2. Content
Introduction
Features
Pin Configuration
Architecture 0f 8085
The 8085 Bus Structure
Instruction Set
Addressing Modes
Timing diagrams
Reference
3. Introduction
Microprocessor is a Central Processing Unit (CPU) etched
on a single chip.A single Integrated Circuit (IC) has all the
functional components of a CPU namely Arithmetic Logic
Unit (ALU), Control Unit and registers.The 8085
microprocessor is an 8-bit processor that includes on its chip
most of the logic circuitry for performing computing tasks
and for communicating with peripherals.
4. Features
8 bit microprocessor(8085 microprocessor can read or write or
perform arithmetic and logical operations on 8-bit data at time)
It has 8 data lines and 16 address lines hence capacity is 216 =
64 kB of memory
Cock frequency is 3 MHz
It requires +5V power supply.
It is a single chip NMOS device implemented with 6200
transistors.
It provides 74 instructions with five addressing modes.
It provides 5 hardware interrupt and 8 software interrupts.
5. Pin Configuration
40 pins classified into
6 groups:
1. Data bus
2. Address bus
3. Control & status lines
4. Externally generated
5. Serial interface
6. Power supply & clock
6. Pin Configuration cont…
1) Address Bus (A15-A8 and AD7-AD0):
The microprocessor 8085 has 16 bit address lines from
A15-A8 andAD7-AD0.These lines are used to transfer 16 bit
address of memory as well as 8-bit address of I/O ports.
2) Data Bus:
The lower 8 lines (AD7-AD0) are often called as
multiplexed data lines.
7. CONTROL LINES
RD : Read: This is active low signal which indicates that the
selected I/O or memory device is to be read and also is available on
the data bus.
WR :Write:This is active low signal which indicates that the data
on data bus are to be written into a selected memory location.
IO/ M : (Input / Output / Memory):This is used to select either
Input / Output devices or memory operation.When it is high it
indicates an I/O operation and when it is low, it indicates a
memory operation.
STATUS LINES
Status Pins (S1, S0):The microprocessor 8085 has two status pins
as S1, S0 which is used to indicate the status of microprocessor or
operation which is performed by microprocessor.
8. SPECIAL SIGNAL
ALE (Address Latch Enable): TheALE signal is used to
enable or disable the external latch IC (74373/8212).
The external latch IC is used for the de-multiplexing of
AD7-AD0 lines, i.e., it is used to separate the address and
data fromAD7-AD0 lines.
IfALE = 1/0 then external latch IC is enabled / disabled
respectively.
13. Architecture 0f 8085
1. ALU
2. Timing and Control Unit
3. General Purpose
Registers
4. Program Status word
5. Program Counter
6. Stack Pointer
7. Instruction Register and
Decoder
8. Interrupt Control
9.Serial I/O Control
10.Address Bus
11. Data Bus
15. REGISTERS
The Registers are of 8-bit & 16-bit size used for different
purposes
A- Accumulator – This is an special
purpose register. All the ALU operations are
performed with reference to the contents of
Accumulator.
B,C,D,E,H,L – General purpose registers.
These registers can also used for 16-bit
operations in pairs. The default
pairs are BC, DE & HL.
16. F – Flag register – This register indicates
the status of the ALU operation.
PC – Program Counter – This is a 16-bit
register used to address the memory
location from where an instruction is going
to be executed.
17. SP – Stack pointer - This is a 16-bit register
used to address the top of the stack memory
location.
Temporary register, W & Z – These registers
are only used by 8085 and are not available for
the programmer.
18. ALU – Arithmetic & Logic Unit
ALU of 8085 performs 8-bit arithmetic & logical
operations. The operations are generally performed
with Accumulator as one of the operands. The result
is saved in accumulator register.
Timing & Control Unit
This unit works as the brain of the CPU and
generates all the timing and control signals to
perform all the internal & external operations of the
CPU.
Instruction Decoder & Machine Cycle Encoder Unit
This unit decodes the op-code stored in the
Instruction Register (IR) and encodes it for the timing
& control unit to perform the execution of the
instruction.
19. The 8085 Bus Structure
The 8-bit 8085 CPU (or MPU – Micro Processing Unit) communicates with the other
units using a 16-bit address bus, an 8-bit data bus and a control bus.
21. MPU Communication and Bus Timing
21
Figure 3: Moving data form memory to MPU using instruction MOV C, A
(code machine 4FH = 0100 1111)
22. Instruction Set
Broadly classified into two types:
Based on word size:
One word- Opcode only (CMA,ADD B)
Two word- Opcode ,an operand (MVIA,32H)
Three word- Opcode, operand, operand (LDA 4200, STA 4500)
Based on function:
Data transfer group (MOVA,B; MVI A,32H;MOV C,4500)
Arithmetic operations (ADD B, SBI 32H,INC D, DEC B)
Logical operations (ANA B, ORI 05H, RLC, RAR)
Branching operations (JUMP, JMP, JNZ, JC, CALL,
RETURN)
Machine control instructions (HLT, NOP,EI,DI,SIM,RIM)
23. Addressing Modes
Immediate (MOVA,B ;ADD B; SUB E;ANA C)
Register (MVIA,05H;LXI B, 20AEH;ADI 05H;ORI 07H)
Direct (LDA 4500H;STA 7500H;IN 09H;OUT 70H)
Indirect (MOVA, M;MOV M,A;ADD M;ORA M)
Implied(implicit) (HLT; NOP;RST;RET)
24. Timing diagrams
The 8085 microprocessor has 7 basic machine cycle.
They are
1. Op-code Fetch cycle(4T or 6T).
2. Memory read cycle (3T)
3. Memory write cycle(3T)
4. I/O read cycle(3T)
5. I/O write cycle(3T)
6. Interrupt Acknowledge cycle(6T or 12T)
7. Bus idle cycle
