UNIT-IV
INTRODUCTION
The 8085 microprocessor is an 8-bit
microprocessor that was developed by Intel in the
mid-1970s.
It was widely used in the early days of personal
computing and was a popular choice for hobbyists and
enthusiasts due to its simplicity and ease of use.
The architecture of the 8085 microprocessor consists
of several key components, including the accumulator,
registers, program counter, stack pointer, instruction
register, flags register, data bus, address bus, and
control bus.
8085 Microprocessor
It is an 8-bit microprocessor designed by Intel in 1977
using NMOS technology.
It has the following configuration −
8-bit data bus
16-bit address bus, which can address upto 64KB
A 16-bit program counter
A 16-bit stack pointer
Six 8-bit registers arranged in pairs: BC, DE, HL
Requires +5V supply to operate at 3.2 MHZ single
phase clock
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.
Cont..
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.
Stack pointer
It is also a 16-bit register works like stack, which is
always incremented/decremented by 2 during push &
pop operations.
Cont..
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-flops −
Sign (S)
Zero (Z)
Auxiliary Carry (AC)
Parity (P)
Carry (C)
Cont..
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
Control Signals: READY, RD’, WR’, ALE
Status Signals: S0, S1, IO/M’
DMA Signals: HOLD, HLDA
RESET Signals: RESET IN, RESET OUT
Cont..
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.
Serial Input/output control
It controls the serial data communication by using
these two instructions: SID (Serial input data) and
SOD (Serial output data).
Cont.
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.
8085 Microprocessor Architecture diagram
