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UNIT 2
Architecture of 8085 Intel microprocessor, Flag
Register ,Addressing mode, pins diagram of 8085,
Demultiplexing of Address & Data Bus, Generation of
various control signals for I/O & Memory
Organization
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Architecture of 8085 Intel microprocessor

8085 Microprocessor – Functional Units
• Accumulator: It is an 8-bit register used to perform
arithmetic, logical, I/O & LOAD/STORE operations. It is
connected to internal data bus & ALU.
• Arithmetic and logic unit: As the name suggests, it
performs arithmetic and logical operations like
Addition, Subtraction, AND, OR, etc. on 8-bit data.
• General purpose register: There are 6 general purpose
registers in 8085 processor, i.e. B, C, D, E, H & L. Each
register can hold 8-bit data.These registers can work in
pair to hold 16-bit data and their pairing combination is
like B-C, D-E & H-L.

• Program counter: It is a 16-bit register used to
store the memory address location of the next
instruction to be executed. Microprocessor
increments the program whenever an instruction
is being executed, so that the program counter
points to the memory address of the next
instruction that is going to be executed.
• Stack pointer: It is also a 16-bit register works like
stack, which is always incremented/decremented
by 2 during push & pop operations.
• Temporary register: It is an 8-bit register, which
holds the temporary data of arithmetic and
logical operations.

• Flag register: It is an 8-bit register having five
1-bit flip-flops, which holds either 0 or 1
depending upon the result stored in the
accumulator.
These are the set of 5 flip- ops
• Sign (S)
• Zero (Z)
• Auxiliary Carry (AC)
• Parity (P)
• Carry (C)

• Instruction register and decoder: It is an 8-bit register.
When an instruction is fetched from memory then it is
stored in the Instruction register. Instruction decoder
decodes the information present in the Instruction
register.
• Timing and control unit: It provides timing and control
signal to the microprocessor to perform operations.
Following are the timing and control signals, which
control external and internal circuits
-Control Signals: READY, RD’, WR’, ALE
-Status Signals: S0, S1, IO/M’
-DMA Signals: HOLD, HLDA
-RESET Signals: RESET IN, RESET OUT

• Interrupt control: As the name suggests it
controls the interrupts during a process. When a
microprocessor is executing a main program and
whenever an interrupt occurs, the
microprocessor shifts the control from the main
program to process the incoming request. After
the request is completed, the control goes back
to the main program. There are 5 interrupt
signals in 8085 microprocessor: INTR, RST 7.5,
RST 6.5, RST 5.5, TRAP.
• Serial Input/output control: It controls the serial
data communication by using these two
instructions: SID (Serial input data) and SOD
(Serial output data).

• Address buffer and address-data buffer: The
content stored in the stack pointer and program
counter is loaded into the address buffer and
address-data buffer to communicate with the
CPU. The memory and I/O chips are connected to
these buses; the CPU can exchange the desired
data with the memory and I/O chips.
• Address bus and data bus: Data bus carries the
data to be stored. It is bidirectional, whereas
address bus carries the location to where it
should be stored and it is unidirectional. It is used
to transfer the data & Address I/O devices.

Flag Register
• Out of 8 F/F’s of flag register, 5 are used as
flags to store status of result
• The five status flags are CY,ZF, SF,AC & PF
• The flags are affected by the arithmetic and
logical operations
• There position in the flag register is as follows

• CY (Carry flag) : If an arithmetic operation results in a carry,
then CY=1
• AC (Auxiliary carry) : In an arithmetic operation when carry
is generated after D3 and passed to D4, the AC is set.
• P (Parity flag) : After an arithmetic or logical operation, if the
result has an even no. of 1’s then parity flag is set.
• Z (Zero flag) : Zero flag is set when result of operation is
zero (00H).
• S (Sign flag) :It is a copy of 7 bit of result. If S=1,
then negative result. If S=0, then result is positive.

