microprocessor Lec 01

1. Microprocessor Basics

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Microprocessor & Microcomputer
• The complete processor built on a single
chip is called microprocessor (µP) or
micro processing unit (MPU).
• The computer built from microprocessor is
called microcomputer

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Architecture of Microcomputer
Bus Architecture

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MEMORY (Primary & Secondary)
• Primary storage memory is made of RAM & ROM.
• Secondary storage memory is Hard disk, Floppy disk,
USB, CDs, DVDs etc. These devices store data so they
are memory devices but for the microprocessor all
secondary memory devices are I/O devices. For µP,
memory is only the primary memory.

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Peripherals
• All I/O devices are called peripherals.
• The µP, primary memory & the peripherals
are connected to system bus and all the
data exchange among them takes place
over the system bus.

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Primary Memory
• The primary memory is a collection of lot of
registers. One register of memory can hold any
number of bits, but usually it is designed to hold
1 byte (8 bits).
• The complete chip consists of lot of such
register, each having different address by which
µP calls it (or identifies it).
• Remember, the number of registers in a memory
chip are always in the power of 2. For example u
have heard about kilo byte (KB), Mega Byte
(MB), Giga Byte (GB) etc.

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Primary Memory always in power of
2
1 KB 210
Bytes =1024 Bytes
1 MB 220
Bytes =1024 K Bytes
1GB 230
Bytes =1024 Mbytes
1 TB 240
Bytes =1024 GBytes

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I/O peripherals
• Well known I/O devices are printer, scanner, key
board, mouse, joy stick etc.
• The I/O devices are connected to system bus
through peripheral interface IC called I/O
controllers. (u can see these ICs on the mother
board).
• This IC or I/O controller consists of many
registers & like memory each register inside the
IC has a unique address & µP calls them by
their address to exchange data with them.

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Peripheral controller
Hard disk,
floppy disk,
CD-R, DVD
Modem, LAN Card Monitor, LCD,
Multimedia

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System bus
• Bus is a group of wires that transmit a
binary word. The system bus is
composed of 3 buses.
1. Address bus
2. Data Bus
3. Control Bus

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System bus
1. Address bus:
The address bus is used to carry the address from
micro-processor to peripheral or memory. The
address generated by the µP is in the form of bits &
each bus line carries one bit of address. It is a
unidirectional bus.
Each line is represented by “A”
2. Data Bus:
The data from memory or peripheral to µP & vice
versa is exchanged on data bus. It is a bidirectional
bus.
Each line is represented by “D”

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System Bus
3. Control bus:
This bus is used to carry control signals from µP to
memory or peripherals & vice versa. It is not a
bus in the true sense as it is not a common
transmission medium to carry control signal.
Every device is connected to microprocessor
control pins individually.
Multiplexed Bus: In order to reduce external pins, one pin can
be shared to carry both address & data. But both are not carried at
the same time. This kind of bus is called multiplexed bus,
Each line is represented by “AD”. (address data)

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Address decoding of 4 bit address
This register is
selected with
address of 0 H
and its contents
are CD H

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Memory with on chip address
decoding

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Memory Interfacing

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Address space
• The complete set of addresses which can
be created from fix number of bits. e.g.
with 4 bits only 24
(16) address can be
created. The address range will be
– 0000 = 0H (first address) note: addresses are always given in Hex format
– 0001 =1H (Second address)
– 1111 = FH (last address)

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Address mapping
• On maps of houses, we give particular
location of a room, washrooms & kitchen.
• Like wise allocating a unique address to
any register or device on the address
space is called address mapping.
Memory
address
space

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Memory Matrix

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Inside Microprocessor
• There are also registers inside the µP. Some special
purpose registers & few data registers. The purpose of
data registers is to store data & results. These results
can afterwards be displayed on output devices or stored
in memory for later use.
• As there are very few registers inside processor so
instead of address, they got names like register A, B, C
etc.
• The size of processor register defines how much
maximum data, the processor can process at one time.
For example the size of 8085 µP’s registers is 8 bit, so it
is 8 bit processor & that of 8088/8086 is 16 bits, so it is
16 bit processor.

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Inside Microprocessor

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Performance Comparison of µPs
• Performance of microprocessor is
measured in units of MIPS how many
million of instructions they can execute per
second.

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Primary memory Segmentation
• Imagine u visit a library & there are no sections. All
books are randomly placed. Can u find your required
book easily? NO
• Similarly the program (code) & data must be organized
in different sections of memory to help the processor to
find them easily, because it will reduce the processing
time.

