Parul Polytechnic Institute
Subject Code : 3330705
Name Of Subject : Microprocessor and assembly language
programming
Name of Unit : Introduction to Microprossor
Topic : architecture of 8085
Name of Faculty : H.M.Avaiya
Name of Students: (i) Prajapati Shivangi (007)
(ii) Upadhyay Nidhi (019)
Architecture of the 8085
processor
Introduction
• The internal architecture of the 8085 microprocessor is
shown in figure.
• The 8-bit data enters into the microprocessor through
the multiplexed address/data bus AD7-AD0.
• The 8-bit internal bus connects the accumulator,
temporary register, flags, instruction register ,interrupt
control unit, serial I/O unit, general purpose register,
stack pointer, program counter and data/address buffer.
• The ALU is fed from the accumulator and temporary
register.
• Register:
• The 8085 contains both 8-bit and 16-bit register which are
discuss below:
• (1) Accumulator(ACC):
• It is an 8-bit register and focus of all the arithmetic, logic,
load and store and I/O instructions.
• (2)General purpose register:
• There are six 8-bit general purpose registers B,C,D,E,H And
L.
• They can also be used as 16-bit register pairs as BC, DE,HL.
• The HL pair is used for storing 16-bit memory address,
while BC and DE are normally used to store data except in
few instruction.
• (3)Program counter(PC):
• Te program counter is a 16-bit register which stores
address of the next instruction to be executed.
• It is responsible for sequencing the program
execution.
• (4)stack pointer(SP):
• It is a 16-bit register which stores the address of top
of stack.
• Stack is a LIFO RAM to store data temporarily during
the execution of the program.
• (5)FLAG register:
• it is an 8-bit register which contains five 1-bit flags used by
conditional jump and call instructions.
• It is shown in figure.
• The flags are set or reset or according to the result of
execution of last arithmetic or logical instruction .
• The flag register is also known as PSW (program status
word) or F-register.
• There are five flags as follows:
• The carry flag(CY): is an overflow from the 8-bit addition.
• It acts as a “borrow “ flag during subtract.
PARITY FLAG
CARRY FLAG
SING FLAG
S
Z
ZERO FLAG
-
AC
-
AUXILIARY CARRY FLAG
P -
CY
• The parity flag(P): indicates no. of 1’s in the result of
arithmetic or logical instruction which is always stored
in accumulator. If no. of 1’s are even in the accumulator,
then it is called even parity and parity flag is set to 1. on
the other hand if no . of 1’s in the accumulator are odd.
Then it is called odd parity and parity flag is reset to 0.
• The auxiliary carry flag(AC): indicates an overflow from
the bit-3 of the result as like CY which is overflow from
bit-7. the auxiliary carry flag is generally used in BCD
operation.
• The zero flag(Z): is set to 1 when result of arithmetic or
logical instruction is zero . it is cleared when result is
not zero.
• The MSB of the accumulator after the arithmetic or
logical operation is copied into the sing (s)flag.
• The 1 in the sign flag indicates result is negative and 0
in the sign flag .indicates result is positive.
• Arithmetic and logic unit:
• It is a logic circuit which is responsible for performing
all the arithmetic and logical operations. The ALU
receives the inputs from the accumulator . The ALU
also accesses the flag register in order to set or reset
the individual flags according to the result it has
computet.
• Instruction register and decoder:
• During the op code fetch cycle , the 8-bit opcode of an
instruction is transferred into the instruction register . The
contents of the instruction register is received by
instruction decoder and it is decoded to find the meaning
of the instruction.
• After decoding the instruction, decoder directs the timing
and control section to perform sequence of events in
order to complete the execution of the current
instruction.
Thank you………..