ARCHITECTURE OF 8085

Whole architecture of 8085 can be divided into 5 parts namely;

1. Register Unit

2. Control Unit

3. Arithmetic and Logical Unit

4. Interrupt Unit

5. Serial IO Unit.

8085 ARCHITECTURE

1) REGISTER UNIT

Register Unit of 8085 consists of:

* Six General Purpose Register (8 bit). 

General Purpose Register are used to store 8-bit data. They are named as B,C,D,E,H and L. Programmer/user can use these registers to store or copy a data during execution of the program. Though these registers are of 8 bits but whenever microprocessor has to handle 16 bit data, then these registers can be used in pair as BC, DE and HL

* Two Internal Register (8 bit).

Apart from 6 general purpose registers, 8085 also provides two 8-bit internal register. Note that these registers are not available for programmer, it is only used for internal processor as example in the case of CALL and XCHG instruction.

* Two 16-bit Address Registers, Program Counter and Stack Pointer.

Program Counter is a memory pointer. We know that memory locations in 8085 have 16 bit address. Function of program counter is to point to the memory address from which next instruction (byte) is to be fetched. When a byte is fetched, the program counter is incremented by one. 

Stack pointer points to the top of stack. 

* One Increment/Decrement Counter Register 

This counter is used to increment/decrement the contents of various registers available in the register unit. For example- every time microprocessor access the memory, content of program counter is increment by one.

2) CONTROL UNIT

Control unit generates the signals within the microprocessor to carry out the various operations. Control Unit consists of 3 parts:

* Instruction Register 

This register holds the machine code of the instruction to be executed. When microprocessor executes a program, program counter reads the opcode of the instruction from the memory and stores it in the Instruction Register.

* Instruction Decoder 

Instruction Register sends the machine code to this unit and it decodes that opcode and finds out what is to be done in response of it and how many machine cycles are required.

* Control and Timing Unit 

It generates the signal within the microprocessor to carry out the instruction, which has been decoded.

3) ARITHMETIC AND LOGICAL UNIT

As the name suggests, this unit performs various arithmetic and logical operations like addition, subtraction, AND, OR etc. It consists of Accumulator, Flags and Temporary Register.

* Accumulator 

It is an 8-bit register and result of operations performed in ALU is stored in the accumulator. It is denoted by A.

* Flags

It includes five flip-flops, which are set or reset after operation according to the condition of the data stored in the accumulators and others registers. The five flags are Zero Flag, Carry Flag, Auxiliary Flag, Sign Flag and Parity Flag.  

4) INTERRUPT UNIT

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.

There are 5 interrupt signals in 8085 microprocessor: INTR, RST 7.5, RST 6.5, RST 5.5, TRAP.

5) SERIAL IO UNIT

* Serial Input Data (SID)

This line is used in the serial data communication. Through this pin, serial data is received by the microprocessor and is loaded into accumulator's bit 7, whenever RIM instruction is executed.

* Serial Output Data (SOD)

Through this pin, serial data is transmitted by the processor. Output of SOD is specified by the SIM instruction.

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