Status flags
affected
e.g. 89H + 88H
1 0 0 0 1 0 0 1
+ 1 0 0 0 1 0 0 0
• 1 0 0 0 1 0 0 0 1
• CY = 1,P = 1, AC = 1, Z = 0 , S = 0
• Flag register contents :
S Z X AC X P X CY
0 0 X 1 X 1 X 1

Addressing Modes of 8085
• The method by which address of source of data & address of
destination of result(i.e. address of operand) is given in the
instruction is called as addressing mode. There are 5 types of
addressing mode in up 8085
1. Immediate Addressing Mode(IAM)
2. Register Direct or Register Addressing Mode(RDAM)
3. Direct Addressing Mode(DAM)
4. Register Indirect or Indirect Addressing Mode(RIAM)
5. Implicit or Inherent Addressing Mode(IPAM)

Immediate Addressing Mode(IAM)
If 8 or 16 bit data required to execute any instruction
is given directly along with the instruction as
operand then such instructions are called immediate
addressing mode instructions. The last alphabet in
most of immediate addressing mode instruction
mnemonic is I.
Ex: MVI A 75H

Register Direct or Register Addressing
Mode(RDAM)
is present in 8/16 bit register or register pair and the
name of this register or register pair containing data
is given along with the instruction as operand. Such
instructions are called Register Direct or Register
Addressing Mode instructions.
Ex. MOV A,B

Direct Addressing Mode(DAM)
is present in memory location or IO port and 16 or 8
bit address of this memory location or IO port is
given along with the instruction as operand then
such instructions are called Direct Addressing Mode
instructions.
Ex. LDA 9000H

Register Indirect or Indirect Addressing
Mode(RIAM)
is present in memory location. The 16 bit address of
this memory location is present in 16 bit register or
register pair and the name of this register or register
pair containing memory address is given along with
the instruction as operand then such instructions are
called Indirect Addressing Mode instructions.
Ex. LDAX B

Implicit or Inherent Addressing Mode(IPAM)
If address of source of data as well as address of
destination of result both are fixed and it is
accumulator then there is no need to give any
operand along with the instruction, such instructions
are called Implicit addressing mode instructions
Ex. CMA
DAA

Pin Diagram of 8085

The pins of a 8085 microprocessor can be
classified into seven groups
1. Address bus: A15-A8, it carries the most
significant 8-bits of memory/IO address.
2. Data bus: AD7-AD0, it carries the least
significant 8-bit address and data bus.
3. Control and status signals: These signals are
used to identify the nature of operation. There
are 3 control signal and 3 status signals.

Three control signals are RD, WR & ALE
a. RD This signal indicates that the selected IO or memory device
is to be read and is ready for accepting data available on the data
bus.
b. WR This signal indicates that the data on the data bus is to be
written into a selected memory or IO location.
c. ALE It is a positive going pulse generated when a new
operation is started by the microprocessor. When the pulse goes
high, it indicates address. When the pulse goes down it indicates
data.
Three status signals are IO/M, S0 & S1
• IO/M - This signal is used to differentiate between IO and Memory
operations, i.e. when it is high indicates IO operation and when it is
low then it indicates memory operation.
• S1 & S0- These signals are used to identify the type of current
operation.

4. Power supply: There are 2 power supply signals
VCC & VSS. VCC indicates +5v power supply and
VSS indicates ground signal.
5. Clock signals: There are 3 clock signals, i.e. X1,
X2, CLK OUT.
X1, X2 A crystal is connected at these two pins
and is used to set frequency of the internal clock
generator.
CLK OUT This signal is used as the system clock
for devices connected with the microprocessor.

6. Interrupts & externally initiated signals
Interrupts are the signals generated by
external devices to request the
microprocessor to perform a task. There are 5
interrupt signals, i.e. TRAP, RST 7.5, RST 6.5,
RST 5.5, and INTR. We will discuss interrupts
in detail in interrupts section.