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Types of Segments
4 types of segments exists in memory
1. Code Segment  for storing program
2. Data Segment  for storing Data
3. Stack Segment for storing Temporary Data
4. Extra Segment  for storing extra Data in addition to
data in DS

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Code Segment (CS)
• It is that portion of memory which contains
the program (Code).
• The program is set of instructions. The µP
fetches (picks) one instruction from the
memory, decodes it to find what to do, &
process the data according to the
instruction.
• So for each instruction 3 steps are performed
• Fetch  Decode  Execute

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Program Counter (PC) or
Instruction pointer (IP)
• Pointer means address. IP is a register inside
microprocessor which holds the address of next
instruction to be executed.
• When the processor fetches one instruction
from the memory, it automatically points to the
next instruction in the memory which is to be
executed after the current instruction.
• It is like an office secretary who always points to
his officer what to do next in his schedule. Its job
is to send to the memory the address of the next
instruction to be fetched & executed.

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Stored program
• The instructions are already stored by
programmer in the memory. When the
processor is turned on, it automatically
takes first instruction from the memory
because initially its IP or PC is 0H.
• The instructions are stored in binary
format in memory & one instruction may
take one byte or many bytes of memory.

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Sequential execution
• Computer is a sequential machine i.e. it fetches
instructions from memory and executes them in proper
order. unless there is any instruction or external event
(interrupt) which breaks the sequence.
Main Program
1st
instruction
2nd
instruction
3rd
instruction
4th
instruction
last instruction
CPU
memory
Data bus
Main Program
1st
instruction
2nd
instruction
3rd
instruction
4th
instruction
last instruction

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Instruction Cycle
Instruction
decoder &
control unit
Adder Inverter AND
logic
PC
A
B
C
0H
Add
res
s
Contents
0H 80 H
1H 12 H
2H 40 H
3H 05 H
4H 75 H
AH 06 H
BH 01H
MOV A,3H
ADD A, 5H
HLT
80 H12H
2H
3H
ADD A,5H
4H
Sum=8H
Memory
=16
Bytes
Data
Segment
Code
Segment
4 bit Address
Bus
8 bit data bus
RD control signal
Note: if the instruction is of 2 bytes, it takes 2 Bus
cycles to Fetch the instruction if the data Bus is 8
bit wide.

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Instruction Cycle
• Cycle means duration. The µP takes certain
amount of time to execute the instruction. The
time required varies from one processor to other
& also depends on System Clock frequency f.
• For example, the instruction ADD A, 5H takes 4
T states.
• If the system clock frequency f=2 Hz, the
• T=1/f = ½ =0.5 s
• So ADD A, 5H  4 T = 4* 0.5= 2 s

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Sub routines/ procedure or
functions
These are small programs written separately from the main
program. These program are written separately
because of many reasons like.
1. It makes programming modular, easy to debug, &
allowing many programmers to work on small units of
their interests.
2. The subroutine developed by one programmer can be
used by other programmers in their own programs.
3. The subroutine can be called by main program many
times without writing the sub routine code again &
again so less memory is required.
The program control is passed to subroutine when the main
program calls it, & returned back to main program
once the subroutine is finished.

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Stack Segment (SS)
• It is the portion of the RAM used for temporary storage of Data.
• It is the data structure which works on LIFO (last in first out
principle) like a stack of books. The last data pushed on the stack is
the one which is popped first.
• In Intel µPs, the stack storage starts from the highest address &
continue towards the lower addresses.
• In normal data structures the data is stored starting from the lowest
address (the bottom) and continue towards the higher addresses
even in Intel micro controller’s stack.
Higher address
Lower address
Stack grows

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Need of Stack Segment
• When the main program calls sub routine, the program control
shifts to sub routine & the Program counter PC is loaded with the
address of the first instruction of sub routine. However at the end
of subroutine, the control has to shifted back to the main program.
• So before shifting the control to sub routine, the contents of PC
are stored automatically on stack with the CALL instruction. And
when the sub routine is over, the old contents of PC are
automatically retrieved from the stack when the last instruction of
sub-routine RET is executed & execution starts from the next
instruction of the main program. which has called the sub-routine.

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Need of Stack Segment
• As there are very few registers inside µP, so it may be
possible that the main program & sub-routine both
require them. This is not possible if there was no stack.
• So the programmer ensures that before the sub-routine
uses the µP registers, the data inside these registers
related to main program are stored on stack with the
help of PUSH instructions & then POPPED from the
stack at the end of sub-routine with the help of POP
instruction.
• The number of POPs instruction must be equal to
number of PUSHES.
• POPPING must be in reverse order of PUSHiNG to
restore original contents due to LIFO principle.

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Pushing means adding
Popping means removing

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Stack pointer
• This register stores the address of the Top
Of Stack (TOS), the top most valid filled
location of SS.
• As in Intel µPs, the stack grows from the
higher address towards the lower one, so
when the data is pushed on stack SP
decrements & when it is popped from
stack, SP increments.

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Instruction pipelining
• In old processors, one instruction is fetched, decoded &
executed. After that the same process occurs for the
second instruction & so on.
• Fetching of the next instruction, while the current
instruction executes is called pipelining.
• This saves the time & increase processing speed.

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BCD & Signed Numbers
Unpacked BCD
Packed BCD