Study Material On Organization of Intel 8085 Microprocessors For Specialist Officer IT Exam 2015
Notes On Organization
of Intel 8085 Microprocessors For Specialist Officer IT Exam By Das
Sir,Kolkata
The microprocessors that
are available today came with a wide variety of capabilities and architectural
features. All of them, regardless of their diversity, are provided with at
least the following functional components, which form the central processing
unit (CPU) of a classical computer.
1.
Register Section : A set of registers for temporary storage
of instructions, data and address of data .
2.
Arithmetic and Logic Unit
: Hardware for
performing primitive arithmetic and logical operations .
3.
Interface Section : Input and output lines through which the
microprocessor communicates with the outside world .
4.
Timing and Control
Section : Hardware for coordinating
and controlling the activities of the various sections within the
microprocessor and other devices connected to the interface section .
Intel Microprocessors:
Intel 4004 is the first
4-bit microprocessor introduced by Intel in 1971. After that Intel introduced
its first 8-bit microprocessor 8088 in 1972.
These microprocessors
could not last long as general-purpose microprocessors due to their design and
performance limitations.
In 1974, Intel
introduced the first general purpose 8-bit microprocessor 8080 and this is the
first step of Intel towards the development of advanced microprocessor.
After 8080, Intel
launched microprocessor 8085 with a few more features added to its
architecture, and it is considered to be the first functionally complete
microprocessor.
The main limitations of
the 8-bit microprocessors were their low speed, low memory capacity, limited
number of general purpose registers and a less powerful instruction set .
To overcome these
limitations Intel moves from 8-bit microprocessor to 16-bit microprocessor.
In the family of 16-bit
microprocessors, Intel's 8086 was the first one introduced in 1978 .
8086 microprocessor has
a much powerful instruction set along with the architectural developments,
which imparted substantial programming flexibility and improvement over the
8-bit microprocessor.
Microprocessor Intel
8085 :
Intel 8085 is the first
popular microprocessor used by many vendors. Due to its simple architecture and
organization, it is easy to understand the working principle of a
microprocessor.
Register in the Intel
8085:
The programmable
registers of 8085 are as follows -
·
One 8-bit accumulator A.
·
Six 8-bit general
purpose register (GPR’s)
B, C, D , E , H and L.
B, C, D , E , H and L.
·
The GPR’s are also
accessible as three 16-bit register pairs BC, DE and HL.
·
There is a 16-bit
program counter(PC), one 16-bit stack pointer(SP) and 8-bit flag register . Out
of 8 bits of the flag register , only 5 bits are in use.
Apart from these
programmable registers , some other registers are also available which are not
accessible to the programmer . These registers include -
- Instruction Register(IR).
- Memory address and data
buffers(MAR & MDR).
·
MAR: Memory Address
Register.
·
MDR: Memory Data
Register.
- Temporary register for ALU use.
ALU of 8085 :
The 8-bit parallel ALU
of 8085 is capable of performing the following operations –
Arithmetic : Addition, Subtraction, Increment, Decrement, Compare.
Logical : AND, OR, EXOR, NOT, SHIFT / ROTATE, CLEAR.
Because of limited chip
area , complex operations like multiplication, division, etc are not available,
in earlier processors like 8085.
The operations performed
on binary 2's complement data.
The five flag bits give
the status of the microprocessor after an ALU operation.
The carry (C) flag bit
indicates whether there is any overflow from the MSB.
The parity (P) flag bit
is set if the parity of the accumulater is even.
The Auxiliary Carry (AC)
flag bit indicates overflow out of bit –3 ( lower nibble) in the same manner,
as the C-flag indicates the overflow out of the bit-7.
The Zero (Z) flag bit is
set if the content of the accumulator after any ALU operations is zero.
The Sign(S) flag bit is
set to the condition of bit-7 of the accumulator as per the sign of the
contents of the accumulator(positive or negative ).
The Interface Section:
Microprocessor chips are equipped with a number
of pins for communication with the outside world. This is known as the system
bus.
The interface lines of the Intel 8085 microprocessor are shown in the Figure 11.3 -
The interface lines of the Intel 8085 microprocessor are shown in the Figure 11.3 -
Address and Data Bus
The AD0 - AD7 lines are
used as lower order 8-bit address bus and data bus , in time division
multiplexed manner .
