8085
Contents
Microprocessor
Architecture
Instruction Set and
Addressing mode
Microprocessor ?
A microprocessor is multi
programmable clock driven register
based semiconductor device that is
used to fetch , process and execute a
data within fraction of seconds.
PIN CONFIGURATION
THE 8085 AND ITS BUSSES
The 8085 is an 8-bit general purpose
microprocessor that can address 64K Byte of
memory.
It has 40 pins and uses +5V for power. It can run
at a maximum frequency of 3 MHz.
-The pins on the chip can be grouped into 6
groups:
Address Bus.
Data Bus.
Control and Status Signals.
Power supply and frequency.
Externally Initiated Signals.
Serial I/O ports.
The Address and Data Busses


The address bus has 8 signal lines A8 – A15 which are
unidirectional.
The other 8 address bits are multiplexed (time shared)
with the 8 data bits.
 So, the bits AD0 – AD7 are bi-directional and serve
as A0 – A7 and D0 – D7 at the same time.
 During the execution of the instruction, these
lines carry the address bits during the early part,
then during the late parts of the execution, they
carry the 8 data bits.
 In order to separate the address from the data, we
can use a latch to save the value before the function
of the bits changes.
SOD
SID
TRAP
RST 7.5
RST 6.5
INTA
RST 5.5
INTR
8085 ARCHITECTURE
Interrupt controller
Serial I/O controller
8 bit internal Data Bus
Multiplexer
Instruction Reg
Reg array
Temp Reg
Accumulator
Flag Flip
Flops
Instruction
Decoder and
Machine
Cycle
Encoding
ALU
W
temp
Z
temp
B
C
D
E
H
L
Stack Pointer (16)
Program counter (16)
Inc/dec latch
Timing and control
X1
Clk gen
Address Buffer
X2
Reset
out
Reset
Reset in
HLDA
DMA
IO/M
S1
S0
ALE
WR
Status
HOLD
Ready
Clk
out
RD
Control
A15 –A8
Address
Buss
Data/ Address
Buffer
AD0 –AD7
Address/
Data
Buss
Flag Register
The flags are affected by the arithmetic and logical
instruction
D7
D6
S
Z
D5
D4
AC
D3
D2
P
D1
D0
CY
Accumulator
It is an 8 bit register
 For any arithmetic and logical instruction one of the data
should be in this register
 It is used for storing the result of any arithmetic and
logical manipulations.
 It is also called as A register
 All the data which are sent to I/O devices are sent via
A register.

Temporary register
 It
is used to hold the data during the
operation of arithmetic and logical operation
Sign Flag
 If
the D7 bit of the accumulator is set then
this flag is set i.e 1 meaning that the result is
in negative.
 Ex. 7-8 = -1
Carry flag
During the arithmetic operation if a carry occurs then
this flag is set.
1 10
 Ex. F1+1F=

Carry
Zero flag
During
the arithmetic/ logical
operation if the result is zero then
this flag is set.
Ex. FF-FF = 00
Parity flag
 After
the of the arithmetic and logical
operation if the result is even then this flag is
set.
 Ex. 0A-02 = 08
Auxiliary carry flag
 During
BCD arithmetic operation when a carry is
generated by D3 bit and passed on to D4 bit then
this flag is set.
 Ex. 1F+11 = 0000 1111 +
0001 0001
= 0010 0000
Timing and control
 It
synchronizes all the operation with the
clock and generates the communication
between the microprocessor and
peripherals
Instruction Register and decoder
 The
instruction is loaded in the
instruction register
 The decoder decodes them and establishes
the operation that has to be performed
Register array
 The
W and Z register are temporary
registers
 Used to hold the 8 bit data during the
execution and it is used internally .
 It is not used by the programmer.
Control and status signals
Machine Cycle
IO/M
S1
S0
Opcode fetch
0
1
1
Memory read
0
1
0
Memory write
0
0
1
I/O read
1
1
0
I/O write
1
0
1
Interrupt ack
1
1
1
Halt
Z
0
0
Hold
Z
X
X
Reset
Z
X
X
Arithmetic and Logical unit
 It
is an 8 bit register
 It is used for performing addition,
subtraction and logical operation.
 AND, OR, NOT, XOR, CMP are
some of the logical operation.
Program Counter
It
is a 16 bit register
It is used to point out the address of
the next instruction which is to be
executed
Stack pointer
 It
is a 16 bit register
 It points the starting address of the stack .
Register Array
 B,
C, D, E, H and L are general purpose
register
 All are 8 bit register
 If the are combined as BC, DE and HL
they can store 16 bit data
Instruction set
An instruction is a binary pattern designed
inside a microprocessor to perform a specific
function.
 A group of instruction together called as
instruction set.
 Group of instruction set is called as a
program.

Classification of instruction set




According to word size or byte size it is
classified into 3 types.
1 - byte instruction
2 - byte instruction and
3 - byte instruction
1 byte instruction
 It
includes the Opcode and the Operand in
the same byte.
 Ex. MOV A,B
 Ex. CMP B
 Ex. ANA B
 Ex. RAL
Opcode an operand
 The
task to be performed is called Opcode
 The data to be operated is called
Operand.
2 byte instruction
 The
first byte specifies the operation cod eand
the next byte specifies the operand
 Ex. MVI A, 10
 Ex. SUI A, 34
3 byte instruction
 The
first byte specifies the opcode and the next
two bytes specifies the 16 bit address/data.
 Ex. LXI H, 4500
 Ex. JMP 5000
Classification of Instructions
The 8085 instruction are classified into 5 categories.
They are..
 Data transfer operations
 Arithmetic operations
 Logical operations
 Branching operations and
 Machine control operation

Data transfer operations
Group of data form a source location are
copied to the destination location without
changing the original data.
Various types of data transfer are:
 Between registers
 Specific byte to a register or a memory location
 Between memory location and a register
 Between an I/O device and accumulator

Arithmetic operations
 Function
like addition, subtraction, increment and
decrement operation are performed
Ex. ADD B
Ex. SUB B
Ex. INR B
Ex. DCX H
Logical operation
 These
instruction perform various logical operation
with the contents of the accumulator.
 Ex. AND B
 Ex. ORA
 Ex. RAR
 Ex. CMP B
 Ex. CMA
Branching operation
 It
alters the sequence of program executing
either conditionally are unconditionally
 Ex. JMP 5000
 Ex. JNC 4500
Machine control operation
 It
controls the machine operation
 Ex. HALT
 Ex. NOP
 Ex. INTR
Addressing modes
 The
various ways of specifying the operand
are called the addressing mode.
It classified is as
 Immediate addressing
 Register addressing
 Direct addressing
 Indirect addressing
 Implied addressing mode
Immediate addressing mode
Instructions that use immediate addressing
MVI R, DATA
Register Addressing mode
MOV Rd, Rs
Direct addressing mode
Instructions that include a direct address
IN/OUT Port #
LDA 4500
Indirect addressing mode

Register indirect instructions reference memory via
a register pair.
LXI Rp, 16 bit address
Implied addressing mode
Instruction deals with the accumulator
ORA
RAL