• INTA It is an interrupt acknowledgment signal.
• RESET IN This signal is used to reset the
microprocessor by setting the program counter to zero.
• RESET OUT This signal is used to reset all the
connected devices when the microprocessor is reset.
• READY This signal indicates that the device is ready
to send or receive data. If READY is low, then the CPU
has to wait for READY to go high.
• HOLD This signal indicates that another master is
requesting the use of the address and data buses.
• HLDA (HOLD Acknowledge) It indicates that the CPU
has received the HOLD request and it will relinquish
the bus in the next clock cycle. HLDA is set to low after
the HOLD signal is removed.

• Serial I/O signals: There are 2 serial signals,
i.e. SID and SOD and these signals are used for
serial communication.
• SOD (Serial output data line) The output
SOD is set/reset as specified by the SIM
instruction.
• SID (Serial input data line) The data on this
line is loaded into accumulator whenever a
RIM instruction is executed.

Demultiplexing of Address & Data
Bus

• up 8085 has 16 address pins A15-A8 and AD7-
AD0.
• Similarly up 8085has 8 data pins AD7-AD0,as
these pins are common which are used to
transfer 8 LSBs of address as well as 8 bit data
but at different time
• So AD7-AD0 pins are also called Time
multiplexed or Time shared address data pins
• In practical system the address and data of
AD7-AD0bpins are separated using external 8
bit latch(flip-flop)

• When up will transfer 16-bit address on A15-A8
and AD7-AD0 pins then at the same time up gives
logic 1 pulse on ALE pin, clk=1. When ALE signal
changes from 1 to 0 then 8 LSBs of address on
AD7-AD0 pins get stored in 8 bit latch
• When up will transfer 8 bit data through AD7-AD0
pins then up gives ALE =0, clk= 0, hence 8 bit data
is not stored in Latch.
• So complete 16 bit address is available on 16 bit
address bus A15-A8 and A7-A0.
• At the same time 8-bit data is available on 8 bit
data bus D7 to D0

Advantages of Common pins AD7-AD0
• As address and data pins are common i.e.AD7-AD0, so
number of pins of up 8085 required for address and
data gets reduced
Disadvantages of Common pins AD7-AD0
• As AD7-AD0 are common so up has to give some time
gap between address transfer and data transfer, hence
up becomes slow.
• If address & data pins would have been separate then
up can transfer address and data together in parallel so
system will become fast
• We have to connect external 8 bit latch IC74373, so
hardware increased.

Generation of various control signals
for I/O & Memory Organization
• To select one memory location, up will transfer
16-bit memory location address on address pins
and at the same time up will transfer IO/M’ =0.
• This 16 bit address with IO/M’=0 is used to select
one memory location
• To select one IO port, up will transfer 8 bit
address on 8 upper address line A15-A8 as well as
duplicated on lower address line A7-A0.
• At the same time up will output IO/M’=1.
• This 8 bit I/O port address with IO/M’=1 is used
to select one IO port

• When IO/M’ = 1/0 then it indicates that the
address on address pins is for IO port/Memory
location respectively
• To read 8 bit data from selected memory location
or selected I/O port up gives RD’=0 otherwise
RD’=1
• To store or write 8 bit data into selected memory
location or IO port up gives WR’ = 0 otherwise
WR’=1
• These control signals output of up i.e. IO/M’, RD’,
WR’ are decoded using OR-gate or using NAND
gate or using 3:8 decoder to generate 4 control
signals

• These 4 control signals are also given in table below
Sr.
No.
GENERATION OF
up
SIGNALS OUTPUT
BY up
CONTROL SIGNALS GENERATED
IO/M” RD’ WR’ MEMR’ MEMW’ IOR’ IOW’
1. up reading data
from memory
0 0 1 0 1 1 1
2. up writing data
into memory
0 1 0 1 0 1 1
3. up reading data
from IO port
1 0 1 1 1 0 1
4. up writing data
into IO port
1 1 0 1 1 1 0

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