The Intel 8085 Microprocessor is an 8-bit microprocessor developed by Intel in 1976. It is an improved version of the Intel 8080 Microprocessor and was widely used in early microcomputer systems and embedded applications. The 8085 is called an 8-bit processor because it can process 8-bit data at a time. It has a 16-bit address bus, allowing it to address up to 64 KB of memory.
Table of Contents
Features of 8085 Microprocessor
- 8-bit microprocessor
- 16-bit address bus
- 8-bit data bus
- 64 KB memory addressing capability
- Single +5 V power supply
- On-chip clock generator
- Five hardware interrupts
- Serial I/O capability
- Multiplexed address and data bus
Architecture of 8085 Microprocessor
The architecture of the 8085 consists of several functional units:
Main Components
- Accumulator (A)
- Arithmetic Logic Unit (ALU)
- General Purpose Registers
- Program Counter (PC)
- Stack Pointer (SP)
- Flag Register
- Instruction Register and Decoder
- Timing and Control Unit
- Interrupt Control
- Serial Input/Output Control
Register Organization
The 8085 contains six general-purpose registers:
B, C, D, E, H, L
These registers are 8-bit registers and can also be used in pairs to form 16-bit registers.
| Register Pair | Function |
|---|---|
| BC | Data storage |
| DE | Data storage |
| HL | Memory pointer |
The HL register pair often acts as a memory pointer.
Accumulator
The Accumulator (A) is the most important register in the 8085.
- It is 8-bit.
- It stores operands and results of arithmetic and logical operations.
- The ALU performs all operations using the accumulator.
Example:
[
A + B \rightarrow A
]
Arithmetic Logic Unit (ALU)
The ALU performs all arithmetic and logical operations such as:
Arithmetic Operations
- Addition
- Subtraction
- Increment
- Decrement
Logical Operations
- AND
- OR
- XOR
- Compare
- Rotate operations
The result of these operations affects the flag register.
Flag Register
The Flag Register contains five status flags that indicate the result of arithmetic and logical operations.
| Flag | Name | Function |
|---|---|---|
| S | Sign Flag | Indicates sign of result |
| Z | Zero Flag | Set when result = 0 |
| AC | Auxiliary Carry | Used in BCD operations |
| P | Parity Flag | Set if result has even parity |
| CY | Carry Flag | Indicates carry/borrow |
These flags are very important in conditional branching instructions.
Program Counter (PC)
The Program Counter is a 16-bit register that stores the address of the next instruction to be executed.
Key points:
- Automatically increments after fetching an instruction
- Controls the sequence of program execution
Stack Pointer (SP)
The Stack Pointer is also a 16-bit register.
It:
- Points to the top of the stack
- Is used in subroutines and interrupts
The stack operates on LIFO (Last In First Out) principle.
Interrupts in 8085
The 8085 microprocessor supports five interrupts.
| Interrupt | Type | Priority |
|---|---|---|
| TRAP | Non-maskable | Highest |
| RST 7.5 | Maskable | High |
| RST 6.5 | Maskable | Medium |
| RST 5.5 | Maskable | Low |
| INTR | Maskable | Lowest |
Addressing Modes of 8085
The 8085 supports five addressing modes.
Immediate Addressing
Operand is present in the instruction.
Example:
MVI A, 45H
Register Addressing
Operand is stored in a register.
Example:
MOV A, B
Direct Addressing
Memory address is directly specified.
Example:
LDA 2050H
Register Indirect Addressing
The memory address is stored in a register pair.
Example:
MOV A, M
(HL contains memory address)
Implicit Addressing
Operand is implied.
Example:
CMA
Instruction Cycle of 8085
The instruction execution consists of:
- Fetch Cycle
- Decode Cycle
- Execute Cycle
These operations are controlled by the Timing and Control Unit.
The machine cycle consists of multiple T-states.
Example:
Instruction Cycle = Fetch Cycle + Execute Cycle
Pin Configuration of 8085
The 8085 microprocessor has 40 pins.
Major groups of pins include:
Address Bus
- A8 – A15
Multiplexed Address/Data Bus
- AD0 – AD7
Control Signals
- RD
- WR
- ALE
Interrupt Signals
- TRAP
- RST 7.5
- RST 6.5
- RST 5.5
- INTR
Serial I/O
- SID
- SOD
Applications of 8085
Although modern processors have replaced it, the 8085 was widely used in:
- Embedded systems
- Traffic light controllers
- Washing machines
- Industrial automation
- Educational microprocessor training kits
Advantages of 8085
- Simple architecture
- Easy to program
- Low hardware requirement
- Good for learning microprocessor fundamentals