The A8 - A15 lines are
used for higher order 8 bit of address bus.
There are seven memory
and I/O control lines -
RD : indicates a READ
operation when the signal is LOW .
WR : indicates a WRITE
operation when the signal is LOW .
IO/M : indicates memory
access for LOW and I/O access for HIGH .
ALE : ALE is an address
latch enable signal , this signal is HIGH when address information is present
in AD0-AD7 . The falling edge of ALU can be used to latch the address into an
external buffer to de-multiples the address bus .
READY : READY line is
used for communication with slow memory and I/O devices .
S0 and S1 : The status of the system bus is
difined by the S0 and S1 lines as follows -
S1
|
S0
|
Operation Specified
|
0
|
0
|
Halt
|
0
|
1
|
Memory or I/O WRITE
|
1
|
0
|
Memory or I/O READ
|
1
|
1
|
Instruction Fetch
|
There are ten lines associated
with CPU and bus control-
· TRAP , RST7.5 , RST6.5 , RST5.5 and INTR are the
Interrupt lines.
· INTA: Interrupt acknowledge line.
· RESET IN : This is the reset input signal to the
8085.
· RESET OUT : The 8085 generates the RESET-OUT
signal in response to RESET-IN signal , which can be used as a system reset
signal .
· HOLD : HOLD signal is used for DMA request.
· HLDA : HLDA signal is used for DMA grant .
Clock and Utility Lines :
X1 and X2: X1 and X2 are provided to connect a crystal or a RC network for generating
the clock internal to the chip.
SID: input line for serial data communication.
SOD: output line for serial data communication.
VCC and VSS: Power supply.
SID: input line for serial data communication.
SOD: output line for serial data communication.
VCC and VSS: Power supply.
Instruction and data
formats :
Memory used in the Intel
8085 microprocessor is organized in 8-bit, i.e., It is byte organized.
Every byte has a unique
location in physical memory.
The address bus of 8085
is 16-bit wide , so the location of memory is described by one of a sequence of
16-bit binary address.
The 8085 can address up
to 64k(i.e. 65,535) bytes of memory , which may consist of both RAM and ROM.
Data in 8085 is stored
in the form of 8-bit binary integers-
And 8085 program
instruction may be one , two or three bytes in length.
Multiple byte instructions must be stored in
successive memory locations; the address of the first byes is always used as
the address of the instruction.
Addressing Modes :
The 8085 has four
different modes for addressing data stored in memory or in registers -
Direct: Bytes
2 and 3 of the instruction contains the exact memory address of the data item(
the low-order bits of the address are in byte 2 , the high-order bits in byte 3
).
Register: The instruction specifies
the register or register pair in which the data are located.
Register Indirect: The instruction specifies a register pair which contains the
memory address where the data are located .( the high-order bits of the address
are in the first register of the pair and the low order bits in the second ).
Immediate: The
instruction contains the data itself . This is either and 8-bit quantity or a
16-bit quantity (least significant byte first , most significant byte second ).
Unless directed by an
interrupt or branch instruction the execution of instructions proceeds through
consecutively increasing memory locations.
A branch instruction can
specify the address of the next instruction to be executed in one of two ways -
Direct: The branch instruction contains the address of the next instruction to be executed .
Direct: The branch instruction contains the address of the next instruction to be executed .
Register Indirect : The branch instruction indicates a
register pair which contains the address of the next instruction to be executed
.
Instruction Set :
The complete instruction
set of 8085 can be grouped in five different functional groups -
1.
Data Transfer group: Moves data between registers, or between memory location and
registers, includes moves, loads, stores and exchanges.
2.
Arithmetic group: Adds, subtracts, increments , or decrements data in registers or
memory .
3.
Logic group : AND’s , OR’s , XOR’s , compares , rotates , or complements data in
registers or between memory and a register .
4.
Branch group : Initiates conditional or unconditional jumps , calls , returns ,
and restarts .
5.
Stack , I/O and machine control group : Includes instructions for maintaining the stack
, reading from input ports , writing to output ports , setting and reading
interrupt masks , and setting and clearing flags .
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