction must be ah executed sometime
earlier by the processor ok, for the interrupts to be ah enabled. And the 8085 checks an interrupt
during the execution of every instruction. So, what happens is that , what happens is
that; so, this 8085 instruction, so that is so it is taking a number of cycles number
of t states. So, if suppose this is these so these are the different t states that the
processor the processor is taking for executing. So, if this is the last t state so in these
t state so it will check whether any interrupt has ah occurred or not. And then if the interrupt
has occurred then after finishing the current instruction, so it will go to that interrupt
service routine ok. So, this is the point so this last, but one clock cycle the last,
but one clock cycle of the total instruction execution ok. So, in every instruction execution
the last, but one clock cycle it will check the interrupts. And if it finds that some
interrupt is has occurred and it needs to be serviced; then it will be branching to
that ISR at routine after finishing the current instruction.
So, if there is an interrupt the microprocessor will complete the ah execute complete executing
the instruction and start a restart sequence. So, restart sequence is the ah the interrupt,
somehow the interrupt services process . One thing you must note that it always completes
the current instructions. So, it is not that it leave leave the first the original instruction
halfway done, so, it is not like that. It finishes the current instruction and then
it attends to go to the ah interrupt service process. It starts the restart sequence of
operations by which it will be doing the interrupt servicing.
What is this restart sequence? It first resets the interrupt flag that interrupt enable flag
IE so this is disabled, so this becomes DI. So, that is, so this and then it will start
the one cycle which is known as an interrupt acknowledge cycle. So, in case of 8085 if
if if you remember that there were 2 pins, one is ah the one pin was the INTR which is
an interrupt line and then there was another pin going out which is INTA bar interrupt
acknowledge bar. So, this actually tells that this is actually used for to that for that
you to tell the device that, the interrupt has been received by the 8085 processor.
So, if this is your ah 8085 processor and this is the device, now this device has given
an interrupt to the 8085. Now the device must be told that ok that that is that it is interrupt
has been accepted ok, then the processor is going to do the ah the associated operation
somehow this has to be told ok. So, that is the role of this INTR and INTA bar lines.
And as you know that since INTR ah the address is the ISR address is not known, then the
device should somehow supply this ah interrupt address the interrupt service address to the
8085 processor. So, we will see how it can be done .
Now, in case of so, upon receiving this INTA signal, the interrupting device is expected
to return the op code of one of the RST instructions, one of the 8 RST instructions.
So, so in general, this RST instructions are like this , So, this ah so this RST n. So,
this n can be a 3-bit value , this n can be a 3-bit value. And what ah the processor does
is that; it multiplies this this ah what the processor does is that ah it is like this,
so this is the 8085 processor and this is the device .
Now, it has given the interrupt and it has put the interrupt acknowledge line . Then
the processor the 8085 expects that the device will put one 8-bit code here. So, this is
the data bus line, this is the data bus line of the processor and it expects that one 8-bit
value will be coming here, which will correspond to one of this RST n instructions ok. So,
upon getting that; it may be that device has got some special register here one 8-bit register
here , which holds the ISR address, which holds the ISR address. And this interrupt
acknowledge signal when it reaches this register. So, it puts the content of those content of
the register on to the data bus. In this way, the device may have some device
address register; and that address register is nothing but the address of the ISR that
it is executing that, that that that should be executed to ah get the interrupt service.
So, this data bus on the data bus so it will expect that one of the RST instruction code
will be put here and this 8085 processor so it will execute the RST instruction then.
So, so that this RST instruction so that we actually transfers the control to some location
this this whatever value be, whatever be the value of this n. So, depending upon that it
will jump to a predefined pre specified location and from that point onward the processor will
start executing. So, it is expected that this interrupt service
routine for this device is located from that particular address. So, this way this vectored
interrupt can be non vectored interrupt. So, this this non vectored interrupt so this ah
so it will be ah it will be doing it it so so, this will be yeah. So, this INTA receiving,
the INTA signal this interrupting device is will put the output of one of the 8 RST instructions
and upon getting this op code, ah this RST instruction. So, it will save the address
of the next instruction in the stack and jump to the appropriate entry in the interrupt
vectored table. And this interrupt vector table entry it will
it may redirect must redirect the microprocessor to the actual service routine, because the
space may be small. So, in the interrupt vector table, so it may hold a branch instruction
to to a latter part in the memory where it has it actually loads the entire service routine.
Then the service routine will be executed and the at the end of the service routine
there should be one return instruction, that will return the execution to the original
program at where the interrupt was received.

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