8085 Microprocessor

 8085 Microprocessor Architecture

8085 is an 8-bit microprocessor. It mainly contains ALU, timing and control unit and registers. It has 8 bit data bus and 16 bit address bus.

Registers: Used to store date temporarily during the execution of program

                       

General purpose registers: The 8085 has six general purpose registers to store 8-bit data. They are denoted as B, C, D, E, H and L. They can be combined as register pair – BC, DE, HL – to perform 16-bit operations.

 

Special purpose register:

Accumulator (A): The accumulator is an 8-bit register used to store 8-bit data and to perform arithmetic and logical instructions. Accumulator is also used for input/output operations.

Flags:

        

  

There are five flip-flops which can be set or rest after an operation according to data conditions of the result in accumulator and other registers. They are sign, zero, auxiliary carry, parity and carry.

Instruction register: It us used to fetched instructions.

   Program Counter: It is used to store16 bit address of next instruction to be fetched

  Stack pointer: 16-bit register used to store address of most recent stack entry.

 Temporary registers: Programmer cannot access these registers. They are

         a)  W & Z registers: Used to store data temporarily.

         b) Temporary data register: Internally used for most arithmetic & logical instructions

Incrementer /decrementer address latch: 16- bit register used to increment or decrement the content of program counter or stack pointer

Address buffer: 8-bit unidirectional buffer used to drive higher order address bus (A15-A8)

 

Address/data buffer: 8-bit bidirectional buffer used to drive lower order address bus (A7-A0) & address bus (D7-D0)

Interrupt control: It has five interrupt inputs RST 7.5, RST 6.5, RST 5.5, INTR , TRAP and one interrupt acknowledge signal INTA.

Serial input/output control: In serial data communication one bit is transferred at a time over a single line. There are two pins to receive and transmit data. They are SOD and SID. Serial Output Data (SOD) line is used to send data serially. Serial Input Data (SID) line is used to receive data serially.

Timing and control unit: This unit is responsible for all operations in 8085. The functions are:

 

a.         Controls fetching & decoding of instructions.

b.         Generate appropriate signals for instruction execution.

c.         Generate signals required to interface external device to the processor.

 The control signals are:

1.         Status signals: S0, S1, IO/M

2.         DMA signals: HOLD, HLDA

3.         Reset signals: RESET IN, RESET OUT

4.         Control signal: RD, WR, ALE

5.         Clock: CLOCK OUT

Please note: RD,WR,RESET IN, INTA are active low signals.

For detailed class check my YouTube channel: 

https://www.youtube.com/watch?v=w3ytXx0KNoo&list=PLL3CgPb5iniJQymZSWfz8TC2Swun9281H&index=3

Reference: 'Microprocessor Architecture, Programming, and Applications with the 8085' Author Ramesh Gaonkar'